Lidar intensity data

As well as the relative x, y and z position of the point on the ‘ground’ the sensor also records the intensity of the reflected signal. When seen as a simple image file the intensity information translates into a series of tonal differences and provides an image of the return surface similar to that of a true panchromatic orthophoto at the same resolution

It is important to note that, because the lidar pulse is generally in the near infrared (NIR) rather than in the visible spectrum, the reflectance is not what might be expected by those used to working wavelengths in the visible range (e.g. those used to dealing with standard aerial photographs). Whereas a flat, solid surface such as stone or concrete will reflect almost all of the light in the visible spectrum, this is not the case with infrared light; instead asphalt for roads has a low return value, while grass or a green crop will have a high return.

Chlorophyll in plants reflects near infrared radiation, so changes in the chlorophyll content of a single plant species, perhaps as a result of stress such as drought, can be represented in the intensity data in the same way that they are seen in the visible spectrum as cropmarks. In fact, because chlorophyll reflects c 50% of NIR radiation, as opposed to 15% of visible, plant stress (e.g. grass growing over buried walls) is much easier to discern than in the visible spectrum (Verhoeven and Loenders 2006). This is something that has long been recognised by archaeologists and was first systematically investigated by Hampton in the summer of 1970 when he reported that compared to standard film the NIR film ‘showed distinct advantages at the early stages of cereal growth’ (Hampton 1974).

The intensity of the reflection can be affected by a combination of factors such as flying height, laser power and wavelength, atmosphere, direction of laser beam, and the number of returns. If these parameters are known, the data can be calibrated so that the resulting values are largely determined by the nature of the surface from which the pulse is reflected, and so be used to identify the character of these surfaces (Höfle and Pfeifer 2007; Boyd and Hill 2007.

There may be archaeological potential in using intensity values as a method of assessing the moisture content of exposed soils. A project funded by the Aggregates Levy Sustainability Fund (ALSF) investigated whether this could be used to predict the likelihood of preservation of waterlogged archaeological remains, but results have proved inconclusive (Challis et al 2008a). While the results suggested that from a visual standpoint the lidar intensity data proved useful in qualitative analyses of certain areas, the report stated that ‘the application of lidar intensity data to predictively model sediment units of high preservation potential can be deemed at present to be untenable’. However, while the usefulness of the intensity data to identify damp ground seems to be uncertain there is definitely useful information in the data in other circumstances.

A brief history of lidar

Airborne lidar technology

What does lidar provide?

Height data from lidar