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I have ASCII files for a country that map altitude. I'd like to extract information on slopes for specific cities in my country. My goal is to have at least a rough idea of physical constraints in those cities. I thought about using the variance of altitude within each city. I have them in vectorial format.

I have no idea of how to do that. I can use QGIS or R.

EDIT : I successfully created a slope map. My problem now is how to cut my slope map using the polygons for cities, and then compute an average slope value within each city

EDIT 2: I found the solution on the internet, with the Zonal statistics plug-in

JRPV
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  • "ASCII" is not a good description of a file format, its an encoding of characters from binary numbers. An ASCII-encoded file is one that generally presents as plain text. Some geospatial data formats use ASCII-coded plain text - for example the format known as "ESRI ASCII GRID". Do you have one of these? Or is it a comma-separated value file with lines like "2.3, 4.3, 897.34"? – Spacedman Aug 09 '19 at 18:32
  • I have never delt with this kind of data. When I tried opening it, it opened with QGIS. It looks like this when I open it with textedit :

    ncols 1000 nrows 1000 xllcorner 74962.500000000000 yllcorner 6750037.500000000000 cellsize 75.000000000000 NODATA_value -99999.00 -99999.00 -99999.00 -99999.00 -99999.00 -99999.00 -99999.00 -99999.00

     – JRPV Aug 10 '19 at 11:43
  • Now you've worked out how to open it you should create a new question for any other problems you have. Judging by your edits you've solved your problems and now this question is a bit of a hodgepodge of questions and answers, so probably not much use to anyone else so probably worth deleting. – Spacedman Aug 10 '19 at 16:58

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You would need to import the ASCII file as points, then interpolate a DEM. A quick google search will return many explanations. For example, See: this answer to Creating DEM from Point Data using QGIS?

Then create a slope raster from the DEM. Again, google it. i.e.: Creating slope map from DEM using QGIS?

PolyGeo
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Micha
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