shortest distance a point on the line between point and line in wgs84 coordinate system -

in wgs84 coordinate system, know 3 coordinate:a, b , c. need find coord d. there line passing through , b. coord d shortest distance a point(c) on line(a,b).

-(coord) getcoordinateshortest:(coord)lineona :(coord)lineonb :(coord)coordiatec { ... return d; }