Bug report #15202
Raster shows nodata for values near nodata
| Status: | Closed | ||
|---|---|---|---|
| Priority: | Normal | ||
| Assignee: | - | ||
| Category: | Rasters | ||
| Affected QGIS version: | 2.14.3 | Regression?: | No |
| Operating System: | Easy fix?: | No | |
| Pull Request or Patch supplied: | No | Resolution: | end of life |
| Crashes QGIS or corrupts data: | No | Copied to github as #: | 23140 |
Description
QGIS rounds raster values that are near NoData to be presented as NoData in the GUI. I regard this behaviour to be a bug since it is a misleading representation from other software (e.g. ArcGIS), and it can occur as a result of using common open source tools like gdal_calc.py.
For example, the difference between two Float32 rasters A.tif and B.tif with similar values, and a few common pixels with identical values:
gdal_calc.py -A A.tif -B B.tif --outfile=C.tif --calc="A-B"For this example with gdal_calc.py, note that until recently [1] the default NoData value for Float32 was 1.175494351E-38 [2], which is very close (but not equal) to 0.0.
The result C.tif (attached) will show nodata in QGIS for raster locations with values of 0.0 (top left corner). This same raster can be viewed in ArcGIS without any misrepresentation. (Note that the example raster has one pixel near the centre set with NoData).
[1] https://github.com/OSGeo/gdal/commit/c8d725c6f8ef70f217941074e2d2ca737a70a6e9#diff-e2fab2380f03654cf57cff4f7aa94776
[2] https://github.com/OSGeo/gdal/blob/4654aa64c648ab7918df28e47abbb500238cd2a6/gdal/swig/python/scripts/gdal_calc.py#L60
History
#1
Updated by Mike Taves almost 8 years ago
#2
Updated by Mike Taves almost 8 years ago
- File QGIS_2016-07-04_14-22-26.png added
- File ArcMap_2016-07-04_14-21-22.png added
I've attached screenshots from both applications. ArcGIS shows the raster correctly. The value of the upper left pixel should be 0.0, not nodata. I've identified a pixel with value -0.6. QGIS wants to show -0.599999904632568, which is different than what I get from ArcGIS and GDAL, i.e.:
from osgeo import gdal
ds = gdal.Open('C.tif')
ar = ds.ReadAsArray()
print(str(ar[1,2])) # -0.6 good
print(repr(ar[1,2])) # -0.5999999 best
print(repr(float(ar[1,2]))) # -0.5999999046325684 is misleading, since it isn't double precision
#3
Updated by Mike Taves almost 8 years ago
Correction, GDAL shows similar behaviour for the mask band, see https://trac.osgeo.org/gdal/ticket/6578
#4
Updated by Giovanni Manghi almost 8 years ago
- Category set to Rasters
#5
Updated by Giovanni Manghi about 7 years ago
- Regression? set to No
- Easy fix? set to No
#6
Updated by Giovanni Manghi about 5 years ago
- Resolution set to end of life
- Status changed from Open to Closed
End of life notice: QGIS 2.18 LTR
Source:
http://blog.qgis.org/2019/03/09/end-of-life-notice-qgis-2-18-ltr/
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