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<div class="moz-cite-prefix">On 3/17/2020 2:24 PM, Danilo da Rosa
wrote:<br>
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<blockquote type="cite"
cite="mid:CAA3XD6a3DWMUNGSn4zpDkcfTnhZYOsqJK=PGgqxNBKbfhwt5qw@mail.gmail.com">Do
you think it would be a good idea to do some kind of interpolation
to smooth the DEM file or the slope file? Do you have any
recommendations on how to do that using gdal? The idea is to use
the gdaldem color-relief command to generate a coloured and easy
to read map. The problem is that this lines makes the map more
difficult to understand, which is a priority in this case.</blockquote>
<br>
as has already been pointed out, the root of your problem is that
your DEM appears to have been created from contours with no
interpolation, resulting in a series of terraces with steps between
each level, giving the appearance that the terrain consists of flat
areas bounded by 90° slopes. Interpolating the existing DEM would
relieve this problem, but there is no reason to believe the
interpolation represents the actual surface. You would be better off
starting with a new DEM that accurately represents the region of
interest. I used the SRTM downloader plug-in QGIS to download a new
DEM. I saved this DEM in a projected coordinate system to get away
from the problems of dealing with degrees (as issue pointed out by
others). I then used the slope tool (which actually just calls gdal)
to create this slope image:<br>
<img moz-do-not-send="false"
src="cid:part1.4CFA0CA3.9C0E0C7E@stripfamily.net" alt=""
width="587" height="515"><br>
The slope ranges from 0 (white) to about 30° (black). <br>
SRTM data is 1 arc-second, or about 30 meters, which is coarser than
you previous data, but at least it's the actual surface. There are
higher resolution DEMs for much of the US on the US National Map.
Maryland (where your patch appears to lie) is covered by 1/9
arc-second data and parts are covered by 1 meter DEMs derived (I
think) from LIDAR. <br>
<br>
You state your goal is a colored, easy to read map. I've seen
articles about combining hillshade, elevation coloring, and slope,
though I've more often seen just hillshade + elevation (the latter
part being <i>hypsometric tinting)</i>. I think you will find it
easier to achieve this goal in Qgis, which will allow you to
experiment interactively with the various parameters in tinting,
hill-shading, and combining the layers. For most of these operations
Qgis uses gdal behind the curtain, so you can even see the gdal
calls if you want to replicate the results from the command line. <br>
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