[Qgis-user] New User: Cropping makes HUGE files (and I know about COMPRESS-LZW)

[Dennis Grant]

Hi all, I have a very specific problem I'm attempting to solve, and I'm sad
to say it has defeated me.

What I am trying to do is generate a 3D model of the terrain relief in a
certain area so that it can be fed into a CNC machine for subsequent
carving.

So then, step 1 - get a DEM GEOTIFF from EarthExplorer: done.

Here's the thumbnail
https://earthexplorer.usgs.gov/browse/srtm_v3/1arc/w067/n45_w067_1arc_v3.jpg
(but I have the actual GEOTIFF too - I know that the thumbnail is not the
GEOTIFF)

Now what I need to do is crop the area covered by the GEOTIFF down to the
roughly 4 sq km area I want to model.

So I downloaded and installed qgis for Linux. Add new raster layer using
the downloaded GEOTIFF, and that comes in fine.

Problem: I can't narrow down the exact area I need. So I installed the
OpenStreetMap plugin and added an OSM layer. Happily, the OSM layer and the
GEOTIFF raster layer line up as expected. So I'm able to adjust the zoom
window to the area I want to carve.

I go to Raster: Extraction: Clipper. Input file is the downloaded DEM
GEOTIFF. Output file is a filename I pick for the extract. I select the
area I want, and I see that it builds a command line for gdal_translate
accordingly. OK, I get it, QGIS is a graphical front end for command line
tools - that's a common Linux method for building applications. Seen that
before. No worries.

So then I execute, and after 10 minutes, it still isn't done, and the file
it is building is almost a gigabyte in size.

Stop process, google for "qgis extract huge files" and I find that the
issue is likely compressed vs uncompressed TIFF, and there are a couple of
options to feed gdal_translate to turn on compression (via the little
pencil icon) OK, add that, and set it off.

Come back an hour later, still not done, and the file is 150 Gb in size.

Something is not right. The base GEOTIFF is 25 Mb. A drastically cropped
version - even uncompressed - should not be that large.

This happens even if I delete the OSM layer.

What is going on here? What have I missed?

DG
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