Sunshading global datasets

The proliferation of satellites in the last 30 years has resulted in a similar proliferation of global datasets, not only of the Earth but also of other bodies in the solar system. Measured parameters include topography, temperature, water vapour, electron temperature and density, and the strength of the gravity and magnetic fields, to mention just a few. Directional derivatives and sunshading are powerful tools for enhancing linear features in raster datasets. This paper demonstrates modified directional derivative and sunshading algorithms that take into account the curved geometry on the surface of the sphere, and hence are able to be used with global datasets (assuming the Earth or other body to be a sphere). The algorithms can either be used to enhance any linear features (i.e. great circles) than pass through a chosen point, or to attenuate them. They can also be used to enhance small circles of any radii centred on a given point, or to attenuate them. Source code, in Matlab format, is available from the IAMG server at www.iamg.org.