A method comprising: supplying to a graphics processing unit, information about an environment as textures representing elevation data of obstructions in the environment, the obstructions including terrain, wherein the textures are accessed from a real time terrain database; supplying to the graphics processing unit location information comprising object position and object altitude of an object in the environment, and reference point position, reference point altitude and reference point orientation of a reference point, the reference point corresponding to a vehicle moving across the environment or an airborne platform moving above the environment; using a plurality of shader units of the graphics processing unit in parallel to process the textures to determine in real time intervisibility between the object and the reference point, intervisibility comprising whether a line of sight between the reference point and the object is obstructed by the environment; outputting by the graphics processing unit a composite intervisibility texture to a user indicating the line of sight; and updating by the graphics processing unit the outputted composite intervisibility texture when the reference point moves across the environment; wherein the object includes a weapons system or the reference point corresponds to a vehicle armed with a weapons system; and wherein ancillary information is further supplied to the graphics processing unit, the ancillary information including at least one of lethality and detection range of the weapons system, wherein the shader units are further programmed to compute intervisibility from the reference point or object to every point in the environment within the maximum of the lethality or detection range and classify it as lethality or detection and wherein color coding is applied to the composite texture based on lethality and detection range of the weapons system and whether the weapons system is hidden. supplying to a graphics processing unit, information about an environment as textures representing elevation data of obstructions in the environment, the obstructions including terrain, wherein the textures are accessed from a real time terrain database; supplying to the graphics processing unit location information comprising object position and object altitude of an object in the environment, and reference point position, reference point altitude and reference point orientation of a reference point, the reference point corresponding to a vehicle moving across the environment or an airborne platform moving above the environment; using a plurality of shader units of the graphics processing unit in parallel to process the textures to determine in real time intervisibility between the object and the reference point, intervisibility comprising whether a line of sight between the reference point and the object is obstructed by the environment; outputting by the graphics processing unit a composite intervisibility texture to a user indicating the line of sight; and updating by the graphics processing unit the outputted composite intervisibility texture when the reference point moves across the environment; wherein the object includes a weapons system or the reference point corresponds to a vehicle armed with a weapons system; and wherein ancillary information is further supplied to the graphics processing unit, the ancillary information including at least one of lethality and detection range of the weapons system, wherein the shader units are further programmed to compute intervisibility from the reference point or object to every point in the environment within the maximum of the lethality or detection range and classify it as lethality or detection and wherein color coding is applied to the composite texture based on lethality and detection range of the weapons system and whether the weapons system is hidden. wherein the object includes a weapons system or the reference point corresponds to a vehicle armed with a weapons system; and wherein ancillary information is further supplied to the graphics processing unit, the ancillary information including at least one of lethality and detection range of the weapons system, wherein the shader units are further programmed to compute intervisibility from the reference point or object to every point in the environment within the maximum of the lethality or detection range and classify it as lethality or detection and wherein color coding is applied to the composite texture based on lethality and detection range of the weapons system and whether the weapons system is hidden.