Working with raster data

Lavorare con i dati geospaziali in Python

Joris Van den Bossche

Open source software developer and teacher, GeoPandas maintainer

Grid of values illustration

Image source: QGIS documentation

Raster data

Chance of rain map

Raster data with multiple bands

The rasterio package

import rasterio

"Pythonic" bindings to GDAL
Reading and writing raster files
Processing tools (masking, reprojection, resampling, ..)

https://rasterio.readthedocs.io/en/latest/

Opening a raster file

import rasterio

src = rasterio.open("DEM_world.tif")

Metadata:

src.count

src.width, src.height

(4320, 2160)

Raster data = numpy array

array = src.read()

Standard numpy array:

array

array([[[-4290, -4290, -4290, ..., -4290, -4290, -4290],
        [-4278, -4278, -4278, ..., -4278, -4278, -4278],
        [-4269, -4269, -4269, ..., -4269, -4269, -4269],
        ...,
        [ 2804,  2804,  2804, ...,  2804,  2804,  2804],
        [ 2804,  2804,  2804, ...,  2804,  2804,  2804],
        [ 2804,  2804,  2804, ...,  2804,  2804,  2804]]], dtype=int16)

Plotting a raster dataset

Using the rasterio.plot.show() method:

import rasterio.plot

rasterio.plot.show(src, cmap='terrain')

Extracting information based on vector data

rasterstats: Summary statistics of geospatial raster datasets based on vector geometries (https://github.com/perrygeo/python-rasterstats)

Extract raster values with rasterstats

For point vectors:

rasterstats.point_query(geometries, "path/to/raster", 
                        interpolation='nearest'|'bilinear')

For polygon vectors:

rasterstats.zonal_stats(geometries, "path/to/raster",
                        stats=['min', 'mean', 'max'])

Extract raster values with rasterstats

result = rasterstats.zonal_stats(countries.geometry, "DEM_gworld.tif", 
                                 stats=['mean'])

countries['mean_elevation'] = pd.DataFrame(result)

countries.sort_values('mean_elevation', ascending=False).head()

            name    continent                     geometry  mean_elevation
157   Tajikistan         Asia  POLYGON ((74.98 37.41, ...      3103.231105
85    Kyrgyzstan         Asia  POLYGON ((80.25 42.34, ...      2867.717142
24        Bhutan         Asia  POLYGON ((91.69 27.77, ...      2573.559846
119        Nepal         Asia  POLYGON ((81.11 30.18, ...      2408.907816
6     Antarctica   Antarctica  (POLYGON ((-59.57 -80.04...     2374.075028
..           ...          ...                          ...             ...

Let's practice!

Lavorare con i dati geospaziali in Python