GPS SETTINGS AND COMPUTER INTERFACE

by Richard L. Carey, revised Nov. 24, 2002.

Advantages of the UTM Grid

       Most all GPS receivers are set up by the manufacturer to display positions in latitude and longitude using the WGS 84 datum. This is fine if you are using the GPS for boating or for flying since marine and aviation charts have latitude and longitude divisions in the margin which make it easy to interpret your position in degrees and minutes. For the hiker who is working with standard 7.5 minute topographic maps the latitude and longitude tick marks are given only every 2 1/2 minutes with no subdivisions. Worse yet is the fact that the east-west distance of 2 1/2 minutes will change depending on where you are since longitude lines converge at the north pole. Only at the equator is 2 1/2 minutes longitude equal to 2 /12 minutes latitude. It is not easy to think of distance in terms of degrees, minutes and seconds when we are more used to miles or meters. The solution to this problem is to shift to the Universal Transverse Mercator (UTM) coordinate system which most GPS receivers now have.

       When changed over to a UTM display your GPS receiver will show three things to define your position: the UTM zone, the easting value in meters, and the northing value in meters. The zone is a north-south band which is six degrees longitude wide starting at zone 1 on the east side of the International date line (180 to 174 degrees west) and continuing around the earth from west to east. Thus there are sixty UTM zones. The continental US lies in zones 10 in California to zone 19 in Maine. The spherical earth is projected onto strips six degrees wide which may be laid out flat. The narrow zone width keeps the distortion to an acceptable amount. Measurements within the zone are in meters increasing from west to east or left to right on the map. This is the easting value which is always six digits since the center of the zone is given a value of 500,000 meters to keep all numbers positive. The easting will vary from about 200,000 meters near the western edge of a zone to about 750,000 meters near the eastern edge. The northing is in meters also and is the distance north of the equator. This number in the continental US is always seven digits and varies from about 2,700,000 meters to about 5,100,000 meters.

       Looking at a 7.5 minute topo map you will see the one kilometer UTM grid in the margin marked by blue tick marks. On some maps the grid has been drawn in with fine black lines. The grid most likely will be slanted left or right relative to the margins of the map depending on which side of the center of the UTM zone it is located. This is not a problem. The beauty of this grid is that is always the same one kilometer spacing no matter where you are. One kilometer is 0.6 miles so it is relatively easy to estimate distances counting up UTM grid squares. In the margins the grid is marked in kilometers so you need to add three more zeroes to the easting and northing values to enter a point into your GPS receiver. The zone is always given in the lower left corner of the map.

Setting a Garmin Receiver to UTM Mode

       The instructions given here are for a Garmin GPS 12XL, however Garmin has maintained a fairly consistent display on most models so these instructions should be similar for the GPS II or the earlier models 38, 40, and 45. As it comes, the Garmin is set to latitude and longitude display using WGS84. To change to UTM turn the unit on and wait until the globe stops spinning and it starts acquiring satellites. Press the Quit key once and you will come to the Menu Page. Then press the center four-way arrow button down till you see the reverse display on Navigation and press Enter. The Nav Setup page will now appear. The first entry at the top is Position Frmt: which will probably show hddd°mm.mmm’ below it. Move down to this and press Enter and then scroll down several places until you see UTM/UPS and then press Enter. This will set up the position display to UTM mode.

       Next you want to change the datum which is the mathematical model of the earth used in the internal calculations. Using the wrong datum can cause position errors of up to several hundred meters so it is a good idea to have the right one. Most all 7.5 minute topo maps in the US use the North American Datum of 1927, abbreviated NAD27, so you should use this. (Only one state, Delaware, has redone its maps to the newer NAD83 datum at present.) Move down to the next entry which is Map Datum: and press Enter and then scroll through the long list until you see NAD27 CONUS and press Enter.

       The following setup changes are more for convenience, but are useful in my experience. Next move down to Units:. The default setting is Statute which I recommend. If it is not set to this press Enter and scroll through the three choices until you see Statute and press Enter. This will set the display of distance to nearby waypoints to miles which I think is convenient for most people. So even though the UTM grid is metric the receiver will show distances in miles. Next move down to Heading: and press enter and scroll through the choices until you see True and press Enter. This will show bearings to nearby waypoints relative to true north. I recommend this method in which case you correct for the declination in the local area with your compass. The other item that probably needs setting is the time offset so that your receiver will show correct local time. Press Quit to return to the Menu Page and then move up to System and press Enter. Move to Offset press Enter and set in an offset time in hours that is correct for your area. For California during the summer months with Daylight Savings time it is -07:00. Check in the Garmin owners manual in Appendix C for the correct offset for your area.

Connecting a Garmin GPS to Your Computer

       Most of the Garmin receivers have a provision for transferring data to and from a computer by using a serial interface cable. This must be purchased separately and plugs into the four-pin connector on the rear of the receiver under a rubber cover. The other end has a type DB9 nine-pin connector on it which plugs into the connector on serial port COM1 on your IBM compatible computer. This cable is Garmin part number 010-10141-00 and sells for about $27.00. A source for the Garmin cable is Larry James & Associates in Colorado. He has a Web site at: http://www.macgpspro.com which describes the cables. This site also has GPS software for the Macintosh computer.

       The connector used by Garmin is non-standard and this precluded alternatives for a while. But ingenuity won out and there is now an aftermarket source for connectors and cables which work fine and are cheaper. A whole cottage industry has sprung up after low cost molded connectors were made available by Larry Berg of Purple Computing in Murphy, Oregon. For a list of cable suppliers around the US using this connector take a look at the Web page: http://home.cdsnet.net/~purple/projects/g45contr/USA/p_list.htm. For suppliers in other parts of the world look at: http://home.cdsnet.net/~purple/projects/g45contr/country/d_list.htm.

       I have bought cables from Jose Ramos who is in of all places Portugal! Mr. Jose Ramos makes a quite satisfactory cable fully assembled with connectors on both ends for $15.00 US which includes shipping. He can be reached by email at: jose.ramos@mail.telepac.pt. Send him a note with your address and requirements and he will send out a cable in about a week. You send $15.00 cash to him as payment.

Software for Garmin GPS Receivers

       With the cable you still need software and there are several sources for this. Originally from Garmin there was the PCX-5 software for about $75.00. This is a DOS based program which will run under Windows 95. It has a good instruction book and does an adequate job. It does not have a feature for showing a table of waypoints which you can then edit. This must be done separately with a simple text editor. The program does have some features for showing a satellite visibility chart which free programs do not.

       A newer program is Garmin's PC4X which is actually the software with an interface cable. I have not seen this, but it sounds more capable. There is a description of it at Navtech GPS. See their Web page at: http://www.navtechgps.com/. This software with cable is also sold by Larry James & Associates for $80.00.

       A widely used free program for use with Garmin receivers is Waypoint+ written by Brent Hildebrand. This program runs only with Windows and is available for download at the following site: http://www.tapr.org/~kh2z/Waypoint/. To use this you need to download the main program. The latest version is 1.8.03 released September, 2001. Your best bet is to read the FAQ and other notes on the Web page before getting started. If you have trouble getting this program to talk to your Garmin try shifting to COM2 using a 9-pin to 25-pin adapter if you have been trying COM1. This worked for me since there seemed to be an IRQ problem using COM1 even though the port was open.

      Another waypoint program is GARtrip written by Heinrich Pfeifer in Germany. You can download a demo version which is limited to saving up to 30 waypoints at: http://www.gartrip.de. The unrestricted version costs $30.00 and there is information at the web site about how to pay for this product. GARtrip has some unique features, is more powerful than Waypoint+, and is being upgraded regularly.

Waypoint Files

      The waypoint files listed on the main page are text files which you can save and then upload to your Garmin receiver using either Waypoint+ or GARtrip. All of the peaks are set given in UTM coordinates, NAD 27 datum, with proper header information for the Garmin. The exception is the Munros peaks list which is in Latitude and Longitude and WGS84 datum. In order to make use of the Munros list when in Scotland you will want to change the Garmin to the British Grid and the datum to Ord Srvy GB. The display will change to the British Grid system and positions will agree with the Ordnance Survey maps you will most likely be using. Waypoint Files