Combining UK OS Historic Digimap Tiles

Question

I'm assisting a retired user in combining a large number of raster tiles downloaded from Historic Digimap http://edina.ac.uk/digimap/description/historic_overview.shtml into one project in qgis.

I'd like if possible to combine these into one layer, - I'm aware that these can likely be stitched together using an external tool, but am unsure exactly how to go about doing so (or whether this is in fact possible in qgis itself) in a way that the resulting layer is still usable on a standard desktop pc (~4GB ram) or whether this is in fact not possible.

Many thanks in advance

Answer 1

This should be an ideal case for GDAL buildvrt: http://www.gdal.org/gdalbuildvrt.html

Only thing that is necessary is georeferencing information. Geotiff would be best.

For putting Dutch maps into a vrt, I used the following commands on Windows:

for %%N in (D:\Karten\gdal\gdal2tiles\NL50\*.tif) DO gdal_translate -of vrt -expand rgba %%N D:\Karten\gdal\gdal2tiles\NL50\%%~nN.vrt
gdalbuildvrt -allow_projection_difference index.vrt NL50\*.vrt

You can add the index in QGIS just like any other Raster source.

Answer 2

Just to add that the gdalbuildvrt is just creating a virtual mosaic of the tiles using the georeferencing files relating to each .tif file.

If you wanted to create a proper mosaic then you can use your .vrt file that you created and then create a GeoTiff from that.

The code would be something like this

gdal_translate -of GTiff -a_srs "EPSG:27700" -co "COMPRESS=LZW" -co "TILED=YES" index.vrt mosaic.tif

• the a_srs will set the georeferencing projection toe British National Grid
• the -c COMPRESS is useful to drop file size using a set compression method
• the -c TILED builds internal tiles in the geotiff which helps with the performance of reading the tile

Answer 3

If all you want to do is have several images on the QGIS screen you could just load them, they should align properly. You're allowed more than just one raster in a QGIS project! Or, as Andre suggests, create a virtual raster from the required maps, which can be done from within QGIS. Raster -> Miscellaneous -> Build Virtual Raster (Catalog) and select the required raster files. At the end of this you'll get a file with the extension *.vrt which you can load into QGIS like any other raster image.

I don't see much OS historical data but I think it comes in the form of TIFFs rather than GeoTIFFs. So each *.tif must have its own world file (a little file with the extension *.tfw) in the same directory as the image.

N.

Answer 4

Unsure why my account hasn't claimed the question, apologies! Thanks all for the answers.

With the 1st edition data a

gdalbuildvrt index.vrt 1stEdition/*.tif

was sufficient to build the virtual index. OSGeo4W http://trac.osgeo.org/osgeo4w/ was handy in getting gdal's various bits and pieces setup quickly