Zonal Statistics#

Table of Contents#

  1. What is Processor ZonalStatistics

  2. Processor ZonalStatistics Example

    1. Setup BDT

    2. Input Raster

    3. Input Data

    4. Running Processor ZonalStatistics

Part 1: What is Processor ZonalStatistics#

ZonalStatistics is a processor that calculates statistics on the values of the raster cells a given polygon overlaps.

ZonalStatistics Overview

Part 2: Processor ZonalStatistics Example#

Setup BDT#

[1]:

import pyspark
import bdt
bdt.auth("../bdt.lic")
from bdt import functions as F
from bdt import processors as P

BDT has been successfully authorized!

            Welcome to
             ___    _                ___         __             ______             __   __     _   __
            / _ )  (_)  ___ _       / _ \ ___ _ / /_ ___ _     /_  __/ ___  ___   / /  / /__  (_) / /_
           / _  | / /  / _ `/      / // // _ `// __// _ `/      / /   / _ \/ _ \ / /  /  '_/ / / / __/
          /____/ /_/   \_, /      /____/ \_,_/ \__/ \_,_/      /_/    \___/\___//_/  /_/\_\ /_/  \__/
                      /___/

BDT python version: v3.4.0-develop-12-ge79ea377
BDT jar version: v3.4.0-develop-12-ge79ea377

Input Raster#

  • Elevation data from the US Geologic Survey and the National Geospatial-Intelligence Agency

  • Available on ArcGIS Living Atlas

  • Cut down to the LA area as a sample

[2]:

raster_file = "World_Elevation_GMTE_LA.tif"

Drawing

Input Data#

Create a sample polygon that overlaps some raster cells from the World_Elevation_GMTE_LA.tif.

Assign it a unique ID, PARCEL_ID in this example.

[3]:

polygon = """
        POLYGON((
        -117.6878845 34.1968949,
        -117.6756497 34.1968949,
        -117.6754958 34.1939036,
        -117.6877306 34.1942218
        ))
        """

# The raster uses spatial reference 3857 so the polygon must be projected to it.
polygon_df = spark.sql(f"""
                        SELECT
                            ST_Project(ST_FromText('{polygon}'), 4326, 3857) AS SHAPE,
                            '1' AS PARCEL_ID
                        """).withMeta("POLYGON", 3857)

Drawing

Running Processor ZonalStatistics#

Calculate elevation statistics for the polygon.

Parameters:#

  • Polygon DataFrame

  • Path to raster TIF file

  • Unique ID field for the polygon DataFrame

  • mode: Set to “area” (any overlap) or “cell” (centroid overlap)

    • “area” mode considers all raster cells that overlap the polygon. When using this mode, also use the deno parameter.

    • “cell” mode only considers raster cells whose centroid overlaps the polygon.

[4]:

output_df = P.zonal_statistics_table(polygon_df,
                                     raster_file,
                                     miid_field="PARCEL_ID",
                                     mode="cell")

Output statistics:#

  • Count

  • Min

  • Max

  • Area of the overlapping region with the polygon

  • Mean

  • Standard deviation

  • Sum

  • 50th percentile

  • 90th percentile

[6]:

output_df.show(truncate=False)

+---------+-----+-----+------+--------+------+------------------+-------+------+------+
|PARCEL_ID|count|min  |max   |area    |mean  |std               |sum    |pct50 |pct90 |
+---------+-----+-----+------+--------+------+------------------+-------+------+------+
|1        |12   |872.0|1363.0|750000.0|1109.0|178.45074016844723|13308.0|1143.0|1363.0|
+---------+-----+-----+------+--------+------+------------------+-------+------+------+

The resulting dataframe can be joined back to the original dataframe to enrich the polygon geometries with the raster cell statistics.

[7]:

output_df.join(polygon_df, "PARCEL_ID").show()

+---------+-----+-----+------+--------+------+------------------+-------+------+------+--------------------+
|PARCEL_ID|count|  min|   max|    area|  mean|               std|    sum| pct50| pct90|               SHAPE|
+---------+-----+-----+------+--------+------+------------------+-------+------+------+--------------------+
|        1|   12|872.0|1363.0|750000.0|1109.0|178.45074016844723|13308.0|1143.0|1363.0|{[01 06 00 00 00 ...|
+---------+-----+-----+------+--------+------+------------------+-------+------+------+--------------------+

Drawing