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

Tue Nov 18 08:04:38 PST 2014

Hi Carlos,

I reprojected to a different swiss projection system and the expression 
still does not work. I tested various projections, including UTM-32N. 
Very strange ...

There clearly seems to be a bug in the raster calculator. I will open a 
bug with high priority.

Thanks for testing and helping!

Andreas

Am 2014-11-18 15:36, schrieb Carlos Cerdán:
> Hi Andreas:
> 
> It is solved: you have to reproject your TEMP layer. I've tried with
> UTM-32N zone and raster calculator works fine.
> 
> Why raster calculator (and SAGA) doesn't work with your original CRS?
> Well, I don't know, perhaps devs group have the answer.
> 
> It was a good challenge.
> 
> Carlos
> 
> 2014-11-18 4:37 GMT-05:00 Andreas Neumann <a.neumann at carto.net>:
> 
>> 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
>> 
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