Aspect (Spatial Analyst)

Summary

Derives the aspect from each cell of a raster surface.

The aspect identifies the compass direction that the downhill slope faces for each location.

The Surface Parameters tool provides a newer implementation and enhanced functionality.

Learn more about how Aspect works

Illustration

OutRas = Aspect(InRas1)

Usage

• The Surface Parameters tool provides a newer implementation of aspect and is recommended to be used instead of the Aspect tool. The Aspect tool fits a plane to the nine local cells, but a plane may not be a good descriptor of the landscape and may mask or exaggerate natural variations of interest. The Surface Parameters tool fits a surface to the neighborhood of cells instead of a plane, which provides a more natural fit to the terrain.

  The Aspect tool uses a 3 by 3 window of cells to compute the value, while the Surface Parameters tool allows window sizes from 3 by 3 to 15 by 15 cells. Larger window sizes are useful with high resolution elevation data to capture land surface processes at an appropriate scale. The Surface Parameters tool also provides an adaptive window option that evaluates the local variability of the terrain and identifies the largest appropriate neighborhood size for each cell. This can be useful with gradual homogeneous terrain interrupted by streams, roads, or sharp breaks in slope.

  You can continue to use the traditional approach of the Aspect tool if you need the results to exactly match previous tool runs or if fast processing time is more important than a better algorithm.

• This tool uses a 3 by 3 cell moving window to process the data. If the processing cell is NoData, the output for that location will be NoData.

• Of the eight cells neighboring the processing cell, this tool requires that at least seven of them have a valid value. If there are fewer than seven valid cells, the calculation will not be performed, and the output at that processing cell will be NoData.

• The cells in the outermost rows and columns of the output raster will be NoData. This is because along the boundary of the input dataset, those cells do not have enough valid neighbors.

• Aspect is expressed in positive degrees from 0 to 360, measured clockwise from north.

• Cells in the input raster that are flat—with zero slope—are assigned an aspect of -1.

• For the geodesic method, specifying the surface z-unit ensures the accuracy of the output. The Z unit parameter will be enabled only when the geodesic method is selected.

• If a z-unit is available in the vertical coordinate system of the input raster, it will be applied automatically. It is recommended that you define a z-unit for the input raster if it is missing. You can use the Define Projection tool to specify a z-unit. If it is undefined, meter will be used by default.

• The Project geodesic azimuths (project_geodesic_azimuths in Python) parameter is available only when the Method parameter is set to Geodesic.

  For the Geodesic method, if the Project geodesic azimuths parameter is checked (project_geodesic_azimuths is set to PROJECT_GEODESIC_AZIMUTHS in Python), the following are true:

  • North is always represented by 360 degrees.
  • Azimuths will be projected to correct the distortion caused by a nonconformal Output Coordinate System environment value. These angles can be used to accurately locate points along the steepest downhill slope.

  Check the Project geodesic azimuths parameter if you're using the Aspect output as a back direction input for a tool in the Distance toolset.

• When the input raster needs to be resampled, the bilinear technique will be used. An example of when an input raster may be resampled is when the output coordinate system, extent, or cell size is different from that of the input.

• If the Input raster parameter value is high resolution with a cell size of less than a few meters, or particularly noisy, consider using the Surface Parameters tool and its user-defined neighborhood distance option instead of the immediate 3 by 3 neighborhood of this tool. Using a larger neighborhood can minimize the effect of noisy surfaces. Using a larger neighborhood can also better represent landforms and surface characteristics when using high resolution surfaces.

• This tool can be GPU accelerated, which means that if a compatible graphics processing unit (GPU) is available on your system, it will be used to enhance the performance of the tool. Use the Target device for analysis (analysis_target_device in Python) parameter to control whether the GPU or CPU will be used to run the tool.

  See GPU processing with Spatial Analyst for details on compatible GPUs, configuring and working with GPU devices, as well as troubleshooting tips.

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

Parameters

Dialog

Label	Explanation	Data Type
Input raster	The input surface raster.	Raster Layer
Method (Optional)	Specifies whether the calculation will be based on a planar (flat earth) or a geodesic (ellipsoid) method. The planar method is appropriate to use on local areas in a projection that maintains correct distance and area. It is suitable for analyses that cover areas such cities, counties, or smaller states in area. The geodesic method produces a more accurate result, at the potential cost of an increase in processing time. Planar—The calculation will be performed on a projected flat plane using a 2D Cartesian coordinate system. This is the default method. Geodesic—The calculation will be performed in a 3D Cartesian coordinate system by considering the shape of the earth as an ellipsoid.	String
Z unit (Optional)	Specifies the linear unit that will be used for vertical z-values. It is defined by a vertical coordinate system if it exists. If no vertical coordinate system exists, define the z-unit using the unit list to ensure correct geodesic computation. The default is meter. Inch—The linear unit will be inches. Foot—The linear unit will be feet. Yard—The linear unit will be yards. Mile US—The linear unit will be miles. Nautical mile—The linear unit will be nautical miles. Millimeter—The linear unit will be millimeters. Centimeter—The linear unit will be centimeters. Meter—The linear unit will be meters. Kilometer—The linear unit will be kilometers. Decimeter—The linear unit will be decimeters.	String
Project geodesic azimuths (Optional)	Specifies whether geodesic azimuths will be projected to correct the angle distortion caused by the output spatial reference. Unchecked—Geodesic azimuths will not be projected. This is the default. Checked—Geodesic azimuths will be projected. This option supports CPU processing only.	Boolean
Target device for analysis (Optional)	Specifies the device that will be used to perform the calculation. GPU then CPU—If a compatible GPU is found, it will be used to perform the calculation. Otherwise, the CPU will be used. This is the default. CPU only—The calculation will only be performed on the CPU. GPU only—The calculation will only be performed on the GPU.	String

Return Value

Label	Explanation	Data Type
Output raster	The output aspect raster. It will be floating-point type.	Raster

Python

Aspect(in_raster, {method}, {z_unit}, {project_geodesic_azimuths}, {analysis_target_device})

Name	Explanation	Data Type
in_raster	The input surface raster.	Raster Layer
method (Optional)	Specifies whether the calculation will be based on a planar (flat earth) or a geodesic (ellipsoid) method. PLANAR—The calculation will be performed on a projected flat plane using a 2D Cartesian coordinate system. This is the default method. GEODESIC—The calculation will be performed in a 3D Cartesian coordinate system by considering the shape of the earth as an ellipsoid. The planar method is appropriate to use on local areas in a projection that maintains correct distance and area. It is suitable for analyses that cover areas such cities, counties, or smaller states in area. The geodesic method produces a more accurate result, at the potential cost of an increase in processing time.	String
z_unit (Optional)	Specifies the linear unit that will be used for vertical z-values. It is defined by a vertical coordinate system if it exists. If no vertical coordinate system exists, define the z-unit using the unit list to ensure correct geodesic computation. The default is meter. INCH—The linear unit will be inches. FOOT—The linear unit will be feet. YARD—The linear unit will be yards. MILE_US—The linear unit will be miles. NAUTICAL_MILE—The linear unit will be nautical miles. MILLIMETER—The linear unit will be millimeters. CENTIMETER—The linear unit will be centimeters. METER—The linear unit will be meters. KILOMETER—The linear unit will be kilometers. DECIMETER—The linear unit will be decimeters.	String
project_geodesic_azimuths (Optional)	Specifies whether geodesic azimuths will be projected to correct the angle distortion caused by the output spatial reference. GEODESIC_AZIMUTHS—Geodesic azimuths will not be projected. This is the default. PROJECT_GEODESIC_AZIMUTHS—Geodesic azimuths will be projected.	Boolean
analysis_target_device (Optional)	Specifies the device that will be used to perform the calculation. GPU_THEN_CPU—If a compatible GPU is found, it will be used to perform the calculation. Otherwise, the CPU will be used. This is the default. CPU_ONLY—The calculation will only be performed on the CPU. GPU_ONLY—The calculation will only be performed on the GPU.	String

Return Value

Name	Explanation	Data Type
out_raster	The output aspect raster. It will be floating-point type.	Raster

Code sample

Aspect example 1 (Python window)

This example creates an aspect raster from an input surface raster.

import arcpy
from arcpy import env  
from arcpy.sa import *
env.workspace = "C:/sapyexamples/data"
outAspect = Aspect("elevation")
outAspect.save("C:/sapyexamples/output/outaspect01.img")

Aspect example 2 (stand-alone script)

This example creates an aspect raster from an input surface raster.

# Name: Aspect_Ex_02.py
# Description: Derives aspect from a raster surface.
# 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
inRaster = "elevation"
method = "GEODESIC"
zUnit = "FOOT"
# Check out the ArcGIS Spatial Analyst extension license
arcpy.CheckOutExtension("Spatial")

# Execute Aspect
outAspect = Aspect(inRaster, method, zUnit)

# Save the output 
outAspect.save("C:/sapyexamples/output/outaspect02")

Environments

Auto Commit, Cell Size, Cell Size Projection Method, Current Workspace, Extent, Geographic Transformations, Mask, Output CONFIG Keyword, Output Coordinate System, Scratch Workspace, Snap Raster, Tile Size