TransDEM Forum

Hi everyone,

there is a great collection of DEMs available for Austria for free on http://www.data.gv.at - simply search for "DGM" ("Digitales GeländeModell") or use direct link
https://www.data.gv.at/suche/?search-term=dgm&searchIn=catalog&nr=0

Unfortunately, although available as geotiff, I seem to be unable to open the DEMs in TransDEM - I always get the message "PCSCitationGeoKey tag not found". Is there a way round this? Maybe changing/inserting this tag into the tif file, and if yes - how and what value to be chosen?

Thanks and all the best,
Tobi

Possibly. But I have to investigate. I already downloaded the data yesterday (pure coincidence) and will run TransDEM in debug mode to analyse.

Strange thing. GeoTIFF has many options to encode meta data. One of them is "custom". And in custom mode you can do whatever you want. Unfortunately, a data reader application has to cope with this. Now, in this case, the projection appears to be "MGI Austria Lambert", one of the two classic projections in use in Austria. And because it's classic and well-known, it has a standard numeric encoding. Why they are not using that, I do not know. Instead they define all the projection parameters manually. The only other case where I have seen this customized approach before was the 25m DEM for New Zealand. I have to find out how I managed to decode that and then do something similar for the Austrian DEM.

Some progress. In my lab environment, I have managed to extract data:



After the initial confusion with the custom definition for a well-known projection, the remaining meta information all made sense. Unfortunately, it points to a branch in TIFF encoding which TransDEM had not encountered before. Some additional code is required, that will also imply some refactoring in the existing GeoTIFF handling. It will take a bit of time.

In any way, the overall amount of data in that tif file is far too big to be opened in one go. But TransDEM already has an option to only extract a subset of the data and that will be obligatory when using the 10m source. But from the results in my lab test, that subset dialog also needs some adjustment.