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I've got a DEM raster that I've analysed for slopes. I now need to generalise it, and it sounds like using gdal_sieve is the best way.

However, whenever I try to run it, the resulting raster is always entirely black. It seems to set each pixel to the NoValue value (in my case, -9999). My data is in floats, and goes from 0 to about 34.3 or something like that (apart from NoValues obviously).

I have used Raster>Conversion>Translate to get it to an integer, and also the Raster>Calculator to multiply the values by 100, but the results are still 0.

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  • Which was the command that you use?
    – xunilk
    Mar 17, 2017 at 9:40
  • I didn't use QGIS to get resulting raster. I directly use bash console in GNU/Linux Debian.
    – xunilk
    Mar 17, 2017 at 11:24
  • I thought I had reproduced the error, but actually it was just a representation problem skewed by -9999 (confirmed by @xunilk, see 4th image). This is probably problem with latest QGIS. However, I am now wondering if it is also your case, as you have 0 with -9999. In both tests (@xunilk and myself ) original values were retained. Can you try it again?
    – Kazuhito
    Mar 19, 2017 at 1:31

1 Answer 1

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I don't know which was the command that you use but, in my case, I don't have any problem with mine. With next slope raster (Data Type: Float 32):

enter image description here

I ran next command with default values (in this case equivalent to -4):

gdal_sieve.py  utah_demUTM2_slope.tif new_output.tif

and I got:

enter image description here

It can be observed that new raster values are integers and between 1-100.

A detailed analysis by using Value Tool plugin of QGIS (see next image), it allows to observe that raster was well generalized.

enter image description here

Editing Note:

Based at Kazuhito commentary, I tried the command out by using GRASS console of QGIS 2.18.3. I got "apparently" no raster:

enter image description here

However, it's only a visualization problem. With pseudocolor and 7 new classes of next image, I have:

enter image description here

When I test it in my Windows 7 System (GDAL/OGR version'2.1.2'), I got again a visualization "problem". However, changing the minimum value of -9999 to 0, I have:

enter image description here

There is not a bug.

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  • GDAL/OGR version of my windows QGIS - both 2.18.4 and 2.14.12 are 2.1.3. Older one (2.14.12) is compiled against GDAL/OGR 2.1.2. Can I ask your GDAL version?
    – Kazuhito
    Mar 17, 2017 at 11:37
  • Please, see my Editing Note. It could help. By the way, my GDAL/OGR version is '1.10.1'.
    – xunilk
    Mar 17, 2017 at 11:53
  • Woot, thanks! Then by (1) Going to Transparency tab, set nodata (e.g. -9999), then (2) going back to Style tab, reload min/max values by [Load] button, it seems to work. (At least for me.) Thanks again!
    – Kazuhito
    Mar 17, 2017 at 12:04
  • I tested it in my Windows 7 system. I got good results. It isn't a bug.
    – xunilk
    Mar 17, 2017 at 12:27

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