TERRAIN REFERENCED NAVIGATION SYSTEM

A terrain referenced navigation system comprising: at least one sensor arranged to measure the elevation of terrain below a current position of the navigation system; a terrain elevation map comprising a plurality of map posts that each comprise terrain elevation data; and a land-water detection unit arranged to calculate a proportion of land and/or water of at least a portion of the terrain elevation map based on the terrain elevation data of said portion of said terrain elevation map. Terrain maps store a fixed elevation for each entry, including entries that correspond to both areas of land and areas of water. The elevation of bodies of water, however, rises and falls over time (e.g. as a result of tides, seasonal filling and evaporation of lakes, etc.). This can cause the actual elevation of water to differ from stored elevation values in the terrain map, which can cause errors in position estimates obtained from terrain-referenced navigation systems. The land-water detection unit provides the ability to estimate a proportion of land and/or water of the terrain elevation map, using the terrain elevation data stored therein. The land-water detection unit allows the navigation system to detect areas of the map that contain transitions between land and water, and compensate accordingly when passing over these areas.