Route Length Accuracy using OSM Snap

matthiaswatkins's Avatar


26 Jul, 2020 04:54 PM

I'm planning a High Sierra Trail trip (70 miles.) If I draw a line for the whole length of the trail snapping to OSM points, the total length of the route comes out to about 10 miles short. (60 miles vs 70.) I suspect this has something to do with the sampling interval. Is there a way to get a more accurate route length estimate? Should I break up the route into shorter segments?

  1. Support Staff 1 Posted by Meghan on 27 Jul, 2020 04:41 PM

    Meghan's Avatar


    Going from Crescent Meadow to Whitney Portal, having snap to configured to OSM and using the MapBuilder Topo base layer, I got 66.3 miles for the High Sierra Trail. GPX tracks I found online had the length at 66.5 miles. The short answer is as long as you have drawn your line accurately (making sure its continually snapping to the correct trail), changing the sampling interval won't make it any more accurate because its based on the underlying map data (although I always like to "click as I go" with snap to in order to ensure I am snapping to the correct trails).

    Long answer is MapBuilder topo is largely based on OSM (in addition to other data sources), which is a crowdsourced compendium of mapping information. OSM allows users to upload GPX and overlay them on the map but you have to hand draw any additions to the database. GPX tracks have too much noise because they are “real world” and while you can still draw really accurate lines in OSM, you have to draw a lot of points to do so which takes time. So a lot of times OSM lines are very close but subject to small straight jumps if you zoom way in because users don’t want to add 100 vertexes for a corner to get it spot on. These are imperfect measurements on imperfect databases using imperfect original source data. There is a fair amount of error in the entire process, especially across 70+ miles, I would expect a 5mile route to be fairly close between theoretical drawn line and measured distance across several devices.

    Also, GPS is prone to 20m or so shifts particularly in steeper areas (harder to get 5+ sat fixes). Over time, these add up.

    My suggestion would be to consult more than a single source of data (in other words look at what snap to tells you but also look at other data sources) in order to get the most accurate idea of your route. I hope this helps shed some light on your route planning!


  2. Meghan closed this discussion on 27 Jul, 2020 04:41 PM.

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