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Learn to Use GPS in the Backcountry (Intro) | Learn About Your GPS Receiver | Set Up Your GPS Receiver | Practice Recording Locations | Interpret your Data: Recording and Using Tracks

Interpret your Data (Recording and Using Tracks)

by Phil Romig Jr.

Analyze and Compare the Locations with Each Other and with Map Locations

The waypoints you are recording as you walk around your home territory will allow you to address several questions that become important when you are on the trail:
  1. How precise are the waypoint locations (e.g. what is the smallest difference detectable)?
  2. How consistent are the waypoint locations from day to day?
  3. What factors (weather, time of day, nearby structures, trees, etc.) affect consistency?
  4. How accurate are the locations (how close are they to the “true” location)?
  5. What factors affect accuracy (map datum, etc.)
  6. How do you know what is the “true” location? Does it matter?

This analysis will be done in two steps. First, you will use other resources to determine the “true” location of each waypoint, then you will do some calculations to get results that will help you develop a better understanding of how much you can trust your GPS in the back country.

When you have some time, determine the coordinates for each location from a map.
  1. I start with Google Maps, accessible in any web browser. It can superimpose satellite images and maps, allowing you to put the curser precisely on the street corner or other reference point you used for the GPS measurements. Right click on the reference point, and it gives you the coordinates.
  2. You can do the same thing with Google Earth, a free download for most computers.
  3. Another option would be old-fashioned paper topographic maps from the USGS. They aren’t as precise as online maps, but you can depend on their accuracy.
After you have a dozen or so waypoint readings at each location and have determined the map coordinates, compare the waypoints with the maps.
  1. How much do the individual GPS coordinates (N/S, E/W) vary?
  2. Calculate the averages of the individual GPS coordinates.
  3. How does the average compare with the coordinates from the maps? If there is a consistent difference for each location, check that datum for the map and the GPS.
  4. Use the “square root of the sum of the squares” to calculate the distance from each reading to the average.
  5. Compare the distances for the various readings to things like time of day, type of weather, nearby structures, etc. Is there a pattern?

These experiments takes very little time out of your busy day, but doing them regularly over different routes will make a huge difference in your ability to use GPS effectively on the trail. If you have stuck with us this far, you now are ready for the next step — learning to use tracks.

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