« on: September 18, 2008, 12:26:19 AM »
I am a happy user of TopoFusion and use it several times a week to track mountain bike rides. I have created a pretty good set of trail maps based on merging individual rides at each location using the Make Network capability.
I noticed something that seems odd. I went on one of the rides I do pretty frequently and then did a network analysis using the new track (New) and the original merged data (Original) that was created using v3.38. I had a short bit of new trail in the new track, but the vast majority of New is well represented in Original. In this case, Original is superset of New except for the short new track. This is confirmed in when the New merged data is compared with Original. However, I noticed that the New merged file is significantly smaller. I checked a few things in the GPX files and found:
Original New 3.38 New 3.41
File size 3677 KB 1385 KB 931 KB
Waypoints ("<wpt") 75 92 89
Tracks ("<trk>") 103 115 109
Track Points ("<trkpt") 43152 16148 10582
TopoFusion #Points 43227 16240 10671
I read the June 2004 ACM paper and followed your explanation of parallel and face reductions. As I understand the algorithm, the number of points is not reduced below the maximum number from the polyline with the most points when performing a parallel/face reduction. Since serial reductions are always performed, the Original file should not have any points (vertices) of degree two. Based on this, I would have expected New to be slightly larger (due to the points in the short new track) than Original, but by only a very small amount. As the numbers indicate above, New was slightly more than 1/3 the size of Original using v3.38. I installed 3.41 and the problem is even worse (or better, depending on your point of view) but it seemed to run much faster.
The procedure I used was to select the new track and Original and then run Network Analysis. I did not do an explicit data reduction, spline or any other steps. I am using the same reduction and contraction setup parameters in all cases.
Looking closely at the tracks suggests that using re-using a previously generated network does some sort of a spline because the New tracks are much smoother when I zoom in a lot. This smoothing effect only occurs along the path of my most recent ride (the new track is smoother than the Original track). If there is some sort of reasonable data reduction being performed, why wasn't this also applied when Original was generated?
Am I really losing resolution using a previous generated network as one of the inputs? Is it safe to merge data using previously generated networks?