The distances listed in the "properties" and "Graph" tabs never agree, often by quite a lot. Viewing a hike from yesterday, distance is listed as 19.5 miles in the properties tab but only 18.4 in the graph tab. This disparity is present in virtually every track imported into BaseCamp. Can anyone explain this, and if so, what do I need to be doing (or not doing) during my hikes to get accurate and consistent distances.
The discrepancy is most likely due to different sets of points used to calculate the distance. On some models, Garmin has begun to include summary data for tracks. If your device does this, it can be verified by exporting a track as a gpx file and viewing it with a text editor like notepad or wordpad. The summary data is at the beginning of the track. Times are in seconds, distances are in meter and speed is meters/sec. These values should agree with what is seen on the trip computer or similar page that displays the distance covered on your device.
This summary data is displayed on the properties tab in BaseCamp. It comes from the real-time change in position during your activity. The real time data is updated every second and, at least on my device, there is some logic that makes a guess as to whether or not you have moved (it is very rare for a device to show no movement even when stationary). So, the moving time in the summary data represents the number of points used to calculate the distance.
The graph tap uses the actual stored positions in the track file to create the graph (the Points on the properties tab are the number of stored positions). Thus a different set of points used to calculate the distance.
There is nothing you can do on your device to alter this. But, what you can in BaseCamp is force a recalculation by deleting a single point. A good point to choose is one in a straight section that shows the same direction in elevation change as the adjacent points (up or down) or a minimal change in elevation between the deleted point and one of the adjacent points.
The GPS system does not give accurate positions, so things like total distance can not be the true values. Even if positions were true, distance would not because the distance value is in the horizontal direction only (changes in elevation are not taken into account) on the model of the Earth (WGS4 ellipsoid). In general, fewer points will provide the more accurate distance provided that they do a good job of capturing the curves in your track. Most of the time, this will be the stored track and not the summary data. An exception might be for slow walkers who take lots of breaks during their walk.
Garmin had a bug in some of their handhelds that would cause this. They have released software for some (I think all) units that had this problem. The new software is working on my eTrex 20.
