This document was written in an effort to help those who either have a unit of are thinking of buying one. Its objective is to explain the UTM system and why you need it, particulary on land and to save you buying a unit which you will later find less than useful.
For purposes of better transmission, I gave broken the document into three parts. This is part one.
As for usefulness, if I had to make choices, I would choose in this order:
1.A GPS (with spare batteries!)
2. A compass
3. A map.
Ideally, you should have all three, but if I can have only one in strange territory, it would be the GPS. While, I always travel with a compass, force of habit, the compass compliments the GPS and saves its batteries. I don't always have a map with me.
I've had a Garmin 75 for over 2 1/2 years, having used it on 3 moose hunt and 3 deer hunts as well as small lake fish- ing in the bush. It is extremely useful and I find accurate enough for my needs. If I can get a clear spot without too much tree cover, I feel I can be within 50 yards. They will NOT penetrate a building,( glass yes,) not even heavy leaf cover.
Accuracy- I have set mine up on a government bench mark on a hill over Ottawa and averaging out 10 readings over 3 or 4 minutes, came within 6 yds of the mark! I don't however expect this accuracy in the bush.
I have some advice:
Be sure the unit you choose can handle the Universal Transverse Mercator grid system, comonly called UTM! Note the later refernce to MGRS. This is most important, at least on land. Don't waste time fooling around with Lat. & Long. unless you are on the water. If you can't get a definitive answer to this question when buying, DO NOT BUY.
If you already have one that does not handle UTM, all is not lost but you will need to use latitude/longitude This is fine if you are not referring to a map. For bush work without a map, the unit will do it fine. But for ease of conversion from map to GPS and back, you can't beat the UTM grid system.
Use only maps that have the grid. The 1:50,000 topos, in Canada at least, have a grid 2 cm. square, which is 1 kilometre on the ground. If your map is not 1:50,000, just remember that between the grid lines on maps of between 1;50M or 1:100M or in between, the grid is 1 kilometre, that is 1,000 metres or about 1,100 yards. I leave my GPS set on metric for this reason but a person can use UTM and still use yards/miles for units with no problem.
The important thing is to understand the grid system. A good book is:
"Map Use" by Phillip C. Muehrcke, JP Publications, Madison, WI
another is a small Canadian book, "Map Reading" L.M. Sebert which is quite very readable.
I spent three months wasting time before I realized how to equate the map grid to the GPS.
Batteries- I like the new rechargeables like Renewal alka- lines and also carry a spare set.
Do not travel with the unit on unless, plugged in to the battery of a car or boat. Instead, stop frequently, take a reading, save as a way- point if necessary and turn off. I can make batteries last several days on a hunting trip this way.
The literature, at least with the Garmin, while complete enough for nautical use, leaves much to be desired for land use. If the boys in Desert Storm had to rely on the Garmin manual, they would still be there!
When traveling to a new area, when I stop for gas or a coffee, I get a clear spot and take a 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. This might take a minute or so. There is no need to save this point. By doing this on the trip, when you arrive at your destination area, the GPS will not take long to get a good reading.
When you are scouting a new area, the GPS will positively tell you on which road you are on from the map if any doubt exists. This can save a lot of time if camp is not set up and it is getting dark.
The first thing to do in camp with the GPS is to get a good reading and store it as "camp". Way-points may be added as the hunt unfolds. But be selective of what way-points you save as too many can we worse that a few well chosen. Also, before you start to save way-points, decide how you going to name them. For example, a series on a trail could be TR1, TR2, TR3....A series of RD1, RD2, RD3...could be a road. With a series like this, you could plot them on the map later and draw the trail or road.
With a GPS, a person may explore the territory as he wishes, without having to worry about returning to base. The GPS will tell you the distance and direction of all way-points, give you a compass course to follow, and up-date it as necessary when you again turn it on. It is extremely hard on battery life to leave it on while traveling and not at all necessary. Don't get hung up on the nice graphics (unless you are connected in a car or boat).
Use the compass to point you in the right direction, turn it off and walk around the swamps, hills etc. Your next GPS position (which you don't need to store) will permit you to correct your heading and update how far you need to go.
Here is the exercise that I went through to discover how to use UTM.
For those of you who have way-points stored in Lat-Long, have no fear in losing them in the following exercise. For the Garmin 75, from the AUX screen, choose Units/ Heading and cycle thru the options for Posn, choose UTM/UPS.
Now with a gridded map of your location, go OUTSIDE and take reading. (I am reasonably certain that these direct- ions, with some modifation will work with all hand-held units). Wait until the reading seems more os less stable and appears like thus:
18T 0439535
5024750
Record the reading, turn off GPS and go inside to a topo map with the grid on it, put a pencil mark on your location (hopefully you know where you are)! 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. 0439535 above. Do the same with the numbers along the sides of the map and the second number, example above, 5024750. CAN YOU SEE IT NOW?!!!
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.
The UTM grid is that which is used by the military and others doing work on the ground. To understand the grid, imagine instead of longitude, the north-south lines are based on meridian lines 6 degree apart. These strips or srips are 6 degrees wide, centered on these meridians, starting at 180 degrees west and numbered towards the east.
Thus, all of north america is covered by those zones numbered from #7 to #22. Here in Ottawa, I am in zone 18. Where we hunt moose is zone 16, to the west. In referring to the grid, it is important to know what zone you are in and in giving a UTM reading, the Garmin 75 (and Iam sure all others) will give the zone first. In normal use in a small area, you can dis- regard the zone number, in my case 18T.
In using UTM grid coordinates, the "easting" from the zone meridian is given first, and then the "northing", always. Please note that east/west is the opposite to longitude which is measured from the east and called West, at least in the Americas. In reading coordinates, you READ RIGHT UP. In other words, the easting is always given before the northing. The coord- inates of a point could be given a number of ways. Here is an example:
18T 0439535 5024750
This is the location of my home in Ottawa to the best precision the Garmin 75 will give. The 18T refers to zone 18 and normally is not used. The other 14 digit number must be broken up:
0439535
is the EASTING from the zone 18 meridian, down to the last metre. If you place a decimal thus:
0439.535
to the right of the decimal is 0.535 of a kilo- metre or 535 metres and to the left are whole kilometres, 439. In practice, 0439.535 would be used as 395, dropping the 04 at first and rounding the 0.535 to 0.5 and ignoring the decimal.While the decimal in not written and the 5 refers to 500 metres or 0.5 of a kilometre, this gives a ground precision of 100 metres.
Thus 5024750 for the NORTHING becomes 24.7
I could therefore say I live at 395247 in Ottawa area which is an accuracy to withing 100 metres (110 yards).
To be more precise, I could say 39542475 which is an accuracy to within 10 metres.
A grid coordinate usually has an even mumber of digits, the first half being the easting and the balance, the northing. But , when it is a full listing e.g. 18T 439535 5025750 the Northing will have the extra digit.
If you refer to a UTM grid on a map, of either 1:50,000 or 1:100,000 scale, the grid squares will 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. 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 much harder to explain in words than to take a map and show you.
I waisted 3 months before in frustration I experimented, setting my Garmin from the UNITS|HDG page to Posn UTM/UPS and Navmetric, and Hdgauto mag variation.
As I said earlier, if you are happier with miles etc., leave the Nav at statute miles (or nautical for the water in which case, you are better with hddd.ddddd or a variation).
Here is another point, this same AUX page of the Garmin for Hdg permits you to choose the automatic magnetic variation so you have no compass calculations.
The other Hdg choices are grid heading, true, etc. If you are plotting a bearing on the map by grid, set the Hdg to grid and input the bearings. While for practical purposes, you can disregard the small difference between Grid north and True nort, for accurate bearings, it should be used.
In my own case, for Hdg, I choose the automatic magnetic variation which is presumably downloaded from the satellites almanac data for my location.
There is one other setting which is best left at the factory setting until you fully understand it. From the Garmin AUX menu MAP DATM page, you will see that it is set at WGS 84. Unless you are out of North America, leave this setting alone. There are many other map datums to choose from, depending on the area in which you intend to use the unit. I have tried NAD27 (Canada 1927) and NAD83 (N. America, Alaska, Canada, Central America & Mexico) but reverted to the WGS 84.
My intention is to test these 3 datums against my bench mark and see is there is a "best" one for me. First however, I need warm weather!
If you have followed me this far, you are well on the way to truly making good use of your GPS. The more that you use it, the easier that is becomes and the more confidence you will gain.
I will make my apologies now to those more technically mind- ed out there who may find fault with this simplified explanation. I am sure many could do it better BUT, the fact is there is a great dearth of knowledge and a thirst for many who are now hearing of the wonders of GPS.
There are some 25 or more navigation satellites in orbit, I forget the exact number. The more there are however, the better the GPS will work.
The various models of GPS track from 5 to 8 satellites. The Garmin 75 tracks 8. This is all very well in theory but it is rare that 8 are within sight at the same time. I think that the number of satellites that a unit will track, above 5 is not too important. The limiting factor is the number that are in view when you want a reading.
There is one other factor which can be important. I think all of the small hand held units can only process the data from one satellite at a time. This being the case, the unit must cycle through the satellites one by one before giving a reading. My understanding is that the better units will use more than one channel for processing simultaneously rather than cycling. This means a faster and more accurate position reading.
Each satellite is traveling in an orbit and from reference to land based stations, it knows where it is at any given instant. Likewise, it knows the exact time. Be sure to set your unit to local time, subtracting or adding the number of hours difference from you and Greenwich MT.
I think each one also has the "almanac" which gives the position data not just for itself, but the other satellites as well.
When you turn on your GPS and point it to the sky, it starts picking up signals from the satellites, starting presumably with that with the strongest signal. It then cycles through the others gathering more info. What it does, as I see it, is measure the time of signal transmission (radio) from the satellite to itself. Let's say it reads 3 satellites and knows the distance from each to itself. If you picture a sat for a moment not moving, but with a line the length of which is the same as the distant from GPS to sat, if you extend this line to the earth, you could trace a circle on the earth's surface and the you and the GPS are on that circle some place. Now with the line from another sat, you have another circle on earth. You are at either of the two points where the two circles intersect. But which one? Easy, the third sat settles the matter. Here is where more sats are useful.
Thinking of the circles, it is evident that the position of the sats in reference to the GPS is critical for accuracy. The farther apart they are and the closer to the horizon, the better accuracy that you will achieve. This is why the GPS will not work in the house! It will however pick up satellites through glass. Neither will it give the best accuracy through tree cover, leaves, branches etc. It will however give useful readings in the bush even though the accuracy is off somewhat.
Because the sats are moving across the sky, under tree cover, they will move in and out of sight quite rapidly. A good GPS will give you an idea of the accuracy of the reading. Garmin call it DOP for dilution of precision, and will also give a distance error.
A DOP of 2.5 or 3 is OK, at least for me in the bush. I have seen it as good as 1.2 but this is with an almost clear horizon and good sat coverage.
There are at times a brief period when not enough satellites are in sight and well positioned to give an acceptable read- ing. Usually within 10 minutes or so, a useful reading is possible. Fortunately, this is not common.
For those readings taken on water, with a clear horizon and good geometry of the satellites, the reading will be much better, as will the altitude (but who needs altitute at sea?!)
As for the degradation introduced by those responsible for the sat system, it means nothing on land (unless you are a surveyor and therefore you know all about it!) As for coast use where the GPS is boat mounted, you are of course running it on engine battery power and can leave the unit on all the time. Also, with a radio connected to the GPS, you can receive the differential corrections for the degradations which greatly increases the accuracy.
Size is a factor, but they are getting smaller.
While most of the better ones will handle UTM, some lower priced ones will not. I know our Canadian forces have used the grid system for many years, using six figures to locate a spot, say for artillery fire.
I note that the description in some catalogs refers to M.G.R.S. which stands for Military Grid Reference System. This I believe to be UTM.
-Mine uses 4 AA cells. They are light to carry. I like the Rayovac Renewal with the small charger. In hunt camp, when the generator is on at night, I recharge.
If you are going to use the unit in a vehicle or boat, then be sure to get whaever you need to hook into the electrical system and save you AA pack.
-If I was buying today, I would also buy a cig. lighter connection for the truck.
-WARNING, for those of you who are fortunate enough to own a hand-held altimeter, don't expect the GPS to take its place. You must have a very low DOP to achieve anything near a useful altitude reading to relate to a map. For the bush here in the eastern Canada, I forget it. If I was in the mountains of western Canada, then I am certain that the overhead coverage would be good enough to give meaningful elevations.
I know that the Garmin 75 will do all that is required and probably many of the others will too but be sure before putting out the money.
The fact is few adverts I have seen mention UTM and its usefulness. Most are slanted towards nautical, fishing off-shore use. While some of the magazine rated them, no article that I saw even mentioned UTM.
We have tried my Garmin 75 on the dash board of a vehicle at 60 MPH and the speed reading was more accurate, I am sure, than the speadometer of the car.
Because the sats are moving, a GPS left on and stationary will give a continuous but variable reading. I am fortunate in Ottawa to have access to a bench mark, the location of which is exactly known. While testing my unit, I took ten readings over 2-3 minutes. By using the Pythag theorem, I worked out the hypotenuse (or error) for each of the 10 readings and the average was about 5 meters (5.5 yards) off! If you are fortunate enough to find a BM, the exact location of which is known, you can conduct the same test.
With respect to power needs, the Garmin 75 comes with a universal mount for the dash. I have taped a 4 D cell battery case to mine and so can use it without draining the AA cells in the unit's own battery pack.
Units that will handle UTM (MGRS for Miliitary Grid Reference System)
[NOTE: I received the following information from [email protected]. MGRS and UTM are not the same. They are similar, but not close enough. MGRS uses a 2 letter grid reference system. A full MGRS reference would appear as: 12BH1234567890. This would be (12) Grid( 100,000 square meters) BH,zone (10,000 square meters), and 1234567890 which would give a position within 1 meter within 1,000 square meters.]
Garmin 75
Garmin 40
Trimble Scout GPS (MGRS)
Magellan Recon Trailblazer (MGRS)
(the lower priced Trailblazer "geared more towards the civilian, hiker, hunter, tourist, or boater" says U.S. Cavalry latest catalog but DOES NOT DO IT. I don't know who wrote this but it is the sort of stuff that will lead the uninitiated to make a bad purchase.
I note that the Cabela's Fall catalog lists the better Magellan cheaper than U.S. Cavalry.
As for sources, in 1992 when I was looking and knew little. E & B Discount Marine,
toll free USA 1-800-533-5007 international 1-908-819-4600
had the best prices and the most selection. They also have many retail stores and an excellent catalog (which does not completely answer my question of will they handle UTM).
I would appreciate hearing the experiences of others in GPS use, details on units, sources, and prices.