LIDAR, which stands for Light Detection and Ranging, is a surveying method that measures distance to a target by illuminating that target with a pulsed laser light, and measuring the reflected pulses with a sensor. Differences in laser return times and in frequency,  can then be used to make digital 3D-representations of the landscape, even removing vegetation and buildings if required, and can reveal features such as Roman roads which are impossible to detect by eye on the ground.

The lidar data used in this gazetteer is provided by the Environment Agency, whose priority is to combat flooding, and consequently coverage can be poor in upland areas, often just where we need it!