Areas of forest, mountain, desert and ocean are clearly recognised in their natural colours, offering an alternative to traditional topographic and political maps. The Mississippi Delta at a 1 km resolution. The satellite systems used offer frequent global coverage, but do not acquire images in true colour. To simulate natural colours over the land, red and near-infrared wavelengths are used to differentiate land cover types on the basis of their spectral reflectance characteristics. Ground-level, aerial and orbital photography are then used to assign red, green and blue colour values to known feature types, giving realistic land surface colour for anywhere in the world.
Ocean colour is derived from a combination of ocean depth and ocean chlorophyll observations.
Polar areas covered by sea ice are derived from individual optical images. On land, the courses of rivers are merged in from a digital map database and the appearence of physical features has been enhanced by hill-shading using a digital terrain model. Software was written to select the most cloud-free parts from many overlapping images. So far, two man-years of effort have been spent on the Satellite Imagemap and it is continually being improved as new data becomes available. The Satellite Imagemap is held in a geographical coordinate system based on a latitude-longitude grid for easy combination with ancillary data layers, e.g. digital terrain, water and political masks, nighttime imagery.