Goal: given a route as a set of ordered coordinates, measure the overall distance of the route and be able to access the coordinate pair at any given distance along the route.
Application: I'm building software to analyze driving routes and I would like to be able to get data for any particular section of the route that I want. For example, if I want to know the elevation profile between 50.3 and 58.7, I need the points from the route that are included between those distances.
Problem: I have tried this in Google Earth and various online route building websites like MapMyRide with absolutely horrible results. The process has been to take the file produced by the service, which is a set of points, and interact with the route based on the points in the file. When measured from start to finish, the distance is the same as the service claims. BUT, if I request a point at a distance along the route, using the same calculations for distance between points, I get a points that is anywhere from .5 to 2km off as compared to the distance from the service (basically, the km markers along the route).
Other info: This is a python project and I'm using Vincenty's ellipsoid to measure distance. This question is stemming from a conversation that started at Calculated distance doesn't match Google Earth.