A color ramp allows you to render a raster using just a few colors to represent different ranges of cell values that have similar meaning, in order to group them. The approach that will be used in this recipe is the most common way to render elevation data.
You can download a sample DEM from https://geospatialpython.googlecode.com/files/dem.zip, which you can unzip in a directory named rasters in your qgis_data directory.
In the following steps, we will set up objects to color a raster, create a list establishing the color ramp ranges, apply the ramp to the layer renderer, and finally add the layer to the map. To do this, we need to perform the following steps:
- First, we import the
QtGuilibrary for color objects in the QGIS Python Console:from PyQt4 import QtGui - Next, we load the raster layer, as follows:
lyr = QgsRasterLayer("/Users/joellawhead/qgis_data/rasters/dem.asc", "DEM") - Now, we create a generic raster shader object:
s = QgsRasterShader() - Then, we instantiate the specialized ramp shader object:
c = QgsColorRampShader() - We must name a type for the ramp shader. In this case, we use an
INTERPOLATEDshader:c.setColorRampType(QgsColorRampShader.INTERPOLATED) - Now, we'll create a list of our color ramp definitions:
i = [] - Then, we populate the list with color ramp values that correspond to elevation value ranges:
i.append(QgsColorRampShader.ColorRampItem(400, QtGui.QColor('#d7191c'), '400')) i.append(QgsColorRampShader.ColorRampItem(900, QtGui.QColor('#fdae61'), '900')) i.append(QgsColorRampShader.ColorRampItem(1500, QtGui.QColor('#ffffbf'), '1500')) i.append(QgsColorRampShader.ColorRampItem(2000, QtGui.QColor('#abdda4'), '2000')) i.append(QgsColorRampShader.ColorRampItem(2500, QtGui.QColor('#2b83ba'), '2500'))
- Now we assign the color ramp to our shader:
c.setColorRampItemList(i) - Now, we tell the generic raster shader to use the color ramp:
s.setRasterShaderFunction(c) - Next, we create a raster renderer object with the shader:
ps = QgsSingleBandPseudoColorRenderer(lyr.dataProvider(), 1, s) - We assign the renderer to the raster layer:
lyr.setRenderer(ps) - Finally, we add the layer to the canvas in order to view it:
QgsMapLayerRegistry.instance().addMapLayer(lyr)
While it takes a stack of four objects to create a color ramp, this recipe demonstrates how flexible the PyQGIS API is. Typically, the more objects it takes to accomplish an operation in QGIS, the richer the API is, giving you the flexibility to make complex maps.
Notice that in each ColorRampItem object, you specify a starting elevation value, the color, and a label as the string. The range for the color ramp ends at any value less than the following item. So, in this case, the first color will be assigned to the cells with a value between 400 and 899. The following screenshot shows the applied color ramp.
