A PRACTICAL GUIDE TO GPS (with emphasis on UTM use on land) Fri Jan 5 15:12:35 1996 Message : #37119099 From: Don Bartlett Address : ae374@FreeNet.Carleton.CA Subject : "A Practical Guide to GPS" (UTM) Jan.5/96 Org. : The National Capital FreeNet revised Jan.5,1996 Most of the articles and literature on the Global Positioning System have only described the use of a GPS unit in a nautical setting. As valuable as they are on water, they are equally valuable in the bush, desert, mountains or tundra. This is written 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 map. 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. 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). This is the most important message in this whole article. While all GPS units handle latitude and longitude, not all are capable of converting lat/long to the UTM coordinates. Most of us travel off the beaten track with a compass and a map. When we have heights of land, mountains, rivers and lakes, these are often sufficient along with the map and compass so that we can, with a reasonable degree of accuracy, know where we are on the map. In those featureless areas however, where all looks the same for miles and miles of slowly rolling bushland, the GPS takes up the slack and will give a definitive location which may be plotted on the map. As for usefulness, if I had to make choices, I would choose in this order: 1. A GPS Hand Held Unit (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 unit. 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 3 years, having used it extensively in northern Ontario. It is extremely useful and I find it more than accurate enough for my needs. If I can get a clear spot without too much tree cover, my readings will be within 50 yards or so, usually less. A GPS antenna will not receive good signals from within a building. Likewise, leaves and branches of trees will interfere and degrade the readings. From three seasons hunting and fishing in the Canadian bush however, I have found that the readings are always quite sufficient for my needs, if I take the time to pick a clearing with as much sky and horizon showing as possible and take readings about every 1/2 hour. Moving about while trying to get a reading helps also. ACCURACY I have tested mine while set on a government bench mark on a hill overlooking Ottawa and averaging out 10 readings over 3 or 4 minutes, came within 6 yards of the mark! This was under the best of circumstances and I don't however expect this accuracy in the bush. In fact my results were as much by chance as the system is not that accurate. The GPS satellites are controlled by the US Department of Defence and this agency has the perogative to degrade the accuracy. When this program is activated, which is almost always, accuracy still is no worse than 100 meters. While I did not have my unit during Desert Storm during the Gulf War, I understand that the system accuracy was not degraded at that time so that commercial civilian units could be used. If you intended to bury a treasure and come back in five years to dig it up, then a 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 50 to 100 yards. Then take a compass bearing and measure with a tape measure to where you want to bury the loot. Record the coordinates, the bearing and distance for future use. If you want to travel however over the ground and relate back and forth to the map without trouble, and not worry about losing camp or the truck, then a GPS is for you. (see the story later about losing a car.) HOW DOES THE GPS WORK? 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 25 navigation satellites in orbit about 12,000 miles from the earth, and more will hopefully be added. Each makes an earth orbit every 12 hours. The more there are, the faster the units will give a reading and the accuracy should also improve because of the choice of better positioned satellites. Perhaps those controlled by the former USSR will be integrated into the US system eventually. Each satellite knows from reference to land based control stations where it is at any given instant. Likewise, it knows the exact time. Each one also has the "almanac" which gives the positions, not just for itself, but the other satellites as well and of course the correct time. When you turn on your GPS and point the antenna to the sky, it starts picking up signals from the satellites, starting with with the strongest signal. It then cycles through the others in sight gathering more info. It measures the times of signal transmissions (radio) from the satellites to itself. Let's say it reads three satellites and 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 where it touches 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. This is my over simplified understanding and normally, four sats are required. Thinking of the circles, it is evident that the position of the satellites in reference to the GPS is critical for accuracy. Within certain limits, 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 well in the house! It will however pick up satellites through glass. Neither will it give the best accuracy through tree cover, leaves, branches etc. With respect to speed of getting readings, the oftener a GPS is used, the faster it will give a reading. The first time that a new unit is used it could take up to 15 minutes for a reading! This is termed a "cold" reading 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). DILUTION OF PRECISION (DOP) Some GPS units will give you an idea of the quality of the reading. Garmin calls it DOP for dilution of precision. Also given is a average error in metres or feet. The lower the DOP number, the more accurate is the reading. A DOP of 2.5 or 3 is OK, at least for practical bush travel. I have seen it as good as 0.9 but this is with an almost clear horizon and good satellite coverage. 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. It is the moisture in the leaves which absorbs the signal. There are at times a brief period when not enough satellites are in sight and positioned well enough to give an acceptable reading, especially in hilly wooded country. 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 altitude at sea? Normally, the altitude error will be 2 or 3 times that of the position error. As an example, if the ground error is 50 feet, then the altitude error could be as much as 150 feet. As for degradation introduced by those responsible for the satellite system and termed Selective Availability (SA), it means little on land. When it is on which is the "normal" situation, accuracy worldwide should be no worse than 100 meter (110 yds) and will often be much better. For coast use, where the GPS is boat mounted and wired up to the electrical system, the radio may be connected to the GPS, and you can then receive the differential corrections broadcast by the Coast Guard which will give increased accuracy. I don't view differential corrections of any practical use for hiking, ever if they were available. LATITUDE/LONGITUDE VS. UNIVERSAL TRANSVERSE MERCATOR (UTM) GRID 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. This is most important. Don't waste time fooling around with Lat. & Long, at least on land, when the UTM grid maps are available. If you already have a GPS 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. If you are buying, be sure the unit you choose can handle the Universal Transverse Mercator grid system, commonly called UTM. If you can't get a positive answer to this question when buying, DO NOT BUY. This does not apply to marine use. MAPS 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 grid too course for practical work. The newest USGS topos show a 7.5' X 15' rectangle on a scale of 1:25,000, metric, using NAD 27 UTM. They fold like a road map. Thet are available from: USGS Map Sales, Box 25286, Denver, Co. 80225 abd cost $4 each plus $3.95 shipping for each order. I leave my GPS set on metric because the UTM grid is metric but a person can use UTM and still use yards/miles for units with no problem for distances. Just remember that the standard UTM grids are metric, usually 1 kilometre (1000 metres or about 1100 yards). Other Map Sources: Phone 1-800 USA MAPS for topos The following two books explain the grid system clearly: "Map Use" by Phillip C. Muehrcke, JP Publications, Madison, WI Another is a small Canadian book, "Map Reading" by L.M. Sebert which is more readable. TRAVELLING When travelling any great distance, when I stop for gas or a coffee, I take 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). If you have hooked up the unit to the car electrical system, then you will 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 data from a map. This can be done before ever leaving home. Or more usually,they may be entered directly by taking a reading with the unit at the location itself, giving it a name and then saving. Arriving at a new camp, the GPS should be turned on, and the location stored at CAMP. 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 one than too many poor ones. In this regard, before you store any, think out a nomenclature. For example, a series on a trail could be TR1, TR2, TR3..... and RD1, RD2, RD3.... for a road. With a series like this, you could plot them later on the map and draw the trail or road. 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, 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 120 degrees more or less and walk the easy route. You do not need to travel 120 degrees exactly. Walk around each hill and swamp. After travelling for a while in the general direction, 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 100 or so metres. You are now within the circle of error which is due to SA and camp should be within sight. UTM - AN ATTEMPT TO EXPLAIN Here is the exercise that I went through to discover how to use Universal Transverse Mercator grid system. For those of you who 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 75, from the AUX screen, choose UNITS/HEADING and cycle through the options for Posn>, choose UTM/UPS. Now with a gridded map of your location, go OUTSIDE and take a reading. (I am certain that these directions, with some modification will work with all hand-held units). Wait until the reading seems more or less stable and appears like this: 18T 0439535 5024750 (Please note, the above coordinates disagree slightly with those given in my current E-Mail signature. The difference is I believe do to using NAD 27 for one and WGS 84 for the other.) 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 botton 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. 0439535 above. 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 a 1:24M topo. 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 west- ern 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. 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: 18T 0439535 5024750 This is the full location description of my home in Ottawa to the best precision the Garmin 75 will give. The 18T refers specifically to the area of the earth where I am located, 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 closest metre. If you place a decimal thus: 0439.535. (The leading 0 is redundant as EASTING only needs 6 digits). To the right of the decimal is 0.535 of a kilometre or 535 metres and to the left are whole kilometres, 439. In practice, 0439.535 could 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 kilometer, this gives a ground precision of 100 metres. Thus 5024750 for the NORTHING becomes 247 I could therefore say I live at 395247 in the Ottawa area (or on a map showing the 18T UTM area) which gives an accuracy to within 100 metres (110 yards). (This six digit reference system has been used for years by the military to designate targets, objectives, rendezvous points etc. I learned it in high school cadet classes during WW11 but forgot all about it until I rediscovered it with GPS). Taking into account the possible 100 yards error due to SA, a 6 digit UTM coordinate is really all that is needed. To be more precise, I could say 39542475 which is an accuracy to within 10 metres. In both cases, you would need to know to what map I was referring. 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. 18T 0439535 5024750 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 (18T here) is odd, e.g. 18T 439535 5025750, the extra digit belongs to the northing. As a matter of interest, the northing, 5025750 in the above example, is the distance in metres north from the equator. The metric system defines the metre as 1/10,000,000 of the distance from the equator to the north pole. As a verification of this, I live close to 45 degrees north. 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. 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 show you. 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. I will use the Garmin 75 as an example, going through the more important auxiliary functions. OP MODE SCREEN OP MODE - Operating Mode I leave it set at Battery Saver mode except when I am doing a simulation or extracting data when I set it at Simulator. Note- if you are connected to the car electrical system, then use Normal. FILTERS - I leave them on automatic PLOTTING SCREEN ORIENTATION - I like north up ROUTE & TRACK - No route or track. (For this to operate, you must leave the unit on with antenna clear. This uses too much battery power!) Don't get hung up on the graphics! unless you have hooked up to an external power source. (see below) RESOLUTION - .100 KM TRACK - OFF Note- by all means when you first buy a GPS, after taking a reading leave it on take and go for a walk with the route and track recording turned on. The graphics are great and you may be able to zoom in and out on the plotting screen. This exercise is very interesting and fun but it is hard on batteries. A WARNING- don't try this for the first time in a strange bush unless you have lots of batteries with you. If you bought the unit with a cigarette lighter plug, then try this with your vehicle. (see below) UNITS/HEADING SCREEN POSN - UTM/UPS Note: This is where you cycle through various lat/long choices and then choose UTM/UPS. On some newer units you may also choose the MGRS for the latest grid system. (See below) NAV - I like Metric because the UTM grid is metric but statute will give miles and will not interfere with UTM, except in converting back and forth, map to GPS. You also may choose nautical. HDG - Auto mag Var (I have a West variation of 14 degrees and to make an error could be bad. I therefore leave it on auto GPS. This gives me courses in magnetic and I can forget about the variation). The other Hdg choices are grid heading and true heading. if you are plotting a bearing on the map by grid, then set the Hdg> to grid. While for most purposes, you can disregard the small difference between grid north and true north, for the most accurate bearing, they should ne noted. ALARMS SCREEN I have all of this turned off. It is great if you are operating on a boat with external power. For the alarms to be used the unit must be left on. I had one man tell me though, that the alarms frightened his horse so he turned then off for that reason. DATE/TIME SCREEN LOCAL OFST - This is the difference in hours between GMT (it isn't really GMT as it is off by slight bit- a technicallity) and local time. Here in Ottawa on EST, the offset is -5.00 This must be changed when you go to DST. In summer, my offset is -4.00. DISPLAY - Local time. The alternative is Greenwich ( GMT). As I said above, it is not exactly GMT but very close to it. The important thing for the unit to function accurately is that all sats and your unit are using exactly the same time.You do not need to set the clock in the unit. It is done automaticaly when you turn it on and it starts picking up satellite signals. Instead, set your watch from the GPS! COUNT - Up or down. Great for racing sailors! This might me of use to those orientiering. AUDIO/DISPLAY SCREEN TONE - None CHANGE CONTRAST- I have set midway BKLGHT TIMEOUT - 10 seconds is adequate when using in hand at night. The Garmin 75 asks for a delay in seconds and by default is 10 seconds, to save power. But when on external power, I set the delay to 999 seconds. INTERFACE NONE/NONE - Used to exchange and receive data with another GPS or computer. I can't give any help with this yet as I haven't hooked up yet. MAP DATUM PRE-DEFINED - I am using NAD27 Canada for most of my maps. 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 very close to 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 unless there is good reason to change it. SUNRISE/SUNSET There is nothing here to change but you can change the date and get sunrise / sunset for other dates (for the location where the GPS was last active). Alternately, you may get the sunrise/sunset for another waypoint (WPT). To check for another location, create a WPT with the location first and then call up the WPT on this screen, changing the date as necessary. TRIP/FUEL PLANNING I don't use it at all but great for sailors. In summary, for my needs, I have all turned off that is not needed so as to save batteries. BUYING A GPS Size is a factor, but they are getting smaller. The Garmin 45 is about the size of a 110 camera! It should track at least 5 satellites and 8 or more is better. 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. This will only improve when more satellites are sent up. Multi channel satellite processing is a positive asset as it means that the four best sats are being processed at the same time which in practice means faster fixes, particularly under difficult conditions e.g. wet trees. The trade -off however,is that they use much more battery power! Most of the small hand held units can only process the data from one satellite at a time, no matter how many they are tracking. This being the case, the unit must cycle through the satellites one by one before giving a reading. It should store 100 or more way-points, (250 is common). The capability to store several Routes is an asset. Most store 10 or 20 at least. Being waterproof is an asset. For all uses except nautical, you should be able to use UTM grid data (and the newer MGRS now becoming available). DO NOT BUY ONE THAT WILL NOT USE UTM. Most will handle UTM, a few however will not. 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: -Most will operate on either 3 or 4 AA cells. None of them are easy on batteries. My Garmin uses 4 AA (I like the Rayovac Renewal Alkalines, see below). The Magellan Trailblazer only uses 3 AA cells whereas the Eagle AccuNav Sport needs 6! Remember, you will be carrying spares and these units, even the smallest are heavy enough without batteries. Some makes go through batteries faster then others. Another point against a unit using more than 4 AA cells is that the small chargers like the Rayovac Renewal only hold 4 AA cells. More than 4 cells could mean twice a long to recharge! In choosing batteries, the best bet would be Ray-o-Vac Renewals or Pure Energy (sold in Canada), which are rechargeable alkalines. (I have been using Renewals now for 3 years, same ones, 3 sets.) Nicads I have tried and don't like. They don't put out the volts and then there is the "memory" problem. With my rechargeable alkalines, I top them up when ever possible and they will hold a full charge in storage better than a nicad. They are also cheaper. ALWAYS CARRY EXTRA BATTERIES Warning- Some units, e.g., the Garmin 40 & 45 (but not the 75), have no internal battery to maintain data when the unit is off. They therefore need to have the batteries in place at all times, even when turned off. They will hold data for a little while (I don't know how long) while you change batteries. This makes it all the more important to carry extra batteries. As an extra thought to the above, it would be prudent to record on a card the coordinates of important way-points just in case your internal battery in a GPS like the Garmin 75 fails or you run out of batteries in the case of a unit such as the Garmin 45 which has no internal battery. In either case, the way-points may be reentered upon next powering up. The Garmin 45 will then maintain them when turned off but the Garmin 75 would lose then again when turned off and would still need to have the internal battery replaced at a service center. HOOKING UP TO AN EXTERNAL POWER SUPPLY Since this article was first written early in 1995, I have mounted my G75 in my Exporer, 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 made as well. This is a great set-up for mapping the bush roads we use in the north in the fall. The passenger notes the coordinates and plots them directly to the map while travelling. If you only record a six digit WP (to a precision of 100 metres), you can plot very quickly while travelling. 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. ALTITUDE READINGS 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. Perhaps in the mountains the satellite coverage would be good enough to give meaningful elevations, if you are high enough. Since writing the above, I heard from a man who travellled through the Alps on a motorcycle and who obtained altitude reading in the passes of within 25 metres- remarkably good but the altitude error will normally be at least 1.5 times the position error. SPEED 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. Note however that the Garmin 40, 45 & 75 are limited to 99 MPH, a speed that is useless for flying. I have tried my Garmin 75 on the dash board of a truck 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. (The Garmin 75 will operate on any voltage between 5 and 40 volts, DC. The Garmin 45 is more tolerant of voltage than the G40). VARIABLE READINGS A GPS left on and stationary will give a continuous but variable reading. This is not an error in the unit, just that SA is continously altering and/or degrading the accuracy. A series of readings over many hours when averaged will come very close to the correct figures. 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 3-4 minutes. By using the Pythagorean theorem, I worked out the hypotenuse (or error) for each of the 10 readings and the average was about 5 metres (5.5 yards) off! This was not a scientific test and my good results were as much chance as anything. If you are fortunate enough to find a BM, the exact location of which is known, you can conduct the same test. It is important to stress however that one should not depend on obtaining accuracy of better than 100 yards/110 metres. Generally, the accuracy will be better than this but you can't count on it. Last fall, I read an account of a man with a new GPS who took a reading (WP) of his car, then went on a long hike. To return to the car, he did a GOTO on it, leaving the unit on to give direction and distance. When he got within 100 yards or so of the car (which was hidden in bushes), his GPS was pointing every which way. It took him some hours to find it. Why? First of all, he should have parked near some feature which could be identified from 50 or so yards away. His car could not be seen from 20 or 30 feet away! Secondly, he had no compass and relied on the GPS completely for direction. When he got close to the car, he was expecting accuracy better than the system will yield because of SA. He also used up his batteries. This story shows an example of getting hung-up on the graphics which I warned about above under Route & Track. Comments on Specific Models: While I use the Garmin 75 (essentially a marine unit)s an example here, I am not promoting it over other models. The fact is few ad- verts that I have seen mention UTM and its usefulness. Most are slanted towards nautical, fishing and off-shore use. While some of the magazines have rated various units, no article that I have seen even mentioned UTM and its usefulness. Units that will handle UTM: Garmin 40 Garmin 45- I have handled the Garmin 45 and find it much lighter and more compact than the 75. The 45 has an external antenna whereas the 40 does not. The 45 also will handle a wider voltage supply. Otherwise they seem to be the same. If I was buying today, it would probably be the 45 as the 75 is about $180 more and is heavier. Garmin 75 Handles more waypoints and routes than the 40/45. This model also comes with an excellent mount, and a cable for external power and interfacing whereas with the 45, there are extras. Trimble Scout GPS (MGRS) Magellan Meridian GPS Magellan Meridian XL Magellan Recon Trailblazer (MGRS) (the lower priced) Trailblazer "geared more towards the civilian, hiker, hunter, tourist, or boater" says U.S. Cavalry catalog but DOES NOT SAY IT WILL TAKE UTM??? Magellan 2000 I have read many postings of problems with the Magellans and I have read of people having no trouble. If the Magellan 2000 stands up, it is a great buy currently at $199. Note: The Magellans do not have the speed limitation of the Garmin 40, 45 and 75. The Trimble line is first rate but not inexpensive. The Garmin and Magellan seem to compete on price and features. From reading the comments of many users on the Usenet, it seems that the Garmin line is favored with the 45 being most popular and widely available at $299. The Eagle AccuNav (a multiplex unit) I do not recommended for two very good reasons. While it is accurate, fast, very good under difficult conditions, it DOES NOT HANDLE UTM and less important, takes 6 AA cells, which it eats very quickly. If you can manage with only Lat/Lon, have good battery backup and can afford the price, this would be a good unit. Undoubtedly, new units will be coming on the market as the interest in these units rises. Three years ago, I bought my unit from E & B Discount Marine: toll free USA 1-800-533-5007 international 1-908-819-4600 In 1992 they had the best prices and the most selection. They also have many retail stores and an excellent catalog. They make an good bench-mark to judge other suppliers' prices. Other sources are: Boat/U.S. who may be reached at: mail order 800-937-2628 International 800-937-9307 TDD 703-461-2884 Product Info 813-573-1114 West Marine, P.O. Box 50050, Watsonville, CA 59077 USA phone 800- 538-0775 Catalog E-Mail request to baja@netcom.com or interactive at http://www.gsn.com/westmarine.htm Mud Hole Custom Tackle, Forked River, NJ http://www.vitinc.com/nn/mud/cat/home1m.html Here are phone numbers for many of the manufacturers from whom you may obtain detailed information concerning specific models and their capabilities: Apelco 800-247-7681 Micrologic 818-998-1216 Eagle 800-324-0046 Motorola 800-292-0477 Garmin 800-800-1020 Panasonic 201-348-7966 Trimble 800-487-4662 Magellan ? 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. The information I have given is thought to be accurate but I assume no liability for errors or omissions. I have no connection with Garmin or any retailers. I am far from an expert but only a user. I can't answer techical questions. As yet, I have not even tried to hook up to my computer. For those looking for more information, check out: sci.geo.satellite-nav on Usenet There is a wealth of information in the postings to this group. Often FAQ's (frequently asked questions) will appear and any time that I update this article, it will be posted to this group. This is the place to ask techical questions. Also Peter Bennett in Vancouver maintains an excellent WWW site where much useful information and computer software is available at: ftp://sundae/triumf.ca/pub/peter/index.html Peter posts a GPSFAC and a NMEAFAC to the Usenet group named above and I believe that it is also available at his above site. Good Luck! Don Bartlett E-Mail ae374@freenet.carleton.ca Ottawa, Ontario, Canada. Jan. 5,1996 Don Bartlett, ae374@freenet.carleton.ca (at 18T 043940 5024350 for those in the know) (Don Bartlett) ------------------------