Re: [GRASS-user] r.proj manual clarification

[Micha Silver]

On 07/09/2018 06:43 PM, Rich Shepard wrote:   In the Notes section of the 7.5 r.proj manual the second and third paragraphs read, "To avoid excessive time consumption when reprojecting a map the region and resolution of the target location should be set appropriately beforehand."   Isn't the target's location and region set when that location is created? Yes, but you can change both anytime you need a different extent or resolution. What happens to other maps (vector and raster) in that location if the Nothing region and resolution are changed to match that of the source map? I thought that r.proj was the tool to convert the source's region and resolution to No, r.proj transforms a raster from one projection (location) to a different projection (location) that of the target. "A simple way to do this is to check the projected bounds of the input map in the current location's projection using the -p flag. The -g flag reports the same thing, but in a form which can be directly cut and pasted into a g.region command. After setting the region in that way you might check the cell resolution with "g.region -p" then snap it to a regular grid with g.region's -a flag. E.g. g.region -a res=5 -p. Note that this is just a rough guide."   The first sentence checks the source (current) location's projection. The You should be only in the *target* location. First run:  r.proj -g location=<source location> mapset=<source mapset> input=<source raster> Then copy/paste the output to the g.region -ap command. Then rerun r.proj without the -g flag. third sentence suggests snapping the source grid's region and resolution to a regular grid. Two questions about this: 1) aren't all raster maps on a regular grid? Yes, but every projection transform creates a new regular grid, requiring a "warp" of the original regular grid cells. That's why it's a good idea to use the '-a' flag to g.region to align the cells and extent settings. and 2) how does this change affect the source location? None whatsoever   I'm working on understanding how to correct r.proj failures because the source map/location is outside the bounds of the target location. As Markus said, the source region settings are irrelevant here. One final comment: I noticed in a previous post: >   This I did: > > r.proj loc=elwood map=PERMANENT in=elwood_dem2013 -g > WARNING: Input and output locations are the same    <<==== ???? Obviously, the source and target locations must be different. HTH, TIA, Rich _______________________________________________ grass-user mailing list grass-user@lists.osgeo.org https://lists.osgeo.org/mailman/listinfo/grass-user -- Micha Silver Ben Gurion Univ. Sde Boker, Remote Sensing Lab cell: +972-523-665918

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