[GRASS-user] r.mapcalc: compare two maps [RESOLVED]

[Laurent C.]

Rich,

If I understand well, you have a 'raw' DEM and you want to open the
culvert in that DEM.
If it is an under-road culvert, your 'raw' DEM will show the road
elevation, and the culvert will not be shown. i.e the culvert is
blocked.
If you want to 'open' the culvert, you'll have to cut through the
road. You can do this with r.carve, for example.

Regards,
Laurent

2017-03-30 10:04 GMT-06:00 Rich Shepard <rshepard at appl-ecosys.com>:
> On Thu, 30 Mar 2017, Nikos Alexandris wrote:
>
>>> For the record: there is a "differences" color table which is useful for
>>> this.
>
>
>> Don't forget to try r.colors with -e or -a -g if the first attempt does
>> not
>> show much differences.
>
>
>   I _had_ not noticed the 'differences' color table. Applying that produces
> interesting results (see attached .png).
>
>   The four blue cells are ~0.6m higher in the blocked map than in the open
> culvert map. I don't know the reason for the lighter blue cell to the
> southwest of the group of four. I suspect, but don't know, that the pinkish
> cells represent slightly higher water accumulation in the blocked map than
> in the open map.
>
>  Germane to what Markus M. wrote, "The point locations used to edit the
> original DEM are probably wrong (not close to the center), ..." I now think
> that's where I made one mistake. When I isolated those four cells and
> queried them I truncated the easting and northing coordinates to the integer
> value assuming that all elevations in a 1m x 1m cell were the same. If this
> assumption is incorrect then I'll re-create the change and start the model
> running again with a slightly different set of elevations for those cells.
>
>   If anyone can suggest other changes to the model input to better show
> where water accummulates behind a blockage please let me know.
>
> Rich
>
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