Detecting whether a GPS coordinate falls within a polygon on a map

Question

As stated in the title, the goal is to have a way for detecting whether a given GPS coordinate falls inside a polygon or not.

The polygon itself can be either convex

Answer 1

Runner.Java code in VB.NET

For the benefit of .NET folks the same code is put in VB.NET. Have tried it and is quite fast. Tried with 350000 records, it finishes in just few minutes. But as said by author, i'm yet to test scenarios intersecting equator, multizones etc.

'Usage

If coordinate_is_inside_polygon(CurLat, CurLong, Lat_Array, Long_Array) Then
    MsgBox("Location " & CurLat & "," & CurLong & " is within polygon boundary")
Else
    MsgBox("Location " & CurLat & "," & CurLong & " is NOT within polygon boundary")
End If

'Functions

Public Function coordinate_is_inside_polygon(ByVal latitude As Double, ByVal longitude As Double, ByVal lat_array() As Double, ByVal long_array() As Double) As Boolean
    Dim i As Integer
    Dim angle As Double = 0
    Dim point1_lat As Double
    Dim point1_long As Double
    Dim point2_lat As Double
    Dim point2_long As Double
    Dim n As Integer = lat_array.Length()
    For i = 0 To n - 1
        point1_lat = lat_array(i) - latitude
        point1_long = long_array(i) - longitude
        point2_lat = lat_array((i + 1) Mod n) - latitude
        point2_long = long_array((i + 1) Mod n) - longitude
        angle += Angle2D(point1_lat, point1_long, point2_lat, point2_long)
    Next

    If Math.Abs(angle) < PI Then Return False Else Return True
End Function

Public Function Angle2D(ByVal y1 As Double, ByVal x1 As Double, ByVal y2 As Double, ByVal x2 As Double) As Double
    Dim dtheta, theta1, theta2 As Double
    theta1 = Math.Atan2(y1, x1)
    theta2 = Math.Atan2(y2, x2)
    dtheta = theta2 - theta1
    While dtheta > PI
         dtheta -= TWOPI
    End While

    While dtheta < -PI
        dtheta += TWOPI
    End While
    Return (dtheta)
End Function



 Public Function is_valid_gps_coordinate(ByVal latitude As Double, ByVal longitude As Double) As Boolean
        If latitude > -90 AndAlso latitude < 90 AndAlso longitude > -180 AndAlso longitude < 180 Then
            Return True
        End If

        Return False
End Function