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Another guess:<br>
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The vertical resolution of your DEM is low, say height is measured by 1m intervals, and your horizontal resolution is relatively high, say the pixel size is 25m. Then the minimum slope is 1/25. For many pixels in flatter area, this means adjacent pixels are
of exactly <br>
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<div id="divRplyFwdMsg" dir="ltr"><font face="Calibri, sans-serif" style="font-size:11pt" color="#000000"><b>From:</b> gdal-dev <gdal-dev-bounces@lists.osgeo.org> on behalf of Joaquim Manuel Freire Luís <jluis@ualg.pt><br>
<b>Sent:</b> Tuesday, March 17, 2020 12:32:13 AM<br>
<b>To:</b> Danilo da Rosa <daniloide@gmail.com>; gdal-dev@lists.osgeo.org <gdal-dev@lists.osgeo.org><br>
<b>Subject:</b> Re: [gdal-dev] Slope obtained with GDAL has weird lines</font>
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<p class="x_MsoNormal"><span lang="EN-US" style="">Ah, let me guess. The dem grid was interpolated from contour lines and you seeing the contours again.</span></p>
<p class="x_MsoNormal"><span lang="EN-US" style=""> </span></p>
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<p class="x_MsoNormal"><b><span lang="EN-US">From:</span></b><span lang="EN-US"> gdal-dev <gdal-dev-bounces@lists.osgeo.org>
<b>On Behalf Of </b>Danilo da Rosa<br>
<b>Sent:</b> Monday, March 16, 2020 11:09 PM<br>
<b>To:</b> gdal-dev@lists.osgeo.org<br>
<b>Subject:</b> [gdal-dev] Slope obtained with GDAL has weird lines</span></p>
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<p class="x_MsoNormal">Hi!</p>
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<p class="x_MsoNormal">This may be a very basic question, but I can't find the answer anyware.</p>
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<p>I am working with this DEM obtained using <a href="https://pypi.org/project/terraincache/">
terraincache</a>, it uses mapzen data. The SRC is WGS 84.</p>
<p><a href="https://i.stack.imgur.com/Sp4Sg.jpg"><span style="text-decoration:none"><img border="0" width="822" height="539" id="x__x0000_i1026" src="https://i.stack.imgur.com/Sp4Sg.jpg" alt="DEM File in QGIS" style="width:8.5625in; height:5.6166in"></span></a></p>
<p>I ran this GDAL command to get the slope:</p>
<p><code><span style="font-size:10.0pt">gdaldem slope -s 111120 dem.tif slope.tif</span></code> (I had to do scaling because of the SRC)</p>
<p>The resulting image looks fine but it's not smooth, it has weird lines over it, like contour lines.</p>
<p><a href="https://i.stack.imgur.com/POcHR.jpg"><span style="text-decoration:none"><img border="0" width="821" height="540" id="x__x0000_i1025" src="https://i.stack.imgur.com/POcHR.jpg" alt="Slope file on QGIS" style="width:8.5541in; height:5.625in"></span></a></p>
<p>You can download the DEM file here: <a href="https://gofile.io/?c=Du7WnM">https://gofile.io/?c=Du7WnM</a></p>
<p>Does anyone have an idea of how to solve it? </p>
<p>Thanks in advance for your help,</p>
<p>Danilo</p>
<p>P.S. I made this question here too: <a href="https://gis.stackexchange.com/questions/353983/slope-obtained-with-gdal-has-weird-lines">
https://gis.stackexchange.com/questions/353983/slope-obtained-with-gdal-has-weird-lines</a></p>
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