Comparison of GPS units using TopoFusion

As a part of the Tucson Mountain Park data collection project, we have acquired a number of GPS units for volunteers collecting data. Before we started loaning them out, we took them for a short bicycle ride in order to compare them. We tested the following units:
  • eTrex Vista v3.01 beta
  • Magellan Sportrak Map v3.01
  • eTrex Legend v3.10
  • eTrex Basic (yellow) v2.11
The data: (17k) in GPX format.

(Use the free version of TopoFusion to view and playback the tracks)


All four units were placed in the mesh pocket of a camelbak mule and given as equal as possible view of the sky. The legend and vista were set to Auto and "most often." The eTrex Basic was on Auto. The sportrak was set to "Auto - Detailed".

Above is a screen shot from the playback of the four tracks. Notice that the Magellan (green) is far ahead of the other tracks. This is the most interesting result of the comparison by playback. This behavior is the result of the Magellan not dropping points on speed changes--it only drops them for deviations from course. It is most apparent in places where the track is a straight line, but the GPS (bike rider in this case) was stopped or slowed down along that line. Because the playback interpolates the time values between points, the Magellan will travel at a constant speed between straight line points, while the eTrex units dropped points at slow downs. The result is that tracks collected by the eTrex series receivers will playback much more realistically.

To see this behavior, view a short clip of the playback in mpeg format (4.7 MB)

Or, you can download all four tracks in GPX format for use with TopoFusion's Multi-track playback feature.

Another interesting result, shown above, is the difference between the Vista/Legend and the basic eTrex. Throughout the track the Vista and Legend seem to match each other very closely, while the basic eTrex wanders a bit. One key difference, of course, is that the basic eTrex is dropping fewer points. I am not convinced that this explains the difference completely, though.

Also note the differences between the tracks in distance, number of points and climbing/descending numbers. The difference in the track log capacities is apparent here too. The legend and vista are clearly doing a better job of capturing the path traveled. There may be other conclusions to draw from these four tracks as well. Check them out.

Our set up: eTrex Vista or Legend with Nickel Metal Hydride rechargeable batteries. To date, the eTrex line cannot be beat for size, battery usage and quality of data collection. The Sportrak fails based on its smaller and less detailed track logging, as well as its size. We have tested the Magellan vs. the eTrex in extreme tree cover, and though the Magellan performs better, it does not seem to be too large of a difference.

We use the handlebar mount for the eTrex to give the unit the best view of the sky. Some eTrex units have problems shutting off, especially on the handlebars during descents. Our advice is to find the largest batteries possible. Another solution is to put something between the battery cover and the batteries.

Note that we use GPS primarily for data collection. We have topo maps loaded, but rarely refer to them. We also rarely mark waypoints. Bear this in mind when considering our GPS recommendations. The Sportrak, for instance, has waypoint averaging, whereas the eTrex does not.