creating a right angled rectangle on google maps from a line

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I have an application where the user draws a line, then automatically the software should create a rectangle where the line intersects the midpoint of two adjacent sides of a right angled rectangle. See below, the red line is user drawn and the purple rectangle is calculated. My algorithm almost works, in some directions the rectangles aren't right angled. I want to know why, there is a fair amount of approximation in my code, which should be fine based on the scales I use (a few km). I shouldn't have to worry about the curvature of the earth.

Anybody know how I can improve my code?Here are my references:calculate pointslat and long to meters



public static MapRectangle CreatMapRectangle(BalingZone zone, double thickness) { MapsCoordinate rectpoint2; MapsCoordinate rectPoint1 ; MapsCoordinate rectPoint3 ; MapsCoordinate rectPoint4 ; var point1 = zone.Coordinates[0]; var point2 = zone.Coordinates[1]; var latitudeDiff = LatitudeDiffToMeters(point2.Latitude - point1.Latitude); var longitudeDiff = LongitudeDiffToMeters(point1.Longitude - point2.Longitude, point1.Latitude); var slopeB = longitudeDiff / latitudeDiff; double latOffset = thickness * (slopeB / Math.Sqrt(1 + slopeB * slopeB)); double longOffset = thickness * (1 / Math.Sqrt(1 + slopeB * slopeB)); double p3Lat = CalculateLatitude(point1.Latitude, latOffset); double p3Long = CalculateLongitude( point1.Longitude, p3Lat , longOffset); rectPoint1 = new MapsCoordinate(p3Lat, p3Long); double p4Lat = CalculateLatitude(point1.Latitude, -latOffset); double p4Long = CalculateLongitude(point1.Longitude, p4Lat, -longOffset); rectpoint2 = new MapsCoordinate(p4Lat, p4Long); double p5Lat = CalculateLatitude(point2.Latitude, latOffset); double p5Long = CalculateLongitude( point2.Longitude, p5Lat , longOffset); rectPoint4 = new MapsCoordinate(p5Lat, p5Long); double p6Lat = CalculateLatitude(point2.Latitude, -latOffset); double p6Long = CalculateLongitude( point2.Longitude, p6Lat , -longOffset); rectPoint3 = new MapsCoordinate(p6Lat, p6Long); return new MapRectangle(rectPoint4, rectPoint3, rectPoint1, rectpoint2, thickness); } //use the quick and dirty estimate that 111,111 meters (111.111 km) in the y direction is 1 degree (of latitude) // and 111,111 * cos(latitude) meters in the x direction is 1 degree (of longitude). private static double LatitudeDiffToMeters(double latitudeDiff) { return 111111.0 * latitudeDiff; } private static double LongitudeDiffToMeters(double longitudeDiff, double latitude) { return 111111.0*Math.Cos(latitude)*longitudeDiff; } private static double CalculateLatitude(double latitude, double offset) { return latitude + offset/111111.0; } private static double CalculateLongitude(double longitude, double latitude, double offset) { return longitude + offset/(111111.0*Math.Cos(latitude)); }}

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