I have a problem where I need to work out a value of a specific point that lies in the quadrilateral that is formed by four seperate points. For each point that makes up the quadrilateral, I know the X and Y coordinates and their value, Z. For the point I am trying to find the value for, I only know the X and Y coordinates. I want to use linear interpolation to determine the value at that point, yet am not sure of the best mathematical technique to do it. For simplicity, I can assume that the quadrilateral is, for lack of a better term, 'squareish' - no inner angle is more than 180 degrees. The four points are also given in clockwise order. I have thought about using some simple line equations to do this, where I take the point and find the equation of the line that passes through one of the quadrilateral points. I then find the equations of the lines that form the quadrilateral and find the point where the original line intersects with one of the quadrilateral lines. I can interpolate the value at this point and then use that interpolated value to interpolate the value at the point I need. Would this technique work or is there a better one. The most important aspect is that if the point lies on a known value, it takes that value. For all other points, it needs to be interpolated. A drawing of the problem and how I propose to sovle it can be found at http://www.benthegeek.com/benstuff/interpolation.gif I appreciate your time.