GPS Tracker FAQs

  1. What is a "track" in GPS Tracker?
  2. What defines a "segment"?
  3. Why is the distance that GPS Tracker calculates shorter than what the trail guide says?
  4. When I create a loop track, the total climb is not equal to the total descent. Why is that?
  5. Why can't I create a profile for a track that's built from more than 30 segments?
  6. Why don't all the point numbers show up on the Print Map?
1)  Question:    What is a "track" in GPS Tracker?

In the GPS sense of the word, a "track" is a series of points collected by a GPS unit and strung together to show where you've been. In GPS Tracker, you build a track that you're interested in analyzing by adding together pre-defined GPS track "segments" to form a larger track. You can add these segments together in any order that you want as long as they form a contigous track.

As you add the segments together, GPS Tracker analyzes the track and displays the distance, the amount of climb and descent, and the maximum and minimum elevation. After you've built the track you can display it in a print map or generate an elevation profile for it. If you have Google Earth installed on your computer you can display it in Google Earth. Or you can save it as a .gpx file to read it into another mapping program or upload it to a GPS unit.

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2)  Question:    What defines a "segment"?

"Segments" are pieces of GPS tracks that have been divided up into smaller parts. Segments are added together to form a track describing the trails that you're interested in. The definition of what constitutes a segment in rather arbitrary. Generally speaking, the original GPS tracks are divided up into segments at any trail interesction or trail head. If the resulting segment would be less than about 1/10 mile, it will often just be included in an adjacent segment.

Segments can cover single track trail, dirt roads, or paved roads, however paved roads are not the focus of GPS Tracker and are usually only included to connect other trail segments.

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3)  Question:    Why is the distance that GPS Tracker calculates shorter than what the trail guide says?

That's partly due to the way the GPS unit calculates the distance of a track. A GPS track consists of a series of points and the calculated distance of that track is the straight line distance between the points. In a trail that has lots of twists and turns the points are going to occasionally "shortcut" part of a curve and so the calcuated distance can be shorter than the actual trail.

I think it also has something to do with the way I cut up the GPS tracks into segments. I might be loosing a little bit of the distance each time I cut the track. In spite of that I think the distance that you get from GPS Tracker is pretty close to the distance you would get if you collected track with a GPS unit, and that distance is going to be pretty close to the actual distance of the trail.

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4)  Question:    When I create a loop track, the total climb is not equal to the total descent. Why is that?

Of the three dimensions that a GPS unit measures (latitude, longitude, and elevation), elevation if by far the least accurate. Latitude and longitude are ususally accurate to within about 10 to 15 feet but I've seen the elevation be off by as much as 200 feet. I think that's partly due to the calibration of the altimeter in the GPS unit that I use when I collect the track. My GPS unit has a barametric altimeter that records the elevation when I collect a track and the elevation that it reads can vary depending on the barametric preasure. It's set to be calibrated automatically from the GPS elevation but I'm not sure how accurate that is. It should be calibrated to a known altitude before I begin collecting the track but unfortunately, sometimes I forget to do that.

I think what happens is that when you build a track from segments that were collected on different days when there was a difference in the altimeter calibration, that can show up as a discrepency in the calculation of the total climb and decent of the trackl. Also, when you generate an elevation profile, you'll sometimes see a discontinuity in the curve where the trail suddenly drops off or climbs straight up by a certain number of feet. That's where two segments that were collected on different days come together and that can throw the eleveation calculations off.

Given all of that, it's still a pretty easy way to get a fairly accurate measurement of the amount of climb and descent in a track.

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5)  Question:    Why can't I create a profile for a track that's built from more than 30 segments?

GPS Tracker Profiles are image files that are created dynamically by running a graphical program on the server. This tends to be a resource drain on the server which can slow down the response of the server and that's the kind of thing that web hosts don't like to see.

When I first started developing this web page I was doing a great deal of processing on the server, impacting the server to the point where my web host finally kicked me off. So I rewrote everything to do as much pre-processing of the GPS track data as I could and now I think the only thing that might affect the server is this graphical processing to build the profile images. Unfortunately that has to be done on the server because it can only be done after the user has selected the segments that make up the track. By limiting it to 30 segments I'm hoping that will limit the impact it has on the server.

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6)  Question:    Why don't all the point numbers show up on the Print Map?

The point numbers on the Print Maps show up as little numbered markers on the map that correspond the dots on the profile at the bottom of the page. When a track does an out and back or makes a loop and comes back around to an earlier point on the track then those two markers will be at the same place on the map and one will be drawn on top of the other covering it up, so the earlier marker will not be seen.

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