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My first thought is that the shapefile I accessed is just incorrect or out of date, but the file was last updated in August 2017 by a Bigfoot Trail board member and this same shapefile is hosted on the official Bigfoot Trail website (LINK).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