Python implementation of zonal statistics function. Optimized for dense polygon layers, uses numpy, GDAL and OGR to rival the speed of starspan.

zonal_stats.py

"""
Zonal Statistics
Vector-Raster Analysis

Copyright 2013 Matthew Perry

Usage:
  zonal_stats.py VECTOR RASTER
  zonal_stats.py -h | --help
  zonal_stats.py --version

Options:
  -h --help     Show this screen.
  --version     Show version.
"""
from osgeo import gdal, ogr
from osgeo.gdalconst import *
import numpy as np
import sys
gdal.PushErrorHandler('CPLQuietErrorHandler')


def bbox_to_pixel_offsets(gt, bbox):
    originX = gt[0]
    originY = gt[3]
    pixel_width = gt[1]
    pixel_height = gt[5]
    x1 = int((bbox[0] - originX) / pixel_width)
    x2 = int((bbox[1] - originX) / pixel_width) + 1

    y1 = int((bbox[3] - originY) / pixel_height)
    y2 = int((bbox[2] - originY) / pixel_height) + 1

    xsize = x2 - x1
    ysize = y2 - y1
    return (x1, y1, xsize, ysize)


def zonal_stats(vector_path, raster_path, nodata_value=None, global_src_extent=False):
    rds = gdal.Open(raster_path, GA_ReadOnly)
    assert(rds)
    rb = rds.GetRasterBand(1)
    rgt = rds.GetGeoTransform()

    if nodata_value:
        nodata_value = float(nodata_value)
        rb.SetNoDataValue(nodata_value)

    vds = ogr.Open(vector_path, GA_ReadOnly)  # TODO maybe open update if we want to write stats
    assert(vds)
    vlyr = vds.GetLayer(0)

    # create an in-memory numpy array of the source raster data
    # covering the whole extent of the vector layer
    if global_src_extent:
        # use global source extent
        # useful only when disk IO or raster scanning inefficiencies are your limiting factor
        # advantage: reads raster data in one pass
        # disadvantage: large vector extents may have big memory requirements
        src_offset = bbox_to_pixel_offsets(rgt, vlyr.GetExtent())
        src_array = rb.ReadAsArray(*src_offset)

        # calculate new geotransform of the layer subset
        new_gt = (
            (rgt[0] + (src_offset[0] * rgt[1])),
            rgt[1],
            0.0,
            (rgt[3] + (src_offset[1] * rgt[5])),
            0.0,
            rgt[5]
        )

    mem_drv = ogr.GetDriverByName('Memory')
    driver = gdal.GetDriverByName('MEM')

    # Loop through vectors
    stats = []
    feat = vlyr.GetNextFeature()
    while feat is not None:

        if not global_src_extent:
            # use local source extent
            # fastest option when you have fast disks and well indexed raster (ie tiled Geotiff)
            # advantage: each feature uses the smallest raster chunk
            # disadvantage: lots of reads on the source raster
            src_offset = bbox_to_pixel_offsets(rgt, feat.geometry().GetEnvelope())
            src_array = rb.ReadAsArray(*src_offset)

            # calculate new geotransform of the feature subset
            new_gt = (
                (rgt[0] + (src_offset[0] * rgt[1])),
                rgt[1],
                0.0,
                (rgt[3] + (src_offset[1] * rgt[5])),
                0.0,
                rgt[5]
            )

        # Create a temporary vector layer in memory
        mem_ds = mem_drv.CreateDataSource('out')
        mem_layer = mem_ds.CreateLayer('poly', None, ogr.wkbPolygon)
        mem_layer.CreateFeature(feat.Clone())

        # Rasterize it
        rvds = driver.Create('', src_offset[2], src_offset[3], 1, gdal.GDT_Byte)
        rvds.SetGeoTransform(new_gt)
        gdal.RasterizeLayer(rvds, [1], mem_layer, burn_values=[1])
        rv_array = rvds.ReadAsArray()

        # Mask the source data array with our current feature
        # we take the logical_not to flip 0<->1 to get the correct mask effect
        # we also mask out nodata values explictly
        masked = np.ma.MaskedArray(
            src_array,
            mask=np.logical_or(
                src_array == nodata_value,
                np.logical_not(rv_array)
            )
        )

        feature_stats = {
            'min': float(masked.min()),
            'mean': float(masked.mean()),
            'max': float(masked.max()),
            'std': float(masked.std()),
            'sum': float(masked.sum()),
            'count': int(masked.count()),
            'fid': int(feat.GetFID())}

        stats.append(feature_stats)

        rvds = None
        mem_ds = None
        feat = vlyr.GetNextFeature()

    vds = None
    rds = None
    return stats


if __name__ == "__main__":
    opts = {'VECTOR': sys.argv[1], 'RASTER': sys.argv[2]}
    stats = zonal_stats(opts['VECTOR'], opts['RASTER'])

    try:
        from pandas import DataFrame
        print DataFrame(stats)
    except ImportError:
        import json
        print json.dumps(stats, indent=2)

zonal_stats_example.txt

$ time python zonal_stats.py test.shp terrain/slope.tif
    count  fid  max       mean  min        std    sum
0     203    0   96  65.876847    3  17.968489  13373
1     130    1   90  60.100000    3  16.728994   7813
2    1341    2  102  53.211037    2  17.901655  71356
3     130    3   90  60.100000    3  16.728994   7813
4     132    4   64  15.962121    1  15.360519   2107
5     132    5   53  31.515152   17   7.970100   4160
6     131    6   42   9.893130    0   8.168317   1296
7     132    7   64  28.712121    2  14.853594   3790
8     133    8   54  35.548872   11   8.878856   4728
9     131    9   82  52.297710    4  17.349877   6851
10    131   10   11   3.030534    0   1.781752    397
11    134   11   57  10.156716    1  11.960042   1361
12    133   12   45  19.000000    0  13.727750   2527
13    132   13   64  26.507576    1  18.848075   3499
14    132   14   94  52.787879    1  22.297585   6968
15    131   15   84  19.450382    1  15.992944   2548
16    132   16   52  11.583333    0  11.538501   1529
17    132   17  108  53.515152    6  18.198603   7064
18    341   18   76  39.117302    9  11.540482  13339
19    337   19   57  19.988131    4   9.593512   6736
20    336   20   78  48.636905   11  13.357014  16342
21    338   21    3   0.855030    0   0.527067    289
22    337   22   34   5.347181    0   7.069888   1802
23    341   23    0   0.000000    0   0.000000      0
24    341   24   42  16.612903    0   9.041271   5665
25    337   25  128  78.848665    5  18.689028  26572
26    341   26   29   7.973607    1   5.341357   2719
27    339   27   78  35.616519    5  14.455317  12074
28    341   28   65  20.199413    0  16.636394   6888
29    340   29   84  35.855882    1  17.022989  12191
30    338   30   96  61.440828    2  16.703587  20767
31    340   31  101  57.832353    8  18.161971  19663

real    0m1.311s
user    0m0.372s
sys     0m0.752s


#### Starspan equivalent
$ time starspan --vector test.shp --out-prefix testout --out-type table \
--summary-suffix _stats.csv --raster terrain/slope.tif \
--stats avg mode median min max sum stdev nulls && \
cat testout_stats.csv
  1: Extracting from /usr/local/apps/land_owner_tools/lot/fixtures/downloads/terrain/slope.tif
Summary:
  Intersecting features: 32
      Polygons: 32

  Processed pixels: 8379

real    0m1.440s
user    0m0.944s
sys     0m0.296s

Hi, it would possible to add something like:

try:
do stats
except Exception as e:
print(e)
continue

I am trying to ruin this with several thosusands of lines, but at sometimes it seems that some of them have geomtri erros, so the whole process stop. It would be nice to be able to get the valid values instead of lose all of them for some invalids.

Maybe it could be in the start of the while? I am gonna try it