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Area Solar Radiation

Available with Spatial Analyst license.

Summary

Derives incoming solar radiation from a raster surface.

Learn more about how solar radiation is calculated

Usage

  • Calculating insolation can be very time consuming, where the calculations for a large digital elevation model (DEM) can take several hours, and for a very large DEM, even days. You may wish to do some test runs with a coarser resolution or subset of your data to ensure the settings are correct before committing a run with the full-resolution data.

  • The output radiation rasters will always be floating-point type and have units of watt hours per square meter (WH/m2). The direct duration raster output will be integer with unit hours.

  • The latitude for the site area (units: decimal degree, positive for the northern hemisphere and negative for the southern hemisphere) is used in calculations such as solar declination and solar position.

    The analysis is designed only for local landscape scales, so it is generally acceptable to use one latitude value for the whole DEM. With larger datasets, such as for states, countries, or continents, the insolation results will differ significantly at different latitudes (greater than 1 degree). To analyze broader geographic regions, it is necessary to divide the study area into zones with different latitudes.

  • For multiday time configurations, the maximum range of days is a total of one year (365 days, or 366 days for leap years). If the start day is greater than the end day, the time calculations will proceed into the following year.

    For example, [start day, end day] = [365, 31], represents December 31 to January 31 of the following year. For an example of [1, 2], the time is inclusive for the first day from 0:00 hours (January 1) to 0:00 (January 2). The start day and end day cannot be equal.

  • The year value for time configuration is used to determine a leap year. It does not have any other influence on the solar radiation analysis as the calculations are a function of the time period determined by Julian days.

  • For within-day time configurations, the maximum range of time is one day (24 hours). Calculations will not be performed across days (for instance, from 12:00 p.m. to 12:00 p.m. the next day). The start time must be less than the end time.

  • For within-day time configurations, the start and end times are displayed as solar time (units: decimal hours). Use the time conversion dialog box window to convert the local standard time and local solar time (HMS). When converting local standard time to solar time, the program accounts for equation of time.

  • The use of a z-factor is essential for correcting calculations when the surface z units are expressed in units different from the ground x,y units. To get accurate results, the z units should be the same as the x,y ground units. If the units are not the same, use a z-factor to convert z units to x,y units. For example, if your x,y units are meters and your z units are feet, you could specify a z-factor of 0.3048 to convert feet to meters.

  • It is recommended to have your data in a projected coordinate system with units of meters. If you choose to run the analysis with a spherical coordinate system, you will need to specify an appropriate z-factor for that latitude. Following is a list of some appropriate z-factors to use if the x,y units are decimal degrees and the z units are meters:

        Latitude     Z-factor
           0         0.00000898
          10         0.00000912
          20         0.00000956
          30         0.00001036
          40         0.00001171
          50         0.00001395
          60         0.00001792
          70         0.00002619
          80         0.00005156
  • The latitude for the site area (units: decimal degree, positive for the northern hemisphere and negative for the southern hemisphere) is used in calculations such as solar declination and solar position. Because the solar analysis is designed for landscape scales and local scales, it is acceptable to use one latitude value for the whole DEM. For broader geographic regions, it is necessary to divide the study area into zones with different latitudes.

  • For input surface rasters containing a spatial reference, the mean latitude is automatically calculated; otherwise, latitude will default to 45 degrees. When using an input layer, the spatial reference of the data frame is used.

  • Sky size is the resolution of the viewshed, sky map, and sun map rasters that are used in the radiation calculations (units: cells per side). These are upward-looking, hemispherical raster representations of the sky and do not have a geographic coordinate system. These rasters are square (equal number of rows and columns).

    Increasing the sky size increases calculation accuracy but also increases calculation time considerably.

  • When the day interval setting is small (for example, < 14 days), a larger sky size should be used. During analysis the sun map (determined by the sky size) is used to represent sun positions (tracks) for particular time periods to calculate direct radiation. With smaller day intervals, if the sky size resolution is not large enough, sun tracks may overlap, resulting in zero or lower radiation values for that track. Increasing the resolution provides a more accurate result.

  • The maximum sky size value is 10,000. A value of 200 is default and is sufficient for whole DEMs with large day intervals (for example, > 14 days). A sky size value of 512 is sufficient for calculations at point locations where calculation time is less of an issue. At smaller day intervals (for example, < 14 days), it is recommended to use higher values. For example, to calculate insolation for a location at the equator with day interval = 1, it is recommended to use a sky size of 2,800 or more.

  • Day intervals greater than 3 are recommended, as sun tracks within three days typically overlap, depending on sky size and time of year. For calculations of the whole year with monthly interval, day interval is disabled and the program internally uses calendar month intervals. The default value is 14.

  • Because the viewshed calculation can be highly intensive, horizon angles are only traced for the number of calculation directions specified. Valid values must be multiples of 8 (8, 16, 24, 32, and so on). Typically, a value of 8 or 16 is adequate for areas with gentle topography, whereas a value of 32 is adequate for complex topography. The default value is 32.

  • The number of calculation directions needed is related to the resolution of the input DEM. Natural terrain at 30-meters resolution is usually quite smooth, so fewer directions are sufficient for most situations (16 or 32). With finer DEMs, and particularly with man-made structures incorporated in the DEMs, the number of directions needs to increase. Increasing the number of directions will increase accuracy but will also increase calculation time.

  • The Create outputs for each interval check box provides the flexibility to calculate insolation integrated over a specified time period or insolation for each interval in a time series. For example, for the within-day time period with an hour interval of one, checking this box will create hourly insolation values; otherwise, insolation integrated for the entire day is calculated.

  • The Create outputs for each interval parameter affects the format and number of output radiation files. When checked, the output raster will contain multiple bands that correspond to the radiation or duration values for each time interval (hour interval when time configuration is less than one day, or day interval when multiple days).

  • The diffuse proportion is the fraction of global normal radiation flux that is diffuse. Values range from 0 to 1. This value should be set according to atmospheric conditions. Typical values are 0.2 for very clear sky conditions and 0.3 for generally clear sky conditions.

  • The amount of solar radiation received by the surface is only a portion of what would be received outside the atmosphere. Transmittivity is a property of the atmosphere that is expressed as the ratio of the energy (averaged overall wavelengths) reaching the earth's surface to that which is received at the upper limit of the atmosphere (extraterrestrial). Values range from 0 (no transmission) to 1 (complete transmission). Typically observed values are 0.6 or 0.7 for very clear sky conditions and 0.5 for only a generally clear sky.

    The value for the energy received at the earth's surface is at the shortest path through the atmosphere (that is, the sun is at the zenith, or directly overhead) and for sea level. For areas beyond Tropic of Capricorn and Tropic of Cancer, the sun can never be at the exact zenith, not even at noon; however, this value still refers to the moment when the sun is at the zenith. Because the algorithm corrects for elevation effects, transmittivity should always be given for sea level.

    Transmittivity has an inverse relation with the diffuse proportion parameter.

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

Syntax

AreaSolarRadiation (in_surface_raster, {latitude}, {sky_size}, {time_configuration}, {day_interval}, {hour_interval}, {each_interval}, {z_factor}, {slope_aspect_input_type}, {calculation_directions}, {zenith_divisions}, {azimuth_divisions}, {diffuse_model_type}, {diffuse_proportion}, {transmittivity}, {out_direct_radiation_raster}, {out_diffuse_radiation_raster}, {out_direct_duration_raster})
ParameterExplanationData Type
in_surface_raster

Input elevation surface raster.

Raster Layer
latitude
(Optional)

The latitude for the site area. The units are decimal degrees, with positive values for the northern hemisphere and negative for the southern.

For input surface rasters containing a spatial reference, the mean latitude is automatically calculated; otherwise, latitude will default to 45 degrees.

Double
sky_size
(Optional)

The resolution or sky size for the viewshed, sky map, and sun map rasters. The units are cells.

The default creates a raster of 200 by 200 cells.

Long
time_configuration
(Optional)

Specifies the time configuration (period) used for calculating solar radiation.

The Time class objects are used to specify the time configuration.

The different types of time configurations available are TimeWithinDay, TimeMultipleDays, TimeSpecialDays, and TimeWholeYear.

The following are the forms:

  • TimeWithinDay({day},{startTime},{endTime})
  • TimeMultipleDays({year},{startDay},{endDay})
  • TimeSpecialDays()
  • TimeWholeYear({year})

The default time configuration is TimeMultipleDays with the startDay of 5 and endDay of 160 for the current Julian year.

Time configuration
day_interval
(Optional)

The time interval through the year (units: days) used for calculation of sky sectors for the sun map.

The default value is 14 (biweekly).

Long
hour_interval
(Optional)

Time interval through the day (units: hours) used for calculation of sky sectors for sun maps.

The default value is 0.5.

Double
each_interval
(Optional)

Specifies whether to calculate a single total insolation value for all locations or multiple values for the specified hour and day interval.

  • NOINTERVALA single total radiation value will be calculated for the entire time configuration. This is default.
  • INTERVALMultiple radiation values will be calculated for each time interval over the entire time configuration. The number of outputs will depend on the hour or day interval. For example, for a whole year with monthly intervals, the result will contain 12 output radiation values for each location.
Boolean
z_factor
(Optional)

The number of ground x,y units in one surface z unit.

The z-factor adjusts the units of measure for the z units when they are different from the x,y units of the input surface. The z-values of the input surface are multiplied by the z-factor when calculating the final output surface.

If the x,y units and z units are in the same units of measure, the z-factor is 1. This is the default.

If the x,y units and z units are in different units of measure, the z-factor must be set to the appropriate factor, or the results will be incorrect.

For example, if your z units are feet and your x,y units are meters, you would use a z-factor of 0.3048 to convert your z units from feet to meters (1 foot = 0.3048 meter).

Double
slope_aspect_input_type
(Optional)

How slope and aspect information are derived for analysis.

  • FROM_DEM The slope and aspect rasters are calculated from the input surface raster. This is the default.
  • FLAT_SURFACE Constant values of zero are used for slope and aspect.
String
calculation_directions
(Optional)

The number of azimuth directions used when calculating the viewshed.

Valid values must be multiples of 8 (8, 16, 24, 32, and so on). The default value is 32 directions, which is adequate for complex topography.

Long
zenith_divisions
(Optional)

The number of divisions used to create sky sectors in the sky map.

The default is eight divisions (relative to zenith). Values must be greater than zero and less than half the sky size value.

Long
azimuth_divisions
(Optional)

The number of divisions used to create sky sectors in the sky map.

The default is eight divisions (relative to north). Valid values must be multiples of 8. Values must be greater than zero and less than 160.

Long
diffuse_model_type
(Optional)

Type of diffuse radiation model.

  • UNIFORM_SKY Uniform diffuse model. The incoming diffuse radiation is the same from all sky directions. This is the default.
  • STANDARD_OVERCAST_SKY Standard overcast diffuse model. The incoming diffuse radiation flux varies with zenith angle.
String
diffuse_proportion
(Optional)

The proportion of global normal radiation flux that is diffuse. Values range from 0 to 1.

This value should be set according to atmospheric conditions. The default value is 0.3 for generally clear sky conditions.

Double
transmittivity
(Optional)

The fraction of radiation that passes through the atmosphere (averaged overall wavelengths). Values range from 0 (no transmission) to 1 (all transmission).

The default is 0.5 for a generally clear sky.

Double
out_direct_radiation_raster
(Optional)

The output raster representing the direct incoming solar radiation for each location.

The output has units of watt hours per square meter (WH/m2).

Raster Dataset
out_diffuse_radiation_raster
(Optional)

The output raster representing the diffuse incoming solar radiation for each location.

The output has units of watt hours per square meter (WH/m2).

Raster Dataset
out_direct_duration_raster
(Optional)

The output raster representing the duration of direct incoming solar radiation.

The output has units of hours.

Raster Dataset

Return Value

NameExplanationData Type
out_global_radiation_raster

The output raster representing the global radiation or total amount of incoming solar insolation (direct + diffuse) calculated for each location of the input surface.

The output has units of watt hours per square meter (WH/m2).

Raster

Code sample

AreaSolarRadiation example 1 (Python window)

The following Python window script demonstrates how to use this tool.

import arcpy
from arcpy.sa import *
from arcpy import env
env.workspace = "C:/sapyexamples/data"
outGlobalRadiation = AreaSolarRadiation("dem30", "", "400", TimeMultipleDays(2008,91,152))
outGlobalRadiation.save("C:/sapyexamples/output/glob_rad")
AreaSolarRadiation example 2 (stand-alone script)

Calculate the amount of incoming solar radiation over a geographic area.

# Name: AreaSolarRadiation_example02.py
# Description: Derives incoming solar radiation from a raster surface. 
#              Outputs a global radiation raster and optional direct, diffuse and direct duration rasters
#              for a specified time period. (April to July).
#              
# Requirements: Spatial Analyst Extension

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

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

# Set local variables
inRaster = "C:/sapyexamples/data/solar_dem"
latitude = 35.75
skySize = 400
timeConfig = TimeMultipleDays(2008, 91, 212)
dayInterval = 14
hourInterval = 0.5
zFactor = 0.3048
calcDirections = 32
zenithDivisions = 16
azimuthDivisions = 16
diffuseProp = 0.7
transmittivity = 0.4
outDirectRad = ""
outDiffuseRad = ""
outDirectDur = Raster("C:/sapyexamples/output/dir_dur")


# Execute AreaSolarRadiation
outGlobalRad = AreaSolarRadiation(inRaster, latitude, skySize, timeConfig,
   dayInterval, hourInterval, "NOINTERVAL", zFactor, "FLAT_SURFACE",
   calcDirections, zenithDivisions, azimuthDivisions, "UNIFORM_SKY",
   diffuseProp, transmittivity, outDirectRad, outDiffuseRad, outDirectDur)

# Save the output 
outGlobalRad.save("C:/sapyexamples/output/glob_rad")

Licensing information

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

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