A Practical Guide to GPS - UTM
by Don Bartlett
Last revision - November 23, 2002 Broken Born Link Fixed
With SA gone since May 2000, your GPS may be used to check boat speed when sailing or trolling and that your track will be much more accurate when slowly traversing rough ground.
With respect to the use of two portions of the Canadian topographical map, BOUCHETTE, 31 J/4, used in this Guide, I hereby acknowledge, with thanks, the copyright of the Canadian Government:
Table of Contents
(N.B. This Guide started as a text posting to rec.backcountry Feb.10,1995) Most of the articles and literature on the Global Positioning System until then had only described the use of a GPS unit in a nautical setting. As valuable as they are on water, they are equally as valuable in the bush, desert, mountains, tundra, or, just in Central Park.
This is written therefore for hikers, fishermen, hunters and all persons who wish to traverse the wilderness in the full knowledge of where they are, where they have been and where they wish to go, primarily in conjunction with a gridded topographical map and a compass.
In this discussion, I will at times use "GPS" to mean the actual hand held unit itself as opposed to the whole system of satellites and their control. I will also use "fix" to mean the actual position indicated by the unit in UTM coordinates or Lat/Lon.
The most basic requirements for land use of a hand held GPS are:
1. The ability to give a position reading when not unduly impeded by overhead tree cover, thus virtually requiring a 12 parallel channel receiver unit such as the Eagle Explorer, Garmin 12XL or Magellan 315 or comparable units by these manufacturers.
2. The capability to store at least 100 way-point (WP) positions in the unit's memory and preferably, many more.
3. The ability to list these WP positions giving their distance and direction from the current position.
4. If to be used with gridded topographical maps, the unit must be able to handle the most common UTM datum. (explained later.)
5. Automatic track recording should be available and as well, a Track Back feature.
6. If there is any thought of hooking the unit up to a computer for uploading, downloading, map making or having a computer map available during travel, then there must be a cable connection for the computer.
7. In regard to the above, the unit must be able to accept and send data in the standard format, I believe that used by Garmin is the most common.
Any added features are a bonus but not necessarily needed and must be judged on their own usefulness.
All of the units have many other features, most of a nautical nature which may or may not be useful on land, depending on the circumstances.
The basic requirement for the ease of use of a GPS on land and the correlation of the readings directly to a map require that the GPS be capable of handling the UTM grid system (Universal Transverse Mercator grid). While there are other grid systems in use, the UTM system is by far the most common. Within the UTM system, there are many map datums specific to the area of interest. (More about this later.)
When we have roads, power lines, heights of land, mountains, rivers and lakes, these are often sufficient along with the map and compass for us to establish with a reasonable degree of accuracy, where we are on the map. In those featureless areas however, where all looks the same for miles and miles of slowly rolling bush-land, grassland, dessert, tundra, sea etc., the GPS takes over and will give a definitive location which may be found and plotted on a map.
The usefulness of a GPS is now well recognized and interest is increasing all the time. As a result, there are those who do not know how to use a compass and map but, who after buying a GPS feel that they now have the ultimate tool for ease of travel in backcounry. If a person does not have a basic understanding of how to use a compass nor the desire and/or ability to learn, a hand held GPS is not for them. Indeed, in a hostile environment, they may well come to grief. There is a learning curve in using a GPS intelligently and until time and effort has been spent, a user may well be a hazard to himself and others.
A GPS is just another aid to traveling, high tech, yes, BUT must be used in conjunction with at least a compass and hopefully a detailed gridded topographical map. While using a GPS greatly assists in traveling in featureless areas, it does not mean that there is no need to still maintain an independent record of way-points, routes and directions in case of a system failure.
See the excellent tutorial available at:
I always travel with a compass in the bush as the compass compliments the GPS and saves its batteries. I don't always have a map with me. I do however always carry a small coil bound notebook and pencil for making very basic notes in case something untoward happens to the GPS unit or the whole system (and I admit not using them enough!). The GPS should not be left on and used in place of a compass when on internal battery power in the wilderness unless there is an ample supply of batteries! In fact, unless the GPS is moving at several miles an hour, a GPS WILL NOT TELL YOU DIRECTION. When it is moving, it will give direction in relation to the direction that you are travelling. Let me reiterate, A stationary GPS will not take the place of a compass. Think about it, it knows its position in relationship to the satellites but it has no idea how it is oriented. Some people know exactly where they are in a large department store but have no idea of north. Others in the same situation instinctively update their internal sense of direction and know roughly where to find north.
Like any other sophisticated instrument, a GPS unit is only as good as the person using it. A lack of knowledge of the limitations of the GPS system is responsible for many of the reports of so-called unit/system failure.
A GPS antenna will not receive good signals from within a building. Likewise, leaves and branches of trees will interfere and degrade the signals with the result that the readings, if any, are slow to obtain, if at all. The sophisticated units (meaning multi-channel receiver units) are better at obtaining readings under these conditions than the cheaper units.
The GPS system has been is use for a number of years and used by sailors and pilots, with great success. BUT, in an airplane or on a boat, the sky is in view, from horizon to horizon. Any GPS, even the least sophisticated, lowest price model single channel multi- plexing unit will give good results under these perfect conditions. In addition in most planes and boats, an external power source is available.
To expect an inexpensive single/double channel hand held unit to perform under conditions never experienced on sea or in the air is ludicrous. Yet, this is exactly what many new users have done. As a result, specific makes and individual units have both been given a bad reputation and said to be unreliable. Much of the blame here belongs directly on the various manufacturers who have not adequately explained in their advertisements, literature and manuals, the limitations in the system and requirements for use. In addition, misleading claims for the accuracy of units and the system are made in some of the mail order catalogs and some staff in stores have been known to give faulty information. The best defence when buying a GPS unit is have a good understanding of the system and the differences in the units - which is no different than buying anything such as a car, computer, stereo system or whatever.
Now that most newer units have at least 12 parallel channels, the speed of obtaining a usable reading is only a fraction of the time that the older multiplexing units needed and the frustrations experienced by earlier users will be minimized.
From four seasons hunting and fishing in the Canadian bush with my single channel unit, I found that the readings were always sufficient for my needs, if I took the time to pick a clearing with as much sky and horizon showing as possible and took readings about every 1/2 hour. Now with my 12 parallel channel unit (Garmin 12XL), I have no difficulty obtaining a fix.
The GPS satellites are owned and controlled by the US Department of Defense and this agency has the prerogative to degrade the accuracy for purposes of national defense. This is done by what is called "Selective Availability" or what is commonly known "SA." SA is now turned "off" and the accuracy of GPS readings are now much better. Preliminary reports suggest 95% time, 10 metre accuracy and 50% of the time 5 metre accuracy. See Sam Worley's site linked below.
As a Canadian, I am very grateful for the GPS satellite system paid for by US taxpayers. And SA never hindered me in any way
The gain in accuracy without SA is still not sufficient for me to do my own surveying, to mark a hidden treasure or to find a bow-hunting tree-stand in the middle of a cedar swamp at 4am in the morning.
On June 16, 2000, I did a easy test on the Babbage site and within 5 minutes, when I knew satellites would be in a good position, obtained a fix, ONLY 5 metres east of the position and with the northing exactly correct. This was posted to sci.geo.satellite-nav today. If anyone wants a copy of this tes, just e-mail me.
DGPS is a system whereby the GPS unit receives corrections by way of radio signals from a GPS unit at a known location. While most hand held GPS units are said to be "DGPS Ready," that is for hook-up, this is most commonly used by boats.
If you intended to bury a treasure and come back in five years to dig it up, then a simple GPS would not be the answer by just taking a reading. Instead, take a reading on a feature that you will recognize easily from at least 25 to 50 yards. Then take a compass bearing and measure with a tape measure to the treasure and record the coordinates (of the recognizable feature), the bearing (compass direction) and the distance. Another way to accomplish this would be to choose two ground features readily recognizable and to take compass bearings from each to the treasure.
There is some evidence that different makes and models of various GPS units are slightly more accurate than others. To explore this, check into Tom Born's website listed in the links at the end. But for all practical purposes, they are equal in accuracy.
HOW THE GPS WORKS
This explanation is not technically exact but will give the basic idea of how it functions.
The GPS unit does not send out radio signals, it only receives them. There are some 28 navigation satellites in orbit about 12,000 miles above the surface of the earth, and more might be added. Each makes an earth orbit every 12 hours.
Each satellite knows from reference to land based control stations where it is at any given instant. Likewise, it knows the exact time in UTC (Universal Time Coordinated, formerly Greenwich Mean Time). Each satellite also has the "almanac" which gives the same information about the other satellites as well their own orbits.
When you turn on your GPS and point the antenna to the sky, it starts picking up signals from the satellites, starting with the strongest signal. It will obtain the almanac information for all of the satellites.If an older multiplexer, it will cycle one by one through the next three satellites best in view, before starting to make its calculation to give a position. If it is a newer multi channel parallel unit, when four channels have good data, it will then calculate its distance from each of the satellites to itself.
Let me digress here to explain just why a 12 parallel channel unit is so much better than a single or double channel multiplexing unit. Here is an analogy to compare a multiplexing unit with a newer parallel channel unit. Imagine two hunters entering a forest to shoot birds (the satellites. The bag limit is four, the number needed for a 3-D fix). The first hunter is normal, two eyes, arms, legs and uses a pump shotgun holding four shells. When a flock of birds take flight, he swings on the first one and it drops. He reloads and starts to aim for a second, but it flies behind a tree just before he shoots, so he picks another and gets it. He must still kill two more but they are getting further away. He will be lucky if he gets the third and fourth. BUT what about the other super hunter, who is endowed with four pair of eyes, ears, arms, legs etc. has a brain capable of processing all of the incoming information at once and has four single shotguns, one for each pair of arms, eyes etc. When his flock of birds get up, each gun, separtately and independently controlled, picks a target and in one loud "bang", four birds drop. Same with the GPS units. A multiplexer is slow and might not get a fix in tough conditions, whereas the multi parallel channel unit has a much better chance of getting the data in a fraction of the time. In my opinion and with no evidence other than my use of Garmin 75, Garmin 38, Magellan 2000, Garmin 12XL and Magellan 315, I would estimate that the latter two units will get a fix between 10 and 100 times faster than the three units, all multiplexers. While a minimum of four receiving channels is necessary for parallel processing, the extra ones are used to keep track of whatever satellites are in view and to be ready to replace one of the four being used.
Back to how the unit works. Let's say it receives data from three satellites and thus knows the distance from each one to itself. If you picture a satellite for a moment stationary, with a line, the length of which is the same as the distance from GPS to the satellite, if you extend this line to the earth, you could trace a circle which touches the earth's surface making a circle or an arc, depending whether or not it is overhead or towards a horizon. You and the GPS are somewhere on that arc or circle. Now with the line from a 2nd satellite, you have another circle or arc on the earth. You are at either of the two points where the two arcs or circles intersect, but which one? The third satellite settles the matter and you will have a position. At this point, you will be given a position reading, Garmin calls this 2D mode, that is, no elevation update. If a fourth satellite is in view, it brings more precision to the calculation and an elevation will be given, that is 3D mode.
For a good positional fix, the required four satellites should be located around yet well above the horizon. Unfortunately, the opposite is true for an elevation reading. In the latter case, the satellites should be more overhead. Generally speaking, the GPS system does not give accurate enough readings of elevation to be useful, the error being normally 150% to 200% of the horizontal error - for example, if the position error is 50 metres, the elevation error could be 75 to 100 metres.
With respect to the delay of giving a reading when turned on, the oftener a GPS is used, the faster it will give a reading when next turned on. The first time that a new single channel unit is used it could take up to 15 minutes for a reading! This is termed a "cold" start and has nothing to do with the temperature. Once it has the initial reading and orients itself, it will take much less time. Normally, after 20-30 minutes, less than a minute is needed for a reading (a "hot" reading). But some of the older units even with a "hot" start are slow. On the other hand, my Garmin 12XL never ceases to amaze we how quickly it will get a position, "hot" or "cold", near or far from where it went to sleep.
When the 12 parallel channel units (Eagle Explorer and Garmin 12XL) first appeared on the market, there was a lot of misinformation floating around, particularly concerning the older and now outdated single or dual channel units.
Tracking 8 or 12 sats is NOT the same as having 8 or 12 parallel receiver channels in a unit! And to make it worse, some sales people either through ignorance or worse, will state that a unit tracking 12 satellites is a 12 channel unit. Not so!
My old Garmin 75 "tracked" up to 8 sats but it only read data from one sat at a time and had to cycle through them to get data from four for a fix. It is was much slower than a Garmin 12XL which has 12 parallel channels.
Do not buy anything less than a 12 parallel channel unit.
DILUTION OF PRECISION - DOP or ETIMATED POSITION ERROR - EPE
Some GPS units will give you an idea of the quality of the reading. Usually it is called DOP for dilution of precision, as in the Garmin 75. Also given is a average error in metres or feet. Some other makes call this the Position DOP or PDOP.
The Garmin 12XL only gives an EPE (estimated position error) in metres or feet, depending on the setup. When the satellites are well positioned, this EPE figure will drop close to 4.0 meteres.
The lower the number, either DOP or EPE, the more accurate will be the coordinates given for the position.
With SA now OFF, much more accurate fixes for waypoints may be obtained from a GPS. Most newer units may be left for a period of time to average out the readings. In my experience using my Garmin 12XL, I find that if after turning it on, I immediately and rapidly, push the arrow keys in a clockwise or counter clockwise direction, then when the satellite status page comes on the screen and a 3D fix is obtained, the DOP figure is displayed on the satellite page in the lower right. Thus, if I want to average a wayoint position reading before saving, I can choose the most advantageous time to do so. I try to average only for the brief of time when the DOP and/or EPE are/is very low. (See last paragraph below under SA and How to Defeat It.)
Because the satellites are moving across the sky, when a reading is attempted under tree cover, they will move in and out of sight quite rapidly. This is where a unit with parallel receivers excels because it takes much less time (about 1/10 or less) to have enough data to work out the fix. As mentioned earlier, it is the moisture in the leaves which absorbs the signal (or more correctly, the dissolved constituents of the moisture.)
You will at this point ask, but what about rain, snow, fog and clouds.
Evidence is that weather does not unduly affect the accuracy of the units.
There are at times brief periods when not enough satellites are in sight and positioned well enough to give acceptable readings, especially in hilly wooded country. Usually within 20 minutes or so however, a useful reading is possible. Fortunately, this is not common. When this is seen, it will probably be experienced again about an hour later the next day. This is fortunately rare when using a a 12 parallel channel unit. I only mention it so that if it happens, it will be recognized for what is is, and nothing to be alarmed about
For those readings taken on water, with a clear horizon and good geometry of the satellites, the readings will be much better but you may find the elevation puts you below water level. This is not a system error but involves the distance from the GPS to the center of the earth. Remember, elevation given by a GPS is not to be relied upon for accuracy.
ELEVATION -ERRORS IN
Generally, the elevation errors will be great enough to make the readings useless.
SA AND HOW TO DEFEAT IT - THE BLOB - TAKING A READING OVER TIME
This section is now (May 2,2000) redundant but will be left in , at least for now.
The "blob" method of averaging a fix, to minimize SA, really now only applies to the older GPS units but here is how it is done. Turn the unit on, set it to record "track" on the "Plot" screen and after an interval of at least 60 minutes or longer if possible, check the plot of the track that the unit records. As the "track" is recorded by the pixels on the screen, it will draw an uneven "blob" with the position of the unit in the center of the screen. If you leave it on until the "+" (at least on the Garmin 75) appears to be well centered in the blob, take an immediate reading and you will have the best reading that you are ever going to get with SA "on" and a simple unit. I call this the Blob method of achieving an accurate position or fix.
Some units, such as Garmins, permit the "+" on the plot screen to be moved (panned) and if so, it could be centered on the blob in order to take the reading, not waiting until the blob centered itself over the "+" in the center of the screen.
But I believe that today, all of the currently available 12 parallel channel units will perform this position averaging function automatically, if asked to do so. With the Garmin 12XL, Mark the position then then choose, AVERAGE and leave the unit unmoved for as long as conveniently possible. During this period, the unit will show an estimated error which over time will gradually become smaller. When finally, you can leave it no longer, choose SAVE, and give the WP a name. For this exercize, an hour is good, three hours better and over night (plugged into car) great.
DIFFERENTIAL GPS - DGPS
Where DGPS radio stations broadcasting the corrections to be applied because of SA are locally available, and these corrections are fed to the GPS unit, accuracy is much improved, perhaps giving an error of 20 metres instead of closer to 100.
LATITUDE/ LONGITUDE VS. UNIVERSAL TRANSVERSE MERCATOR (UTM)
When I initially started using the GPS, I worked with latitude and longitude and struggled making the conversions back and forth between the map and the GPS. In realizing that there had to be a better way than this, I found the relationship between the topographical map grid (UTM) and the GPS.
If UTM gridded maps are available, don't waste time fooling around with Lat. & Long, at least on land.
On the other hand, if you already have an older GPS (e.g. Eagle/Lowrance) that does not handle UTM, all is not lost but you will need to use latitude /longitude. This is fine for bush work without a map. But for ease of conversion from map to GPS and back, you can't beat a grid system, UTM being by far the most common.
Although most of the world has been mapped using UTM in one datum or another, gridded maps are not always available being kept to the military in some countries.
On checking Bowditch, The American Practical Navigator, 1977, a standard marine text, reference is made to the fact that some harbours of the world have been mapped with the UTM grid and such maps are available.
Most maps have latitude and longitude and can be used by making the calculations between degrees, minutes and seconds. However, the topographical maps of scales, from 1:100,000 down to 1:50,000 and below usually also have the entire map gridded with the UTM system. Whenever possible, only purchase these maps. The Canadian topos in the 1:50,000 series, have a grid 2 cm. square, which is 1 kilometre on the ground. This means that each mm. on the map is 50 metres (or about 55 yards). If your map is not 1:50,000, check the scale to establish the grid size. While 1:24,000 US topos are gridded at 1,000 metres ( 1 kilometre), I have seen a US Forest Service 1:100,000 map with a 10 kilometre grid. Likewise, the Canadian 1:250,000 maps have a 10 kilometre grid. My aviation advisor tells me that these are great for flying.
The newest USGS topos show a 7.5' X 15' rectangle on a scale of 1:25,000, metric, using NAD 27 UTM. They are available mail-order from:
USGS Map Sales, Box 25286, Denver, Co. 80225 and cost $4 each plus $3.95 shipping for each order. N.B. I am unsure (Dec 7, 1999) if this information is still correct.
Also, many hiking and outdoor shops carry local maps.
There has been some talk of the USGS dropping the UTM grid from their maps and only leaving a + where the intersections occur. If you only have such maps, that is with a + mark at the intersections of the grid lines, then take a very sharp pencil or fine pen and add the lines. (And complain to your maps source.)
Other Map Sources: Phone 1-800 USA MAPS for topos
Previously, when traveling any great distance, when I would stop for gas or a coffee, I took a GPS reading, not saving it. This is because when first turned on, the GPS remembers where it was the last time that it was used. When it finds that the satellites are not where they should be, it must then update its data and work out the new calculations (a cold start). This might take a couple of minutes. There is no need to save this point. By doing this on the trip, when you arrive at your destination, the GPS will rapidly give a reading (a hot reading). My old Garmin 75 would give a fix from a cold start after moving hundreds of miles within 2 to 3 minutes. Now, with the newer units, the delay for a reading with a cold start is no great problem (with a garmin 12XL at least.)
If you have hooked up the unit to the car electrical system, then you may have continuous read-outs right to your destination. (see below)
WAY POINTS -WPS
Way-points are locations worth recording and storing in a GPS. They may be check points on a route, significant ground features, e.g. camp, the truck, a fork in a trail, where Charlie shot his deer, or whatever. They may be put in manually, having taken the coordinates from a map. This can be done before ever leaving home. Or more usually, they are entered directly by taking a reading with the unit at the location itself, giving it a name and then saving. They may also be put into the unit by reference to a WP already stored, giving the reference WP, the distance and compass bearing to the new WP. Most GPS units permit the name and/or an icon to show on the navigaton map screen along with the track and/or route.
Arriving at a new camp, the GPS should be turned on, and the location stored as CAMP. You may choose a tent icon to show on the GPS map screen. As the days unfold and more territory is explored, more way-points are named and stored. But be selective of those that are saved. Too many can be very confusing. Better, a small number of good ones than too many poor ones. In this regard, before you store any, think out a nomenclature. With units now able to record track travelled, you may only wish to save WPs of road/trail intersections, bridges etc.
USING WPs in TRAVERSING THE BUSH
With a GPS, a person may explore a new territory as he wishes, without having to worry about becoming lost. The GPS, after locating itself, will give the distance (in kilometres or miles) and direction (magnetic course preferably, true course or grid course) for the nearest 10 or so way-points. Say that you want to go to camp. The GPS says 3.4 kilometres at a direction of 120 degrees. Turn off the GPS, head out about 120 degrees with compass in hand and walk the easy route. You do not need to travel 120 degrees exactly. Walk around each hill and swamp. After traveling for a while in the general direction, turn on the unit take another GPS reading and check distance and direction now to CAMP. You will find you are now only 2.2 kilometres but must head 145 degrees. You now can turn it off and alter your course further south. You repeat this until the distance drops to within 50 or so metres. Yours camp should be in sight.
Remember, these units are mostly all oriented for nautical use and as such, when left ON, will do a GOTO and give continuous updated readings of direction to go and remaining distance to the destination. They will in effect "navigate" all the way. BUT, this technique presupposes that you have ample battery power (external hook-up) AND a clear view of the sky. This technique is standard procedure on the water and must have worked well for Dessert Storm.
But this "navigation" with a live hand held GPS in the wilderness where topography and tree cover can give intermittently poor reception in my opinion is a NO NO. I believe that much of the frustration of earlier users was because they were asking too much of the system, had only marginal units and were using them under much less than ideal conditions.
While this advice of not "navigating" on land was pertinent with the units of several years ago, since 1997 and the lower battery drain of current units, it is not as important and indeed is acceptable, remembering the need to always carry spare batteries. But in doing so, much that is to be seen will be missed if too much focus is on the GPS and not more on the terrain and scenery. When I am travelling in the bush, my GPS is at the back of my head, securely closed up in a very small belted pack, in a platic bag and turned on to record track.
UTM - AN ATTEMPT TO EXPLAIN
Here is the exercise that I went through to discover how to use Universal Transverse Mercator grid system.
If you already have a GPS with way-points stored in Lat-Long, have no fear of losing them in the following exercise. The GPS will convert them back and forth effortlessly.
For the Garmin 12XL , from the MAIN MENU choose SETUP MENU and from the this screen, choose NAVIGATION.
From the screen now showing, choose for the POSITION FRMT: UTM/UPS.The other choices are various version of Latitude/Longitude degree, minute, second formats. The UPS, by the way, is a type of grid system used in polar regions.
Now, move the cursor down to MAP DATUM and if you know the datum of the map at hand, choose it from the hundred or so presented. They are in aphapebical order and can be scrolled through very fast. If you don't know the correct datum, then use WGS 84, or NAD 83. In Canada, most maps are either NAD 83 or NAD 27 Canada.
Now, from same screen, cursor down to CDI. I leave this as is, at 0.25 and go on down to next field, UNITS. Here I choose METRIC for the UTM system is so based. For those none too familiar with metric, a metre (meter?) is a super yard of about 39 inches e.g.100 metres=110 yards and a kilometre about 2/3's of a mile.
Lastly from this screen, go down to HEADING: and there are several choices. If you choose AUTO, as I have, your unit in giving bearings, will give them directly as compass bearing, magnetic declination being automatically calculated for your location and applied to the true bearing. In my own case, here in Ottawa, the declination is West 15 degrees . The other choices are User, True or Grid. The latter, Grid refers to the UTM grid lines of the map which are not exactlly true north/south, east/west but close enough, they can be accepted as true for all practical purposes.
Now with the gridded map of your location, go OUTSIDE, turn on your GPS. (I am certain that these directions, with some modification will work with all hand-held units). Keep away from trees and buildings as much as possible.
Wait until you get a 3D reading, and the "speed" shown in the Navigation screen drops to zero or close (it will continually vary because of SA) and appears something like this:
Record this reading, turn off GPS and go inside to the gridded map, put a pencil mark on your location (hopefully you know where you are!) If you look along the edges of the map, you will see that the grid lines are numbered. Look at the lower left hand corner of the map. You should see a 6 digit number, printed horizontally with the 2nd and 3rd digits printed slightly larger than the others. The larger numbers are the ones relating to the vertical grid lines and will increase from left to right. Look again at the lower left hand corner and left of map, printed vertically will be another number with 7 digits, the 3rd and 4th printed larger. These are the numbers increasing from bottom to top numbering the east/west grid lines. Now with the grid numbers on the map and what you recorded from the GPS, look for the relationship. Look at the grid numbers along the bottom of the map and see how they match up with the first number, e.g.
Close to where my house is on the map, the grid line 39 appears along the map bottom. Likewise, looking at the grid numbers going up the left (and right) side of the map, I see 24 which is also close. My house is east and north of where north-south grid line 39 and east-west grid line 24 cross. The difference east and north of the 39 and 24 crossing is given by the last 3 digits in each number, e.g. 535 meters east and 750 meters north.
As I said earlier, the UTM grid on the map is the key to coordinating the map with the GPS. By using it, you can easily go back and forth with an accuracy of 50 meters or about 55 yards on a 1:50M map. You will do even better on larger scale map. The UTM grid has been in use for many years. To understand the grid, imagine instead of longitude, that the north-south lines are based on meridian lines 6 degrees apart. These strips are 6 degrees wide, centered on these meridians, starting at 180 degrees west and numbered towards the east.
Thus, all of North America (with the exception of the western Alaskan islands) is covered by those zones numbered from #5 to #22. Here in Ottawa, I am in zone 18. Where we hunt moose is zone 16, to the west. Yellowstone Park is in zone 12. In referring to the grid, it is important to know what zone you are in and in giving a UTM reading, the GPS will give the zone first. The letter, "T" in this case, is the UTM reference to how far north or south of the equator you are. In normal use you can disregard the zone number and letter. Take a look at
Please note that the above explanation is for a position north Lat. and west Lon.
USING UTM COORDINATES
In using UTM grid coordinates, the "easting" from the zone meridian is always given first, and then the "northing". Also take note that for UTM, measurement is from the west, going EAST, the "easting" which is opposite to longitude. In reading coordinates, you READ RIGHT UP. In other words, the easting is always given before the northing. The coordinates of a point could be given several ways. Here is an example:
This is the full location description of my home in Ottawa to the best precision the Garmin 75 will give. (NB-I have redone it by the "Blob" method and it is actually closer to
To the right of the decimal is
I could therefore say I live at
It will be recognized that there will be a repetition in numbers going from one location of the country to another. To alleviate this confusion, in using 6 or 8 digit coordinates, the map must be identified. If not, then the whole number must be given, e.g.
A grid coordinate usually has an even number of digits, the first half being the easting and the balance, the northing. But, when the number of digits after the zone (
As a matter of interest, the northing,
If you refer to a UTM grid on a map, of either 1:50,000 or 1:100,000 scale, the grid squares will usually be 1 kilometre (1000 metres) to a side. It is simple to estimate between grid lines to 1/10 or 100 metres. For closer precision, on a 1:50,000 map, each 1 mm. is 50 metres on the ground. On the newest 1:25,000 US topos, 1 mm. would represent 25 metres on the ground. Note, the most common Silva compasses have a mm. scale on one side and this may be used to measure east from a grid line and north from the E-W grid lines.
All of this sounds much more complicated than it really is. It is harder to explain in words than to take a map and demonstrate.
So lets try it. This is a cut from the lower left corner of BOUCHETTE, QUÉBEC, a 1:50,000 Canadian topo, 31J/4 Edition 2.© N.B. See above Crown Copyright Notice.
Remember, I said that with UTM, you READ RIGHT UP that is you do the Easting and then the Northing.
The blue numbers along the bottom margin are the grid numbers for the Easting. The first whole number is 424000m.E The 24 is emphasized because this is the number which goes up by 1 for each N/S grid line going to the right, e.g. East. The line of 424000m.E is 424 Kilometres East of the base line for Zone 18. Thus going from the line numbered 24 to that to the right, 25, is one kilometre or 1,000 metres.
The same applies for the blue numbers going up the left margin, the first line being 5095000m.N which is 5,095 Kilometres North of the equator.
Looking at the dot that I inked in with the 3 arrows surrounding it, the full UTM coordinates for it would be:
Where did I get the 200 for easting and 850 for northing? Just by eyeballing the map. (Note: If anybody gets to this little lake, give me a report on the fishing.
There are areas of the world where a non standard UTM grid is used. An example in North America is the province of Québec, which has a series of MTM (Modified Transverse Mercator) maps, 1:20,000, in black and white maps which are more detailed than the standard Canadian 1:50k topographical maps.
These Québec maps, instead of using a 6 degree slice of longitude, uses a 3 degree slice. Here is how to set a Garmin 12XL to use this map grid.
You must have such a map available first of all. My Québec map is NAD83 so the GPS must be set to this datum. Now pick the whole degree longitude to your west that is divisible by 3. In my case, this is 75 deg. west. To this figure, add, 1.5 degrees. Go into the Setup Menu and choose Navigation. On this page, NAV SETUP, for Position Frmt: choose User Grid, which is adjacent to UTM/UPS. When the USER GRID screen comes up, set the Longitude of Origin to the figure you calculated earlier, in my case, W076 degrees 30minutes.Now move cursor down to Scale. In this case, use 0.9999 whereas standard UTM used 0.9996.
Now, you must put in a false Easting figure. Use the figure 304800 (same as one million feet).
As for False Northing, just leave it as 0.0 as the False Northing on the Québec maps is the same as standard UTM.
You are now ready to test the settings. Choose a location that may be positively identified on the map. For example, where two roads meet. Using your knowledge of how to read a UTM grid, write down the easting and northing.
Turn on the GPS at this spot and leave it to average out the readings for a few minutes. When you have a reading, write it down and go back to the map. If all has gone well, you will find that your test point GPS coordinates now agree with the the MTM map.
Although the false northings used my UTM and MTM are both 0.0, due to the scale difference, a northing position given in MTM will appear 1,081 metres north of that given by UTM
That's all there is to it.
SETTING UP YOUR UNIT
In setting up a unit, there are a number of choices to be made. What will suit one will not suit another. Furthermore different makes of units use different terms for some of the functions and available screens are not always the same.
Generally speaking, all units will permit certain choices to be made in the SETUP menu. If the unit is being used for other than position finding, e.g. setup, uploading or down loading data, entering waypoints and/or routes, then under the System Setup, the mode chosen should be Simulator.
In explaining how to use UTM, I gave directions in setting a unit to use UTM and some other basic settings.
With the Garmin 12XL, there are several other settings to be made.From the SETUP MENU, for this exercise and any other where you are not outside to receive satellite data, choose, SYSTEM first and at the top of the screen, under MODE:, choose Simulator. This prevents the unit from attempting to obtain a fix and saves batteries. Now, on same screen, go to OFFSET: and input the number of hours that your time zone deviates from UTC (Greenwich Mean Time.) For Ottawa, EST in North America, my setting is "-05.00" hours. In Summer is is -4 hours. I also choose HOURS: as 12, preferring this to the 24 hour system. On this same page, I choose how long the light will stay on in seconds and whether or not I want Tone signals. Being partially deaf, I don't use the audible signal function. This recalls a story I heard of a man who wanted to know how to turn off the beeps, they frightened his horse.
The above is the basic type of setup needed before using a unit and relating the readings to a map.
Much of the setup was done when I explained UTM. There are however a few more odds and ends to mention.
If you examine your topo map carefully, you should find the map datum given . On my 1:50M older Canadian topos, at the bottom in fine print, just to the right of the map name and the scale, it states "North American Datum 1927". The newest map dated 1995 gives North American 1983 (which is essentially same as WGS 84). I therefore have my unit set to the NAD27 Canadian datum but would change for to NAD 1983 or WGS 84 for the newer map. Similarly, on a US Forest Service 1:100M maps, on the lower right hand corner, it states "Universal Transverse Mercator Projection 1927 North American datum". This is sufficient reason to reset the GPS to the NAD27 datum. Therefore, check a map over carefully to find out what datum is used and set the GPS to it. The Garmin comes set with WGS 84 which may be left as is but if you compare a reading with a map using NAD 27, you will find a discrepancy of 100 metres or so in the easting and a smaller one in the northing. As a matter of interest, if you are using a GPS without reference directly to a map, the Datum to which the GPS is set will not matter as all readings will be relative. Only if you try and compare one of these readings with a map will the error appear.
OPT MAP SCREEN AND SETUP
From the Map Screen, cursor over to OPT at the top, right and push ENTER. Now choose the first, MAP SETUP and from this new screen, I like North UP. I keep the RINGS on YES; normally ROUTE is OFF; NEAREST, YES; NAMES usually NO; Tand finally TRACK LOG is a YES.
Going back now to the OPT (0ptions) screen by pushing QUIT, I now choose TRACK SETUP. Because I am often down loading a previously travelled Track to a computer mapping program (QuoVadis) I leave RECORD set to FILL. This means, once all of the 1024 track points in memory are used, I do not overwrite them. If I were not mapping but just wandering, I would set the RECORD to WRAP or even OFF. Too much wandering with the track being recorded makes a cluttered map screen. As for METHOD, I use AUTO rather than a TIME but I have not really explored the difference.
This same Track Setup screen is where you can check how much of the memory is used and where the log can be cleared. N.B. Always CLEAR the Track Log before starting to record a new trail for later uploading and mapping.(see Mapping - Transferring Tracks..below)
To use Route and/or Track, you must leave the unit on with antenna clear. In the earlier units, this used too much battery power but this caveat no longer applies with the new units. For easy mapping, first clear the Track from the map screen then set to record the Track at your point of departure. With my Garmin 12XL ON and no external antenna, I carry the unit in a very small bag at the back of my neck (a minature fanny pack) and my track is kepT until I return home and download it to the computer.
As far as the Route is concerned, I do not normally set a Route for bush travel and so leave it OFF. If however, I had earlier made a Route from a Track, created a new one using Waypoints, I would then turn on the ROUTE plotting on the Mao screen.
Note- by all means when you first buy a GPS, after taking a reading leave it and go for a walk with the track recording turned on. The graphics are great and you may be able to zoom in and out on the plotting screen. A WARNING- don't try this for the first time in a strange bush unless you have lots of batteries with you. This warning is not so important with the newer units. I have no problem saving a track of well over 250 points over 3-4 hours and having battery power to spare with my Garmin 12XL. If you bought the unit with a cigarette lighter plug, try this in your vehicle.
Used to exchange and receive data with another GPS or computer. When your GPS is correctly cabled to a serial port on your computer, desktop or laptop, and you have the proper software installed on the computer, tracks, routes, waypoints may be downloaded, added to maps and saved.
QuoVadis on the WWW
I am using NAD27 Canada for most of my maps although some of the newer one are NAD 83. If you choose a datum different than used by the map, your coordinates will not agree. If you find the GPS does not agree with the map coordinates, check this.
If you examine your topo map carefully, you should find the map datum given . On my 1:50M older Canadian topos, at the bottom in fine print, just to the right of the map name and the scale, it states "North American Datum 1927". The newest map dated 1995 gives North American 1983 (which is essentially same as WGS 84). I therefore have my unit set to the NAD27 Canadian datum but would change for to NAD 1983 or WGS 84 for the newer map. Similarly, on a US Forest Service 1:100M maps, on the lower right hand corner, it states "Universal Transverse Mercator Projection 1927 North American datum". This is sufficient reason to reset the GPS to the NAD27 datum. Therefore, check a map over carefully to find out what datum is used and set the GPS to it. The Garmin comes set with WGS 84 which may be left as is but if you compare a reading with a map using NAD 27, you will find a discrepancy of 100 metres or so in the easting and a smaller one in the northing.
The above is cut is from the bottom of the same map pictured above, topo map, 31J/4, titled . © N.B. See Above Crown Copyright Notice. Underneath the left end of the conversion scale for elevations, you will find in small print, "CONTOUR INTERVAL 50 FEET" and below that in even smaller print
Elevations in Feet Above Mean Sea Level North American Datum 1927 Transverse Mercator Projection
Therefore in using this map with your GPS, you would set the DATUM as NAD 27 (Canada.)
BUYING A GPS
TWELVE PARALLEL CHANNEL UNITS
With the newer units which have twelve parallel channel processing (Eagle Explorer and newer Garmins such as 12, 12XL, II, III are good examples), it means that the 5 (or more) best SATs are being tracked and data from the 4 best processed at the same time which in practice means faster fixes, particularly under difficult conditions e.g. wet trees. Previously, the trade-off was that they used more battery power than the older multiplexers, were usually larger and cost more. But that has all changed and these new units are much easier on batteries than older single channel units!
The descriptions in some catalogs refer to M.G.R.S. standing for Military Grid Reference System. This is a newer grid reference system, similar to UTM if not the same.
Batteries are no longer the problem that they were a few years ago. Most Garmins use 4 AA cells and the newer Magellans only need 2. Furthermore, they use much less power than older units.
ALWAYS CARRY EXTRA BATTERIES WARNING-Leave fresh batteries in your unit at all times and check on a regular basis to be sure they are OK. Not to do so could prematurely run down the internal lithium battery which maintains your data.
HOOKING UP TO AN EXTERNAL POWER SUPPLY
Since this article was first written in February 1995, I have mounted my G75 in my Explorer, drawing on the car battery. I bought a BNC cable from Radio Shack so that I may place the antenna externally on a rear side window or set it on the dash board on the passenger side. In initially getting a fix, the external antenna mount is quicker but once locked on the SATs, the dash board location works fine. It gives me direction of travel in magnetic (or would do so in Grid or True), speed as well as the UTM coordinates as they change. The plot screen will give the track I have covered as well.
With my Garmin 12XL, I use elastic bands to hold it to a small sand bag and leave it usually beside the front seats on the low console. Even this low, it has never failed to hold a lock once obtained.
This is a great set-up for mapping the bush roads we use in the north in the fall. Simply, clear the map screen TRACK settings, then leave the unit to record as you travel. The unit will faithfully record your track over the ground which later may be uploaded directly to the map in your computer through the interface cable.
This 5 year old clip shows an old Garmin 75 unit on an ATV and was said by the European motorcyclists then to be the only unit robust enough to take the pounding on the fron of a XC mororcycle. b
If your unit does not have an external antenna and you wish to use it on the dash board of the car, turn the unit on outside the car and wait until it gets a reading before bringing it inside and placing it on the dash or in a mount.
The windshield on some cars has a metallic coating through which the GPS can't receive the weak signals from the satellites. In such a situation, you can change cars or, cheaper, use an external antenna outside the vehicle.
If you are going to use the unit in a vehicle or boat, then be sure to get whatever you need to hook into the electrical system and save your AA pack.
MAPPING - Transfering a Track to a Map
I have only started interfacing my GPS to my computer since buying the Garmin 12XL and obtaing Tourateck QV with the Québec maps on CD. The process is much easier than I had anticipated. I have no external antenna on my Garmin and find that one is not needed. In the vehicle, maybe using the power cable if I am travelling any distance, or on internal batteries, I leave the unit on the low console between the front seats (after always turning on the unit outside the car and getting a position). If I am afoot, I use a very small fanny pack bag with the strap adjusted short and the GPS in the bag and at the back of my neck. It rides there very nicely and I can swivel it around to the front to open and set a WP very easily. Otherwise, the GPS is out-of-site and forgotten although I do occasionally check the battery reserve power and the memory unused. I see no need to have the GPS in my hand when I am traversing new country or trails. Generally speaking, in the hilly forested country that I traverse, I am continually adjusting my path around trees, swamps, rock out croppings etc., following the easiest path. If I am targetting a particular spot and mapping a tril, then before setting out, I have entered the coordinates of the spot as a WP in the GPS. I therefore know from my starting position and and at any subsequent check, exactly what my compass course should be and the distance to travel. I do not do a GOTO as such, just get the info from the entry on the nearest waypoint list. This means that my compass is checked for direction if necessary and not the GPS
This is a very brief explanation to encourage those (like myself) who have held back from interfacing. Good Luck!
Given that the system can guide flying cruise bombs at whatever speed they travel, most units will have no trouble tracking a hiker or boater. I have tried my Garmins in my vehicles at 60 MPH and the speed reading was more accurate, I am sure, than the speedometer of the car. Also, it was giving changes in position, heading, and keeping a plot. This is great fun but drains the AA cells very quickly, unless plugged in to the car electrical system.
HOW NOT TO USE YOUR GPS
This has been written in an effort to assist in making the right purchase decision and to shorten the learning curve in using a GPS. I have been accused of being biased towards Garmin and UTM. I make no apology for these perceived biases. The Garmin 75 was my first unit. I bought another model from a competitor, sold it, bought a Garminn 12XL As for UTM, where gridded maps are available, to use lat/lon is masochistic in the extreme.
The information I have given is thought to be accurate but I assume no liability for errors, omissions or for someone getting into difficulties in using a GPS and this guide.
A Practical Guide to GPS - UTM by Don Bartlett