Querying the value of a raster at a specified point

A common remote sensing operation is to get the raster data value at a specified coordinate. In this recipe, we'll query the data value in the center of the image. It so happens that the raster layer will calculate the center coordinate of its extent for you.

Getting ready

As with many recipes in this chapter, we will again use the SatImage raster, which is available at https://geospatialpython.googlecode.com/files/SatImage.zip.

Place this raster in your /qgis_data/rasters directory.

How to do it...

We will load the layer, get the center coordinate, and then query the value. To do this, we need to perform the following steps:

1. First, load and validate the layer:

   rasterLyr = QgsRasterLayer("/qgis_data/rasters/satimage.tif", "Sat Image")
   rasterLyr.isValid()
   

2. Next, get the layer's center point from its QgsRectangle extent object, which will return a tuple with the x and y values:

   c = rasterLyr.extent().center()
   

3. Now, using the layer's data provider,...