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Trying to set the power coefficient for the IDW interpolation in QGIS 3.10 in pyqgis.

Whatever number I set it has no effect on the interpolation output - always the same output. The default is 2 but I tried 10, 90, 0.5 and all the same result:

enter image description here

Here's the code:

## Get the selected features by ID first and create a clone layer with that selection

it = [f for f in layObj.getFeatures()]
ids = [5,6,7,8,9,10,12,21,24,26,28]  # example of ids only- in reality it's hundreds                                  
layObj.selectByIds(ids) 
clone = processing.run("native:saveselectedfeatures", {'INPUT': layObj, 'OUTPUT': memory:{}'.format(layObj.name())})['OUTPUT'] 

layer_data = QgsInterpolator.LayerData()
layer_data.source = clone
layer_data.zCoordInterpolation=False
layer_data.interpolationAttribute = 'PM10_Emissions_tpa'

# I've tried all of these
layer_data.distanceCoefficient = 10  # https://qgis.org/pyqgis/3.10/analysis/QgsIDWInterpolator.html?highlight=idw#
layer_data.setDistanceCoefficient = 10  
layer_data.coefficient = 10  

layer_data.mInputType = 1 # I've tried 0 and 1- same exact output
idw_interpolator = QgsIDWInterpolator([layer_data])

export_path = rasOut 
rect = clone.extent()
res = pixSizeP
ncols = int( ( rect.xMaximum() - rect.xMinimum() ) / res )
nrows = int( (rect.yMaximum() - rect.yMinimum() ) / res)
output = QgsGridFileWriter(idw_interpolator,export_path,rect,ncols,nrows)
output.writeFile()

Perhaps doing something stupid but can't seem to find a good example that works for me- seen other posts but didn't solve it for me. Can anyone help?

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  • No results by using different 'Distance Coefficients' is because layer_data has not 'setDistanceCoefficient' method. Desired method is in idw_interpolator. Please, see my answer.
    – xunilk
    May 7, 2020 at 20:48

1 Answer 1

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No results by using different 'Distance Coefficients' is because layer_data has not 'setDistanceCoefficient' method. Desired method is in idw_interpolator.

So, by using same code in your previous question about QgsIDWInterpolator, I had to add only one code line:

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idw_interpolator.setDistanceCoefficient(10)
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in following code:

layer = iface.activeLayer() 
layer_data = QgsInterpolator.LayerData()
layer_data.source = layer
layer_data.zCoordInterpolation = False
layer_data.interpolationAttribute = 1  # the 2nd field in the layer contains the data to interpolate
layer_data.mInputType = 1


idw_interpolator = QgsIDWInterpolator([layer_data])
idw_interpolator.setDistanceCoefficient(10) #added line

rasOut = r"/home/zeito/pyqgis_data/BiomassNOx_Emissions_tpa2.tif"

export_path = rasOut # e.g. r"C:\Temp\AnalysisResults\BiomassNOx_Emissions_tpa.tif" 

rect = layer.extent()
res = 10
ncols = int( ( rect.xMaximum() - rect.xMinimum() ) / res )
nrows = int( (rect.yMaximum() - rect.yMinimum() ) / res)

output = QgsGridFileWriter(idw_interpolator,export_path,rect,ncols,nrows)
output.writeFile()

for producing visible changes in interpolated raster layer.

After running above code with same point layer in your previous question, I got result observed in following image (different of my answer in your previous question):

enter image description here

So, you should modify your code as follows:

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idw_interpolator = QgsIDWInterpolator([layer_data])
idw_interpolator.setDistanceCoefficient(10)
.
.
.

for obtaining an equivalent result.

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  • Works, Grand! Thanks again. I just can't seem to really understand the API without a full example somewhere
    – GIS_py
    May 8, 2020 at 14:36

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