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I have a project where there are GPS points that are centered around the Chesapeake bay, with some points extending between North Carolina and southern Canada

I have been using the NAD83 Virginia South state plane because I was told that a projected coordinate system is more accurate for distance calculations, but I am unsure if this projection would be reliable with the spread of my data points.

I read that outside of a specific state zone errors increase quickly, but I couldn’t find any information on how error accumulates if points extend outside of the bounds of the state zone (for example, for every 5 km outside of the state zone you’d see error increase by 1 m).

Do you know if this information is available anywhere, or am I misunderstanding how the error works? Would a more general projection for all of North America such as the North America Equidistant Conic be a better choice?

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    Test it, is my advice. Do a distance calculation between two distant points in both projections and see what the difference is. – alexGIS Apr 7 '20 at 16:31
  • Canada is way too far north for the Lambert Conformal Conic projection used for Virginia South state plane (the limits for which are 36.53 and 38.27 latitude). The closest parts of Canada are 270 nautical miles distant, nearly three times the nominal height of the projection) – Vince Apr 7 '20 at 17:22
  • Thank you! Where did you get that information about the latitude limits? – Miranda Turner Apr 7 '20 at 17:52
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Because the earth is a 3d sphere and our map is a 2d piece of paper then we will always have distortions when we move from one to the other. These distortions can be in angular conformity (are angles correct from a certain point), distortion in distance (is everything on the map the correct distance from a certain point) or distortion in area (do areas on the map represent true areas in the real world). Some projections just sacrifice one of these components, whilst others sacrifice a little bit of each for general accuracy. Most sections of the world have there own system to limit these distortions and as you get further away from the central point then these distortions get larger and larger. The system that you choose will be dependent on what's the most important to you i.e. angles, distance or area. QGIS have a good guide that might help you decide: https://docs.qgis.org/3.10/en/docs/gentle_gis_introduction/coordinate_reference_systems.html

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  • I think I understand this, but my main question is if anyone knows of any resource that explains how error accumulates for different projections (specifically the Virginia south state plane), because I couldn't find that information. This projection is meant for southern Virginia, so obviously if I was working on points in India there would be high error, but I'm trying to find the "boundary" outside of which where this projection will create high errors. – Miranda Turner Apr 7 '20 at 16:56

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