[GIS] Using QGIS 3.0 raster calculator

qgisqgis-3raster-calculator

I have a lesson/class assignment that teaches the use of format and export environmental raster data, but the issue is the lesson is outdated from the new QGIS version.

Lesson states:

Multiply the values in two layers together, or (as here)
divide all values in a layer by 10. In QGIS, go to Raster > Raster Calculator. Now, select the layer you want to manipulate (bio_1) by double-clicking it, select the name and directory of the Output layer, and then type the expression to divide by 10 – use the buttons, not your keyboard to type / 10:

Then click OK!

The QGIS 3.0 program doesn't allow you to create a new output file (or to my knowledge).

What should I do?

Best Answer

Those instructions seem perfectly valid for the Raster Calculator in QGIS 3.0.

Output file settings are in the upper-right corner of the Raster Calculator window. Enter the filepath and name for your new raster where it says "Output layer."

Related Solutions

QGIS Python Script – Writing a Python Processing Script with QGIS 3.0

With the transition from QGIS2.x to QGIS3.x the whole processing framework has been reworked and large parts of it run now as C++ classes that you can interact with using Python. Unfortunately the simple parameter syntax for data/dataset IO are no longer valid. The new parameter structure is much more orientated after the builtin (Python-) Processing algorithms that you find preinstalled in the toolbox.

As I see, you already followed the description of the new algorithm structure by @underdark. But in order to adjust this structure for your requirements (raster layers, double input, etc.) you have to change the code at multiple locations in the script. I have coded a rough example with a short explanation for you (just an algorithm skeleton based on @underdarks example):

from qgis.PyQt.QtCore import QCoreApplication, QVariant
from qgis.core import (QgsProcessing, QgsProcessingAlgorithm, 
QgsProcessingParameterRasterLayer,QgsProcessingParameterNumber, 
QgsProcessingParameterRasterDestination)

class RasterAlg(QgsProcessingAlgorithm):
    INPUT_RASTER_A = 'INPUT_RASTER_A'
    INPUT_RASTER_B = 'INPUT_RASTER_B'
    INPUT_DOUBLE = 'INPUT_DOUBLE'
    OUTPUT_RASTER_A = 'OUTPUT_RASTER_A'
    OUTPUT_RASTER_B = 'OUTPUT_RASTER_B'

    def __init__(self):
        super().__init__()

    def name(self):
        return "RasterAlg"

    def tr(self, text):
        return QCoreApplication.translate("RasterAlg", text)

    def displayName(self):
        return self.tr("RasterAlg script")

    def group(self):
        return self.tr("RasterAlgs")

    def groupId(self):
        return "RasterAlgs"

    def shortHelpString(self):
        return self.tr("RasterAlg script without logic")

    def helpUrl(self):
        return "https://qgis.org"

    def createInstance(self):
        return type(self)()

    def initAlgorithm(self, config=None):
        self.addParameter(QgsProcessingParameterRasterLayer(
            self.INPUT_RASTER_A,
            self.tr("Input Raster A"), None, False))
        self.addParameter(QgsProcessingParameterRasterLayer(
            self.INPUT_RASTER_B,
            self.tr("Input Raster B"), None, False))
        self.addParameter(QgsProcessingParameterNumber(
            self.INPUT_DOUBLE, 
            self.tr("Input Double"), 
            QgsProcessingParameterNumber.Double,
            QVariant(1.0)))
        self.addParameter(QgsProcessingParameterRasterDestination(
            self.OUTPUT_RASTER_A,
            self.tr("Output Raster A"),
            None, False))
        self.addParameter(QgsProcessingParameterRasterDestination(
            self.OUTPUT_RASTER_B,
            self.tr("Output Raster B"),
            None, False))

    def processAlgorithm(self, parameters, context, feedback):
        raster_a = self.parameterAsRasterLayer(parameters, self.INPUT_RASTER_A, context)
        raster_b = self.parameterAsRasterLayer(parameters, self.INPUT_RASTER_B, context)
        double_val = self.parameterAsDouble(parameters, self.INPUT_DOUBLE,context)
        output_path_raster_a = self.parameterAsOutputLayer(parameters, self.OUTPUT_RASTER_A, context)
        output_path_raster_b = self.parameterAsOutputLayer(parameters, self.OUTPUT_RASTER_B, context)

        #DO SOME CALCULATION

        results = {}
        results[self.OUTPUT_RASTER_A] = output_path_raster_a
        results[self.OUTPUT_RASTER_B] = output_path_raster_b
        return results

Which steps are done?

Import all necessary classes.
Define the algorithm as a class inheriting from QgsProcessingAlgorithm.
First you have to declare the names of the input and output parameters as string variables (parameter names) of the algorithm class (ie. INPUT_RASTER_A = 'INPUT_RASTER_A') in order to reference your algorithm with the parameters provided by the processing framework.
Add the methods that wire your algorithm to the processing toolbox gui and provide helpstrings, etc.
Then you add the parameters of the processing framework. Those are defined as child classes of QgsProcessingParameterType - in the case of your algorithm: QgsProcessingParameterRasterLayer, QgsProcessingParameterNumber, and so on. You can consult the API entries (ie. QgsProcessingParameterRasterLayer) in order to pass the right arguments and construct the parameter objects.
Pass the parameters alongside context and feedback objects to the processAlgorithm() method where you obtain the input datasets from the parameters at runtime (in this case QgsRasterLayer objects by using the parameterAsRasterLayer() method, etc.).
Do your computation.
Add the outputs to the results dictionary and return them as result of calling processAlgorithm().

I hope I could give you some insights on how to design your python algorithms in QGIS3. Whenever you are stuck, it is always helpful to look at how existing algorithms of the processing framework handle the parameters. You can have a look at them here.

[GIS] Create raster out of overlapping buffers within the same shapefile in QGIS 3.0

I was able to achieve this in QGIS 3.2 using the following work flow. There's a few steps involved and several processing algorithms all from the processing toolbox. For the layers produced in the intermediate steps below, I would recommend keeping them as temporary layers rather saving to file.

Start with your layer of overlapping buffers.
On this layer, run the Union algorithm from the processing toolbox. This will produce an output layer which contains multiple duplicate features.
On this layer, run "Delete duplicate geometries". Now you should have a polygon layer with one feature for each of the overlapping sections of the original buffer layer. For some reason I ended up with invalid geometries at this point and had to run GRASS v.clean, so it may be advisable to do this also as an additional step.
Create centroids from your cleaned union layer.
Now run the algorithm "Join attributes by location (summary)". Use the centroids as the input layer and your original overlapping buffer layer as the join layer. Use "intersects" as the geometric predicate and for summaries to calculate select "count". This should produce a new point layer called "join layer" with a new field "value_count" which contains the number of overlapping segments from your buffer layer.
We now need to join that attribute back to the union layer. For this just use the algorithm "Join attributes by location". For the input layer, use your cleaned union layer and the newly created "joined layer" as the join layer. For fields to add select "value count" and for join type select: "take attributes of first located feature (one to one).
This should output a polygon layer also called "joined layer" which also has a field "value count" containing the number of overlapping buffer segments. I would recommend saving and renaming this layer. Here you can see this layer labelled and categorised with the "value count" field.

Finally, rasterize this vector layer using the algorithm "Rasterize (vector to raster)" and use the "value count" field as the burn in value. Here is my resulting raster rendered in single band pseudocolour and coloured according to pixel value.

Best Answer

Related Solutions

QGIS Python Script – Writing a Python Processing Script with QGIS 3.0

[GIS] Create raster out of overlapping buffers within the same shapefile in QGIS 3.0

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