I have ~2000 NASA SRTM .hgt files. I wanted to join them together in a single mosaic, and to save disk space I used GDAL to create a .VRT rather than a very large GeoTiff. This is read by Mapnik.
What I would like to do is “fill in” the areas in the mosiac that aren’t covered by one of the original .hgt files. In other words, I want to pretend as if those uncovered areas are actually covered by a .hgt that has a constant value for all data points. Thus, the resulting .VRT will be able to report a value for those areas. (or, if I end up making a .tif instead of .vrt, the .tif would have a value for each of those areas).
Is this possible?
(to elaborate: the reason I want to do this is because I am generating tiles to cover the entire world – this will be an overlay, not base tiles. My data does not cover everywhere, and for the areas that it doesn’t cover I want to be able to style it in a particular color. Normally, I could use Mapnik’s ‘background-color’ attribute, and it would color any background area in the map. However, one of my styles for the data areas I want to cover is transparent, and setting the background-color would erroneously fill in the transparent areas. Hence, my desire to modify the data set)