QGIS for Archaeologists – A Basic Guide Contents

10. 24LIDAR 2 – AN EXERCISE IN WORKING WITH THE DATA
QGIS for Archaeologists – A Basic Guide
10.1. Using a raster catalogue
For the following exercise, let’s actually add something to the archaeological record. Start a new
QGIS project in a new project folder.
1) Name the project “Dales Lidar”, make sure that the working coordinate reference system is
OSGB1936 [27700] and the file paths are relative. .
We’re going to head to the Yorkshire Dales, where there’s some 1m resolution Lidar data, which we
can use to spot archaeological sites such as earthworks and field systems. I’ll also introduce a new
technique called de-trending which will help to reduce the amount of background noise from very
hilly terrain.
2) Head back to the Environment Agency website and download the composite Digital Terrain Model
for map square SD 87.
3) Once you’ve got it downloaded, extract the tiles into a suitably named sub-folder in the project
folder
There’s a lot of tiles - importing them all
individually is a really bad idea, as you’ll
have to go around changing the CRS for
each individual hectare tile and that will
drive you mad, as well as taking hours!
You can avoid this by changing the CRS
preferences in Settings > CRS
Also, there will be noticeable breaks
around the edges of each tile, as each
one will have its own histogram,
controlling how black and white each
pixel is rendered.
4) Fortunately the solution is to build a
virtual raster catalogue which allows
QGIS to take a number of files and read
them as though they were a single file. A
tool which you can find in the Raster:
Miscellaneous drop down menu.
5) You can choose a directory for your
input, then you’ll have to name the
output file. Every other option should be
set to default.
Figure 41: Build Virtual Raster Catalogue window
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