Data and Deliberations: Part 2 Lidar

As you probably know, the availability of lidar data (lidar stands for Light Detection and Ranging) has taken the archaeology world by storm over the last decade or so. The collection of airborne lidar data involves sending out laser pulses from a plane, which are reflected back from the point at which they hit the ground (or any intervening objects). Given that the speed of the light is known, the distance each pulse has travelled can be calculated from the time it takes to return; in conjunction with very precise GPS positional data for the plane, the result is an accurate 3D location of each point in space, and therefore an accurate model of the land surface. This can then be viewed in a GIS, where it makes a superb tool for detecting subtle earthworks, such as coaxial boundaries, against background clutter.

Unfortunately, there is not 100% lidar coverage of the Dales yet. The lidar I am using has come from the  UK Environment Agency, who collect data as part of flood monitoring and environmental asset planning programmes and therefore target specific, required areas. The image below shows the 1m resolution (there is a point value for every square metre) lidar data available for the Yorkshire Dales National Park. It is very clear from this map that most of the data collection has focussed on the river valleys, with coverage often not (or only just) reaching the top of the valley sides. Which makes it difficult to rely on when prospecting for coaxials, which often, frustratingly, survive just above this level!

Figure 1: 1 x 1m lidar data for the Yorkshire Dales National Park (Data copyright Environment Agency 2015).

Figure 1: 1 x 1m lidar data for the Yorkshire Dales National Park (Data copyright Environment Agency 2015).

In places, however, coaxial field boundaries do show up clearly in the lidar data, running across the contour as in figure 2, and I have been mapping them from the lidar in conjunction with aerial photographs and maps, in order to characterise the individual systems. Lidar data can be viewed in various ways: figure 2 contains a simple 2D hillshade plot that ‘makes sense’ to the human eye, but various parameters can be exaggerated, or the point values used to analyse assorted elements of the terrain. The data can also be viewed in ‘3D’ (as in figure 3), which helps to visualise and understand relationships between the archaeology and the landscape. Usefully, it is also possible to drape other data sets over this digital elevation model – such as the aerial photography in figure 4.

Figure 2: Coaxial field system boundaries visible  north of Grassington in 1m resolution lidar.

Figure 2: Coaxial field system boundaries visible north of Grassington in 1m resolution lidar (Data copyright Environment Agency).

Figure 3: Hillshade plot  showing part of Upper Garsdale, viewed as a 3D surface model. It is possible to rotate and 'fly through' the model landscape. (Data copyright Environment Agency 2015.)

Figure 3: Hillshade plot showing part of Upper Garsdale, viewed as a 3D surface model. It is possible to rotate and ‘fly through’ the model landscape. (Data copyright Environment Agency 2015.)

Figure 4: The height data from the lidar has been used as a surface over which to drape this aerial photograph of Halton Gill, Littondale. (Data copyright Bing Maps/Microsoft/Environment Agency 2015.)

Figure 4: The height data from the lidar has been used as a surface over which to drape this aerial photograph of the valley side at Halton Gill, Littondale. (Data copyright Bing Maps/Microsoft/Environment Agency 2015.)

One of the bonuses of lidar data is the facility to alter the direction from which the (artificial) light source is shining on the landscape – depending on the orientation of individual features or their position in the topography, they may not be easily visible when illuminated from any given direction (as is the case in an aerial photograph, for example). Figure 5 shows the same medieval lynchets in Wharfedale illuminated from 3 different directions, and the difference this has on the visibility of the archaeology is very clear. The same goes for the height of the light source above the ground (think of the difference between viewing earthworks at midday and in late afternoon!). Obviously, while particular light directions and heights reveal more detail in landscapes, the flip side of this is that others cause detail to be obscured and it requires a little trial and error to work out which are the most advantageous light source positions.lynchets0lynchets45

Figure 5: Medieval lynchets near Grassington, Wharfedale, illuminated from 0º (top), 45º (middle) and 315ª (bottom) azimuth. Note how the visibility and prominence of features on different alignments varies under different lighting conditions. (Data copyright Environment Agency.)

Figure 5: Medieval lynchets near Grassington, Wharfedale, illuminated from 0º (top), 45º (middle) and 315ª (bottom) azimuth. Note how the visibility and prominence of features on different alignments varies under different lighting conditions. (Data copyright Environment Agency 2015.)

One way to get around going through 8 or 16 different images by hand for each area, is to use a combination of the images, which essentially includes the ‘best bits’ of each one. For my first foray into the world of computer coding – aided and abetted by my infinitely more code-minded colleagues – I am in the process of writing a piece of code that does this, by applying principal component analysis. You can see the difference between the images below…it’s often only subtle, but in places it may make interpretation just that bit more accurate.wharfe135a_06_07 copy wharfe090a_06_07 copy

Figure 5: The first two images show hillshade plots of Knipe Hill, Kettlewell, lit from 135ª and 90ª respectively; the third image shows the results of the application of principle component analysis, in which elements of these plots were combined with 6 others lit from various directions. Note how each emphasise different features. Click  to enlarge. (Data copyright Environment Agency 2015.)

Figure 5: The first two images show hillshade plots of Knipe Hill, Kettlewell, lit from 135ª and 90ª respectively; the third image shows the results of the application of principle component analysis, in which elements of these plots were combined with 6 others lit from various directions. Note how each emphasise different features. Click to enlarge. (Data copyright Environment Agency 2015.)

You can find out more about lidar here: http://content.historicengland.org.uk/images-books/publications/light-fantastic/light-fantastic.pdf/

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