# What projection should I use to find true distance between two points?

I am a beginner of ArcGIS and I encounter a problem of true distance.

I used "create fishnet" to have a grid map within 0-40N, 20E-90E. Suppose each two points are connected with a railway/road (which does not follow the great circle path of the two points), is it still possible to know the true distance in kilometers of this railway/road between two points? Especially when the two points has a huge difference in latitudes. I found this difficult because the fishnet is built using coordinates, and it seems projection use degree as linear unit could not give true distance if not along the Equator or a specific longitude.

• There isn't any projection which would be equidistant between random points across seventy degrees of longitude. You will need to use geodetic distance functions against objects with sufficient geographic accuracy to calculate the values you need. – Vince Dec 27 '18 at 15:53
• Vince thanks so much. Do you mean great circle distance? But this does not tell the road distance right? So the best I can to is to used the geodetic distance to calibrate the map distance I got? (e.g., use the ratio of map distance/geodetic distance of the straight line between two points to calibrate the real road distance) Thanks again. – Stone Jan 3 at 14:38
• No, I meant geodetic distance, which is spheroidal, and follows the path of the object (hence the need for "sufficient geographic accuracy"). We actually have guidance to avoid thanks here. Please take the Tour to better understand our purpose. – Vince Jan 3 at 14:53
• Thanks @Vince, I will mark and vote : ) About geodetic distance, if it's not the projection's problem, which particular tool should I use in ArcGIS? And so far things I found about geodetic distance are distance between points, but here I want to follow some path. Can I have more hints from you on this problem? Thanks again! – Stone Jan 13 at 17:04
• Look at the Geometry documentation and search for "getLength". The option name is `GEODESIC` – Vince Jan 13 at 21:05