I have a specific question but I think it comes from a general confusion about measuring the length of a polyline feature.

I downloaded the polyline shapefile for the Bigfoot Trail (a long distance hiking trail in Northern California) (ArcGIS Bigfoot Trail). The attribute table in ArcMap shows a trail length of 356.5 which is in agreement with the published trail length (Bigfoot Trail Website). If I use the Calculate Geometry tool within the attribute table and add my own length-in-miles column, I come up with 474.5.

My first thought is that the shapefile I accessed is just incorrect or out of date, but the file was last updated in August 2017 by a Bigfoot Trail board member and this same shapefile is hosted on the official Bigfoot Trail website (LINK).

I find it hard to believe the discrepancy could be explained by GPS error causing the tracks to jump back and forth because zooming in close to the map shows the polyline closely tracks roads and trails with minimal jumping. But if that is the likely culprit can someone suggest a more accurate method for measuring the length of the trail based on this shapefile?

My other hypothesis is that it is a projection error. If so, are there any remedies for fixing the error? The shapefile downloaded from ArcGIS is projected in WGS_1984_Web_Mercator_Auxiliary_Sphere (or that's what ArcMap-desktop automatically projects it in on my computer) and I haven't seen any metadata to suggest it should be any different.

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    The problem here is that you don't know who added the length of the trail to the attribute table. An attribute can be manually entered and may just be there for labeling purposes on a map. The attribute does not have to specifically relate to the geometry of a feature. The error you are seeing could arise from a number of factors. The most likely is the trail was digitized at a very high level. Explaining the 100 odd mile difference in length. – Keagan Allan Dec 13 '17 at 22:04
  • Thank you. I figure something close to that explains the problem. But we're talking about a 350 mile line vs a 470 mile line, which seems like a huge difference to me. Assuming the line follows the correct path of the trail how might someone reconcile these hugely different estimates? Can I fix the "digitized at a very high level" problem, which I don't really see when looking at the actual data? – Adam C Dec 13 '17 at 22:11
  • Does the trail information come with any metadata? It might explain how the line was created. An error of 100miles over a distance of 350 is quite large. How detailed is your downloaded line? Does it match in Google Earth? Besides redigitising the line I am not sure how best to try and reconcile the differences. It may be this line was always meant for display purposes on a more regional map. – Keagan Allan Dec 13 '17 at 22:17
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    In what map projection are you measuring the longer value? If it's Web Mercator, that would explain why your computed value is incorrect. – Vince Dec 14 '17 at 1:43
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    @AdamC Please add any additional information into your question rather than in comments. You can edit your question to add this info. – Midavalo Dec 14 '17 at 2:23

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