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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. Read More
