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Zonal Statistics

Available with Spatial Analyst license.

Available with Image Analyst license.

Summary

Calculates statistics on values of a raster within the zones of another dataset.

Learn more about how Zonal Statistics works

Illustration

Zonal Statistics illustration
OutRas = ZonalStatistics(ZoneRas, "VALUE", ValRas, "MINIMUM")

Usage

  • A zone is defined as all areas in the input that have the same value. The areas do not have to be contiguous. Both raster and feature can be used for the zone input.

  • When the Cell size of the Input raster or feature zone data (in_zone_data in Python) and the Input value raster (in_value_raster in Python) is different, the output cell size will be the Maximum Of Inputs, and the Input value raster will be used as the Snap Raster internally. If the cell size is same, but the cells are not aligned, the Input value raster will be used as the snap raster internally. Either of these cases will trigger an internal resampling before the zonal operation is performed.

    When the zone and value inputs are both rasters of the same cell size and the cells are aligned, they will be used directly in the tool, and will not be resampled internally during the tool execution.

  • If the Input raster or feature zone data (in_zone_data in Python) is a raster, it must be an integer raster.

  • If the Input raster or feature zone data (in_zone_data in Python) is a feature, it will be converted to a raster internally, using the cell size and cell alignment from the Input value raster (in_value_raster in Python).

  • If the Input raster or feature zone data (in_zone_data in Python) is a point feature, it is possible to have more than one point contained within any particular cell of the value input raster. For such cells, the zone value is determined by the point with the lowest ObjectID field (for example, OID or FID).

  • If the Input raster or feature zone data (in_zone_data in Python) has overlapping polygons, the zonal analysis will not be performed for each individual polygon. Since the feature input is converted to a raster, each location can only have one value.

    An alternative method is to process the zonal operation iteratively for each of the polygon zones and collate the results.

  • When specifying the Input raster or feature zone data (in_zone_data in Python), the default zone field will be the first available integer or text field. If no other valid fields exist, the ObjectID field (for example, OID or FID) will be the default.

  • The Input value raster (in_value_raster in Python) can be either integer or floating point. However, when it is floating-point type, the options for calculating majority, median, minority, and variety will not be available.

  • For majority and minority calculations, when there is a tie, the output for the zone is based on the lowest of the tied values. See how Zonal Statistics works for more information.

  • The data type (integer or float) of the output is dependent on the zonal calculation being performed and the input value raster type. See how Zonal Statistics works for more information.

  • By default, this tool will take advantage of multi-core processors. The maximum number of cores that can be utilized is limited to four.

    If you want the tool to use fewer cores, use the parallelProcessingFactor environment setting.

  • See Analysis environments and Spatial Analyst for additional details on the geoprocessing environments that apply to this tool.

Syntax

ZonalStatistics (in_zone_data, zone_field, in_value_raster, {statistics_type}, {ignore_nodata})
ParameterExplanationData Type
in_zone_data

Dataset that defines the zones.

The zones can be defined by an integer raster or a feature layer.

Raster Layer; Feature Layer
zone_field

Field that holds the values that define each zone.

It can be an integer or a string field of the zone dataset.

Field
in_value_raster

Raster that contains the values on which to calculate a statistic.

Raster Layer
statistics_type
(Optional)

Statistic type to be calculated.

  • MEAN Calculates the average of all cells in the value raster that belong to the same zone as the output cell.
  • MAJORITY Determines the value that occurs most often of all cells in the value raster that belong to the same zone as the output cell.
  • MAXIMUM Determines the largest value of all cells in the value raster that belong to the same zone as the output cell.
  • MEDIAN Determines the median value of all cells in the value raster that belong to the same zone as the output cell.
  • MINIMUM Determines the smallest value of all cells in the value raster that belong to the same zone as the output cell.
  • MINORITY Determines the value that occurs least often of all cells in the value raster that belong to the same zone as the output cell.
  • RANGE Calculates the difference between the largest and smallest value of all cells in the value raster that belong to the same zone as the output cell.
  • STD Calculates the standard deviation of all cells in the value raster that belong to the same zone as the output cell.
  • SUM Calculates the total value of all cells in the value raster that belong to the same zone as the output cell.
  • VARIETY Calculates the number of unique values for all cells in the value raster that belong to the same zone as the output cell.
String
ignore_nodata
(Optional)

Denotes whether NoData values in the Value input will influence the results of the zone that they fall within.

  • DATAWithin any particular zone, only cells that have a value in the input Value raster will be used in determining the output value for that zone. NoData cells in the Value raster will be ignored in the statistic calculation. This is the default.
  • NODATAWithin any particular zone, if any NoData cells exist in the Value raster, it is deemed that there is insufficient information to perform statistical calculations for all the cells in that zone; therefore, the entire zone will receive the NoData value on the output raster.
Boolean

Return Value

NameExplanationData Type
out_raster

The output zonal statistics raster.

Raster

Code sample

ZonalStatistics example 1 (Python window)

This example determines for each zone the range of cell values in the Value input raster.

import arcpy
from arcpy import env
from arcpy.sa import *
env.workspace = "C:/sapyexamples/data"
outZonalStats = ZonalStatistics("zone", "value", "valueraster", "RANGE",
                                "NODATA")
outZonalStats.save("C:/sapyexamples/output/zonestatout")
ZonalStatistics example 2 (stand-alone script)

This example determines for each zone the range of cell values in the Value input raster.

# Name: ZonalStatistics_Ex_02.py
# Description: Calculates statistics on values of a raster 
#    within the zones of another dataset.
# Requirements: Spatial Analyst Extension

# Import system modules
import arcpy
from arcpy import env
from arcpy.sa import *

# Set environment settings
env.workspace = "C:/sapyexamples/data"

# Set local variables
inZoneData = "zone"
zoneField = "value"
inValueRaster = "valueraster" 

# Execute ZonalStatistics
outZonalStatistics = ZonalStatistics(inZoneData, zoneField, inValueRaster,
                                     "RANGE", "NODATA")

# Save the output 
outZonalStatistics.save("C:/sapyexamples/output/zonestatout2")

Licensing information

  • ArcGIS Desktop Basic: Requires Spatial Analyst or Image Analyst
  • ArcGIS Desktop Standard: Requires Spatial Analyst or Image Analyst
  • ArcGIS Desktop Advanced: Requires Spatial Analyst or Image Analyst

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