Last time, I wrote about the way the aerial scanning of the ground from a plane can produce high resolution digital terrain models which can reveal patterns of faint earthworks. These are impossible to see through aerial photography or visually on the ground, even in low sunlight.
I also mentioned the way that tree cover could be filtered out to see just the ground surface beneath the trees.
LiDAR is short for Light Detection and Ranging and these digital images are best interpreted by comparing them with other layers of information.
Modern digital mapping allows different imaging sources to be superimposed and by using percentages of transparency, the chronology and phasing of earthworks revealed by LiDAR can be better understood.
I will do this by using the LiDAR that the National Trust commissioned for Dyrham Park. A property which lies between Bristol and Bath in South Gloucestershire.
Briefly, Dyrham is a landscape park occupying an amphitheathre of natural terrain with the great late 17th century mansion occupying the valley floor beside a medieval church. The church demonstrates that manor houses have occupied this favoured spot since the Domesday survey of 1086. Archaeological fieldwork has demonstrated that there has been significant Roman and prehistoric occupation at this location.
The LiDAR plot of the whole park shows that the deer park here was enclosed out of a medieval open field system and that no cultivation has taken place for over 400 years. The plough ridges, arable strips and furlongs remain from the last year of ploughing frozen in time.
The farm buildings in the centre of the park have structures dating to the later 18th and 19th centuries shown on the modern aerial photograph.
However, if you upload the LiDAR and fade out the photo you can see that the present buildings do not relate to earlier earthworks though there is a focal significance to the area in which the buildings stand.
Notice the three broad linear features radiating out from this place, a little above where the present buildings stand. These must be quite old as these linears are cut across by trackways and other features.
A 17th century map shows an earlier lodge near this location. This was the keepers house built at the division between the rabbit warren to the upper left and the deer park to the right.
Another layer to be georeferenced and compared with the LiDAR would be a geophysical survey which would show how vague surface earthworks might relate to anomalies detected beneath the ground.
Another quick comparison would be to compare historic maps with the LiDAR. Here the 1921 edition of the Dyrham Ordnance Survey maps is made transparent above the LiDAR. Notice how the trackway heading from the buildings continues towards the top centre respects a large rectilinear earthwork but does not show what it is.
The next slide shows the 1892 OS map over the LiDAR. In less than 30 years the size of changes can be seen. The site of the aviary is not shown in 1921 and in 1892 the enclosures surrounding the farm buildings were more extensive.
So the basic data of the LiDAR provides a great insight into the historic earthworks of a piece of land but it takes time to test them against all available sources and sift out the potential information that might lie there.