[Qgis-user] Raster: replace a single value with another value

[Andreas Neumann]

Hi Carlos,

Thank you for your help!

I tried with

(TEMP at 1=249)*255 + (TEMP at 1<249)*TEMP at 1 + (TEMP at 1>249)*TEMP at 1

My layer is called "temp". However, the resulting file contains all 0.

I tried in both QGIS 2.4 and 2.6.

Is there something wrong with the input file? It was generated from a 
PDF with ghostscript. Here is the metadata:

-----

GDAL provider
GTiff
GeoTIFF
Datensatzbeschreibung
//gis/gis/mapserverdata/uep/release_2014_11/temp.tif
TIFFTAG_DATETIME=2014:11:17 09:50:08
TIFFTAG_RESOLUTIONUNIT=2 (pixels/inch)
TIFFTAG_SOFTWARE=GPL Ghostscript 9.10
TIFFTAG_XRESOLUTION=600
TIFFTAG_YRESOLUTION=600
Kanal 1
STATISTICS_MAXIMUM=255
STATISTICS_MEAN=247.18714741724
STATISTICS_MINIMUM=0
STATISTICS_STDDEV=37.198102288306
Dimensionen
X: 71575 Y: 70867 Kanäle: 1
Leerwert
*Leerwert nicht gesetzt*
Datentyp
Byte - Acht Bit vorzeichenlose Ganzzahl
Pyramidenübersichten
Räumliches Bezugssystem des Layers
+proj=somerc +lat_0=46.95240555555556 +lon_0=7.439583333333333 +k_0=1 
+x_0=600000 +y_0=200000 +ellps=bessel +towgs84=674.4,15.1,405.3,0,0,0,0 
+units=m +no_defs
Layerausdehnung (in urspünglicher Projektion des Layers)
0.0000000000000000,-70867.0000000000000000 : 
71575.0000000000000000,0.0000000000000000
Kanal
Kanal 1
Kanal Nr
1
Keine Statistik
Noch keine Statistik gesammelt

------------

Thanks again if you have any idea what may be wrong with my file or the 
above listed raster calc statement.

Andreas



Am 2014-11-17 18:38, schrieb Carlos Cerdán:
> Hi Andreas:
> 
> I think QGIS raster calculator can make this job. Conditional syntax
> is a little bit... different.  The sentence that can do it is:
> 
> (TIFF=249)*255 + (TIFF<249)*TIFF + (TIFF>249)*TIFF
> 
> Where TIFF is your raster layer. The trick is: if the sentence in
> parentheses is true, it gets value 1, else zero; so we have to
> multiply by 255 to replace the 249 values. Next we have to add the
> other values.
> 
> Good luck,
> 
> Carlos Cerdán
> 
> PD. Please let me know if i'm wrong with this suggestion
> 
> 2014-11-17 12:02 GMT-05:00 Andreas Neumann <a.neumann at carto.net>:
> 
>> Hi,
>> 
>> I have a very simple task (at least I thought it is simple), but am
>> not able to do it properly.
>> 
>> I have a grayscale tiff value where I would like to replace the
>> value of 249 with 255 - all other values should stay as they are.
>> 
>> I tried with gdal "nearblack", but nothing changed - I started a
>> separate email-thread about that at the gdal list.
>> 
>> Then I tried with the QGIS raster calculator - but it does not
>> support conditional syntax.
>> 
>> I tried with Processing and GRASS - but it would stop at 8% - the
>> file is quite big.
>> 
>> Finally I came across SAGA "Reclassify with grid value". The
>> dialogue is very complicated and hard to understand. After a while
>> it would stop with an error message telling me that Saga may not be
>> properly configured - but it says that it is properly installed.
>> Quite confusing ...
>> 
>> I am trying on Win7 64bit.
>> 
>> If the file wouldn't be so big I would try this task in The Gimp -
>> where it is very simple to replace one pixel values with another -
>> using "Select by color" and "fill with a specific color".
>> 
>> Any idea how I could achieve my task with GDAL, QGIS, GRASS or
>> Saga?
>> 
>> Thanks,
>> Andreas
>> 
>> _______________________________________________
>> Qgis-user mailing list
>> Qgis-user at lists.osgeo.org
>> http://lists.osgeo.org/mailman/listinfo/qgis-user [1]
> 
> 
> 
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