But a threshold that's too big will smooth things out too much and cause you to lose sharp corners and switchbacks. Obviously, with a very small threshold, lots of detail will be preserved, but so will some of the artifacts. The question is, what distance should you use as a trackpoint distance threshold? That depends on a few things. points per track" section at the top of this page it's a much better result: Before: That's going to make a difference when calculating slope or creating elevation profiles.īy the way, here's what happens if we apply an appropriate distance threshold filter to the jagged second example in the "Max. The distance measurement based on the raw data was 17 meters, but now we see that it was really about 10 meters. and perhaps more importantly, the distance that was traveled. We're left with only three points, but they more accurately reflect the path that was traveled. Finally, we come to point F, which is 4.82 meters from point B F will not be removed, and if there were any more trackpoints, they would be evaluated based on their distance from F. D is 4m from C, which would have been enough to get past our threshold, but we deleted C D's distance from the last valid point ( B) is only 2.95m, so it comes out too. But C is only 2m away from B, so it needs to be removed. The distance from A to B is 5 meters, so B is OK too. The first point is always considered valid, so A gets to stay. So we'll set the trackpoint distance threshold to 4 meters this means that points are only allowed to stay in the track if they're at least 4m away from the last valid point. If we add up the raw distances between all seven points, it comes to 17 meters. Here's a track with seven points it should start eastward and then simply bend to the southeast, but the dreaded GPS "jaggies" have made an appearance. If you don't supply a distance unit (feet, miles, etc.), the threshold given is assumed to be in meters. The newer "trackpoint distance threshold" filter examines each point in a track, in order, and discards every trackpoint that is not at least X distance from the last non-discarded point you get to choose the value of X. Obviously, we need another approach to cleaning up track data automatically. If we set the maximum number of points to 6, the jaggedness is preserved, because the simplification filter thought that all those sharp angles were important! This is not an ideal result. (Of course, it's also possible that this jagged line reflects reality, but let's assume for now that it's just the result of unwanted artifacts in the raw data.) As you can see, something weird is happening between point #3 and point #8 if this was a GPS track, it's likely that the person recording the track stopped for a minute, and the GPS signal "wandered around" while he or she stood still. Here's another hypothetical track, this time with 11 points. (For example, Garmin's older GPSMap units only allow 500 points per track.) But now let's look at an example where the "preserve the shape of the track" method doesn't work as well. points" setting works well on relatively clean data, and it's very useful if you need to reduce the number of points in a track because your GPS device has a specific limit. If we set the maximum number of points to 6, four of the points on the "straightaway" sections of the track are removed, leaving us with a modified track that looks very much like the original: For example, here's a track with 10 points: points per track" option, which has been available since 2006, reduces the number of points in a track by passing the track through GPSBabel and asking GPSBabel's simplify filter to remove the most redundant points, based on how much those removals will affect the overall shape of the track (in technical terms, it measures "cross-track error"). They both reduce the number of trackpoints (while preserving time stamps and other information), but in very different ways. points per track ( trk_simplify) and trackpoint distance threshold ( trk_distance_threshold). There are two options in GPS Visualizer's mapping and file conversion input forms that allow you to "simplify" your tracks: Max. « GPS Visualizer Tutorials Tutorial: Smoothing & Simplifying Tracks No ads? No problem! You can support GPS Visualizer by making a donation with PayPal instead.
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