3

I have a list of positions like this.

t, x, y, z

where t is a double representing time in seconds and x, y, z is a position expressed in ECEF coordinates.

I would like to interpolate the position between points in the list and I believe I can do that part, but given two points and time, what is the formula to do the interpolation?

lets assume the Position object contains time, x, y, and z

// @param p1 - first position
// @param p2 - second position
// @param time - time to get interpolated position for
private Position interpolateECEF (Position p1, Position p2, double time) {
   // need help here
}
  • 1
    Possible cross posted to: stackoverflow.com/questions/43148203/… – xunilk Apr 1 '17 at 10:02
  • yes, it was. i originally posted there and a comment was made to post here – mike Apr 1 '17 at 13:51
  • 1
    Welcome to GIS SE. Do you assume linear travel and constant velocity? It should be simple linear interpolation solution. What have you tried? Use Edit. – Martin F Apr 1 '17 at 14:44
  • no, travel is not linear as it will start off linear and then make turns. essentially the path looks like a popsicle stick. velocity is not constant either and i have a velocity vector (magnitude would be speed), but that was going to be part 2 after i get this part working. i have not tried anything yet as i do not know where to begin – mike Apr 1 '17 at 15:12
  • Actually, i meant linear or constant between data points, meaning is it simple interpolation, requiring the very basic math of Matej solution below. – Martin F Apr 3 '17 at 15:18
2

Below is a simple implementation in Python to illustrate the concept.

class Point():
    def __init__(self,t,x,y,z):
        self.t = float(t)
        self.x = float(x)
        self.y = float(y)
        self.z = float(z)

    def __str__(self):
        return 'Point: t={},x={},y={},z={}'.format(self.t, self.x, self.y, self.z)


def interpolate(p1, p2, t):
    t_ratio = (t-p1.t)/(p2.t - p1.t)
    new_x = p1.x + t_ratio * (p2.x - p1.x)
    new_y = p1.y + t_ratio * (p2.y - p1.y)
    new_z = p1.z + t_ratio * (p2.z - p1.z)

    return Point(t, new_x, new_y, new_z)


def main():
    p1 = Point(t=100,x=10,y=20,z=30)
    p2 = Point(t=200,x=5,y=30,z=40)

    # interpolate for time 150
    p_interpolated = interpolate(p1,p2,150)

    print p_interpolated

if __name__ == '__main__':
    main()

This returns: Point: t=150.0,x=7.5,y=25.0,z=35.0.

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