Dave Burken submitted the following images to illustrate the importance of using good elevation with sensor models - including RPCs.  RPCs - Residual Polynomial Coefficients - are used by many data providers as a generic way of specify the three dimensional projection of the pixels.  The following  Quickbird image illustrates the difference between using an elevation surface and simply projecting on a constant elevation.  On the right, the image is projected on a constant elevation surface - notice that the left hand side of the image has not been displaced.  On the left image the same image has been properly projected on an elevation surface and the pixels have been properly shifted due to elevation changes within the view.  The left hand side shows the results of hills and valleys being shifted due to the collection geometry of the sensor.