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I create a slope map from a DEM downloaded with SRTM Downloader in QGIS 3.32.0-Lima. I searched in the internet trying to find the way to interpret the values of the left column in percent. For example:

  • When I generate the slope map (Raster - Analysis - Slope) I select the following

Creating the slope map

  • In my country I have 9 types to classify the slopes, so I left 9 classes.
  • In the "Legend setup" I ticketed the "Use continuous legend" and in the "Format number" I left 2 decimals and I specified "The values are percents".

Slope steps

But I still have this crazy values

Up than 100% in the red color

How can I change the scale values to show from 0% to 100%?

Here you can see a view from the top to the bottom of the site

Top view of the site

And here another satellite view from South to North of the site.

South to North view

I need to add this values in the slope options

Slope table

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    Values are crazy because ratio 1:1 between the horizontal and vertical components does not make sense with that source data. The horizontal unit is degrees and the vertical unit is meters. A rough estimate of 111120 is often used gdal.org/programs/gdaldem.html#description but a better option is to re-project the DEM into some projected coordinate system that is good for your area of interest.
    – user30184
    Commented Dec 27, 2023 at 21:41
  • Consider putting this (or a slightly expanded version) as an answer? Commented Jan 15 at 21:09

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The slopes are "crazy" because your horizontal units are degrees and your vertical units are metres.

Consider reprojecting your DEM into a projected co-ordinate system relevant to your area of interest. That will give you horizontal units in metres.

Alternatively you can adjust your slopes using a conversion from degrees to metres. The issue with this is that the conversion factor can vary considerably from the equator to the poles. As per https://gdal.org/programs/gdaldem.html#description, a conversion factor of 111120 can be used near the equator, but the option of reprojecting is better.

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