Intersection between spacecraft viewing vectors and digital elevation models

A method is presented for determining the latitude, longitude, and altitude of each pixel in a remotely sensed image of a planet for the case where there is significant surface topography. The method works by sequentially stepping along the line-of-sight of each pixel in an image until an intersection with the planet surface is detected. The position and altitude of each pixel on the planet can then be used for further analysis and allows comparison with other data sets. For pixels where no intersection occurs the altitude and location of the tangent point are determined. These pixels are important as they provide views of the planet limb and are useful for studying the vertical structure of the atmosphere. Provision is made for reference to an oblate spheroid when calculating the tangent point, which is required for atmospheric applications on oblate planets such as the Earth. The algorithm requires a digital elevation model, along with the viewing geometry and position of the instrument. Illustrative examples are given using the Martian MOLA topography data set for oblique and limb viewing cases.