Measure distance inaccuracy

Mazi's Avatar


09 Jul, 2020 05:55 PM

I recently hiked the Shepherd pass trail on East Sierra. I used your map and measured the distance between the trail head to the pass. I found the distances i was given by caltopo widely inaccurate. The site tells me that is 8.6m to the pass but in reality it was over 12m. And this was not the first time. I am using your service to plan my multiday climbing trips. such a serious discrepancies can be the difference between a successful trip and failure even possibly dangerous. How do you explain this?

  1. Support Staff 1 Posted by Ben Lantow on 10 Jul, 2020 12:23 AM

    Ben Lantow's Avatar

    Thanks for reaching out! We looked into Shepherd's Pass trail (some of the CalTopo staff have hiked it) and it appears that the mileage reported from the TH to the pass depends on the book or resource you are looking at. We found a realworld gpx track online and overlaid it in a CalTopo map which you can view here:

    This track goes from the stock TH, not the hiker's TH (like yours did) so it has some extra mileage but when I use the measure tool (which the Snap-to settings set for OSM, which is the source of trail data for the MapBuilder Topo layer), we measured 9.5 miles, which is the same mileage as the gpx track that is overlaid.

    If you got your data from a gps device/app, keep in mind GPS is real world signals with no smoothing applied, they are prone to "jump". If you are in steep areas (which this trail has plenty of!) or have intermittent visibility of the sky, you may have extra distance added because of these jumps. You can do smoothing when you come back to the web platform if you so wish by reducing GPS points. Different apps and devices do a variable amount of smoothing.

    I think it is also important to note that maps are always representations and that no map is 100% accurate. Part of route planning and navigation is consulting various tools and making field-time decisions.

    CalTopo Team

  2. Ben Lantow closed this discussion on 10 Jul, 2020 12:23 AM.

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