[Qgis-user] [pyqgis] create new raster

[Ujaval Gandhi]

Instead of manually iterating over the raster, you can use the 'Raster
Calculator' Algorithm to set the pixel values which will be much easier and will
resolve your issues. Test it with the processing toolbox and then all via PyQGIS
with the required parameters.


I tested and the following expression works. Replace N27E086 with the name of
your layer



"N27E086 at 1"*("N27E086 at 1"<1500 or "N27E086 at 1">2000) + -9999*("N27E086 at 1">=1500
and "N27E086 at 1"<=2000)





---
Ujaval Gandhi
Spatial Thoughts
www.spatialthoughts.com
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[data:image/gif;base64,R0lGODlhAQABAIAAAP///wAAACwAAAAAAQABAAACAkQBADs=3D]
On Tue, May 9, 2023 at 11:58 AM andrea antonello via QGIS-User
<qgis-user at lists.osgeo.org> wrote:

> Hello,
> 
> I am trying to find out the best workflow to create a new raster.
> As an example I take an existing elevation model raster and loop over it to
> set values between 1500 and 2000 to novalue and write the result to a new
> raster.
> 
> 
> This is the only way I found to do so:
> 
> 
> # create new raster with novalues between 1500 and 2000
> dataType = dtmLayer.dataProvider().dataType(1)
> crs = QgsCoordinateReferenceSystem('EPSG:3003')
> params = {
> 'EXTENT': dtmLayer.extent(),
> 'TARGET_CRS': crs,
> 'PIXEL_SIZE': dtmLayer.rasterUnitsPerPixelX(),
> 'NUMBER': -9999.0,
> # 'OUTPUT_TYPE': dataType,
> 'OUTPUT': QgsProcessing.TEMPORARY_OUTPUT
> }
> newRaster = processing.run('qgis:createconstantrasterlayer', params)['OUTPUT']
> newRasterLayer = QgsRasterLayer(newRaster, 'temp', 'gdal')
> newRasterProvider = newRasterLayer.dataProvider()
> 
> block = QgsRasterBlock(dataType, cols, rows)
> 
> 
> for row in range(rows):
> for col in range(cols):
> point = dtmLayer.dataProvider().transformCoordinates(QgsPoint(col, row),
> transformType)
> value, res = dtmLayer.dataProvider().sample(QgsPointXY(point.x(), point.y()),
> 1)
> 
> if res and value != -9999.0:
> if value < 1000 or value > 2000:
> block.setValue(row, col, value)
> 
> 
> newRasterProvider.setEditable(True)
> newRasterProvider.writeBlock(block, band=1)
> newRasterProvider.setEditable(False)
> 
> 
> 
> 
> This code has two main issues:
> 1. if I uncomment the line containing OUTPUT_TYPE, I am getting an error about
> the type passed. But I can't find the right type needed there, it should be
> the one taken from the original provider.
> 2. the resulting raster is scrambled as if there was a shift in the setting of
> the values. But the QgsRasterBlock seems to be built correctly (rows, cols)
> and the values set properly (col, row).
> 3. in the above example, the dtmLayer has an epsg 3033 crs and when loaded
> manually into QGIS, it is recognized. But when I read the layer's metadata crs
> with pyQGIS , it is not able to read it and tells me it is invalid. 
> 
> 
> Has anyone a hint about what I am doing wrong? 
> 
> 
> Thanks,
> Andrea
> 
> 
> 
> 
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