Archaeologists have interpreted historic sites from humps and bumps visible on the ground or from the air for a long time. However, the height data recorded by lidar is not a straightforward record of the ground surface. When the laser is fired from the plane it travels towards the ground and if it strikes anything in passing, such as leaves on a tree, part of that beam is reflected back to the sensor and forms the first return; the rest of the beam continues towards the ground and may strike other features that produce further returns until it finally strikes the ground, or a surface that allows no further progression. The final reflection that reaches the sensor is known as the last return. In practice, built-up areas and open land act as solid surfaces and the first and last returns are often identical. Woodland, however, functions as a porous surface where the first return generally represents the top of the tree canopy while the last return may be a reflection from the ground surface, although it might equally be from the trunks of the trees or areas of dense canopy or undergrowth.
For many early-generation sensors only a small number of return echoes were collected from each pulse – often just the first and last return, with occasionally an additional one or two in between. The first and last returns were considered the most important: the first being equivalent to the Digital Surface Model (DSM) and the last being used as a means to help calculate a Digital Terrain Model (DTM).
Within the last few years the latest development of lidar sensors has expanded and now, instead of just recording between two and four returns, the new full waveform system digitises the entire analogue echo waveform for each emitted laser beam (Doneus and Briese 2006; Doneus et al 2008).
However it is generated, the most useful product of lidar for archaeologists is the three-dimensional model of the ground; the DTM is particularly useful for the information it can provide in woodland, but in non-wooded areas the DSM may be preferable because of the absence of smoothing effects. The DTM still requires careful manipulation using specialist software, to enable analysis and interpretation of the archaeological features.