Terra Incognito



GPS & Compass

3D Stills | Cartography | GPS & Compass | Remote Sensing


To accurately use a map in conjunction with GPS always check the following cartographic data and set you unit accordingly:

  • Projection
  • Horizontal Datum
  • Vertical Datum
  • Ellipsoid

Global Positioning Systems (GPS) are a fairly new orientation tool for the outdoor enthusiast. The system consists of a constellation of American & Russian satellites orbiting the earth. The satellites broadcast precise measurements of time, and portable units on the ground or in your hand receive these signals. The time measurements allow the receiver to calculate the distance to the satellite based on the speed of light (the speed at which all electromagnetic radiation propagates is 300,000km per second). Once the receiver calculates the distance to three separate satellites it can perform geometric triangulation and determine your position. Contact with a fourth GPS satellite is required for an elevation calculation. Contact with 5,6,7 or more satellites will result in a more robust estimation of location. Locations within a circular error of less than 100m are generally obtainable, provided 3 satellites can be fixed. The radio signals from the satellites are somewhat attenuated by particles in the atmosphere. Thus the signals do not always travel at exactly the speed of light but instead vary slightly in speed depending on the constantly changing atmospheric conditions. These variations limit the accuracy of standard GPS. More advanced Differential GPS make use of a fixed ground station to correct the atmospheric errors, allowing for locating within 1m or less in the best cases.

Sample GPS instructions from the Colorado Springs Map

The map shows UTM coordinates in meters along the margin in purple. Margin ticks mark off every 10,000m or 10 kilometers. Set your GPS receiver to UTM Zone 13, NAD 1927, Ellipsoid Clarke 1866. The UTM coordinates along the top and bottom of the map count METERS EAST of the Zone 13 start and are called "Easting." UTM coordinates along the left and right sides of the map count METERS NORTH of the equator and are called "Northing." You can use a handy square or right triangle scale printed on clear plastic. Use 1:75,00 scale. Lay scale on map and count off distances.

A Couple of Final Notes on GPS

Within deep canyons or close to steep mountains your GPS receiver may have trouble contacting three satellites, and thus you will not get a position. Also the elevations displayed on your unit may differ from those on a map. Maps show official USGS elevations based on a particular vertical datum, such as NGD 1929. This vertical datum is based on the average sea level of 22 US ports. GPS does not use sea level for zero but instead uses a mathematical model called an ellipsoid. Consequently, the "0" starting point of elevations differs slightly, leading to differences in elevations.

Stellar Navigation

If you are outdoors during the night you can use "Polaris," the North Star, to establish direction, check your compass, and measure deviation in the magnetic pole. Polaris is easy to spot. If you know generally which way North is then look North and you will find a single isolated star. This is Polaris. Polaris never strays more than 2 degrees from the true north pole, and hence while all the other stars "appear" to arc westward across the night sky ("appear" because it is the earth itself that is actually rotating) Polaris stays fixed over True North. You can also find Polaris from the Big Dipper. Locate the cup or pan section of the Big Dipper, transect a line from the far (away from the handle) bottom star in the cup to the far top cup star. The line points to Polaris.