perhaps this is a simple question. I have 3 GPS coordinates (one is the current user location). What I want now is to calculate the angle between the user location and the two GPS coordinates. Imagine the user location in the center of the two other points, the three points can be seen as a triangle. And I want to calculate the angle at the user location. I hope someone can help me because I have no idea how to do this with spherical coordinates like the GPS coordinates I have.
THX - nekro
You could use thessien polygons and calculate the geometry on those from a strictly GIS perspective. If you have qgis or arcgis this should be fairly simple. These packages offer APIs which might suit your needs.
You're essentially doing two calculations (bearing to (or from) current position to two other positions) and not crosstrack (distance from a great circle line between to other points).
However, both can be found in Ed William's Aviation Formulary which has the most comprehensive collection of formulas for spherical calculations I've found.
You would be looking for "Course between points" which is listed as:
tc1=mod(atan2(sin(lon1-lon2)*cos(lat2),
cos(lat1)*sin(lat2)-sin(lat1)*cos(lat2)*cos(lon1-lon2)), 2*pi)
For short distances (less than 100km, say) you can safely ignore the spherical nature of the calculation and treat the problem as a 2 cartesian coordinate problem. For large distances the spherical geometry gets pretty gnarly. I could probably figure it out, but I don't want to think that hard right now.
All you need to do is to convert both coordinates to KM, and then treat it as a cartesian problem. (At a small scale, you can ignore the curved nature of the "lines" and treat them as normal cartesian grid lines, since the curvature is small enough to ignore at that scale)
The distance per degree of latitude is constant. The distance for a degree of longitude changes based on latitude.
Do a google search on "KM per degree of longitude" and find a link that explains it clearly. Here's one: http://www.colorado.edu/geography/gcraft/warmup/aquifer/html/distance.html
