How to compute line segment intersections on a map

Question

I am working with Latitude / Longitude coordinates in a google map.

I have two lines :

• Line A : 48.31508162629726, -2.591741396838972 to 48.40216156645915, -2.2218462112093404
• Line B : 48.383816077371215, -2.274292940053768 to 48.66103546935337, -1.7066197241571377

I then use the following formula to find the point where they cross.

var XAsum = A.LngStart - A.LngEnd;
var XBsum = B.LngStart - B.LngEnd;
var YAsum = A.LatStart - A.LatEnd;
var YBsum = B.LatStart - B.LatEnd;

var LineDenominator = XAsum * YBsum - YAsum * XBsum;
if(LineDenominator == 0.0)
    return false;

var a = A.LngStart * A.LatEnd - A.LatStart * A.LngEnd;
var b = B.LngStart * B.LatEnd - B.LatStart * B.LngEnd;

var x = (a * XBsum - b * XAsum) / LineDenominator;
var y = (a * YBsum - b * YAsum) / LineDenominator;

This tells me that the lines do indeed cross and returns the x and y values.

However, when I plot the returned point, it is offset (not much) from the real intersection.

Is there a better and just as fast algorithm I could use which will return me the correct intersection point ?

It needs to be fast as I am iterating over a large number of lines (~1000).

EDIT : Note this is giving me an error offset of around 7.5 meters

Answer 1

I'm assuming the algorithm you're using is the one for finding line intersections on a Cartesian coordinate system (i.e. a plane). Unfortunately, the earth is not a plane (or even a sphere) so using that algorithm will introduce error. Google Maps uses an ellipsoid (specifically WGS84) to approximate the surface of the earth, so you'll need an algorithm for finding the intersections of arcs on an ellipsoid.

This page might contains some helpful information: http://mathhelpforum.com/calculus/90196-point-intersection-two-lines.html