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 specifically 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, you must 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 the 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. For 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 the x,y units are meters and the z-units are feet, you can specify a z-factor of 0.3048 to convert feet to meters.
It is recommended that you have your data in a projected coordinate system with units of meters. If you run the analysis with a spherical coordinate system, you will need to specify an appropriate z-factor for that latitude. The 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 0.00005156The 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, you must divide the study area into zones with different latitudes.
For input surface rasters containing a spatial reference, the mean latitude is automatically calculated; otherwise, the latitude default is 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).
The following are recommended sky size values when a time configuration of a whole year or multiple days is used:
- For a 1 day interval, use a sky size of 1000 and above.
- For a 0.25 day interval, use a sky size of 2000 and above.
- For a 0.1 hour interval, use a sky size of 4000 and above.
Increasing the sky size increases calculation accuracy but also increases calculation time considerably.
When the day interval setting is small (for example, < 14 days), use a larger sky size. 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 the 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 that you use higher values. For example, to calculate insolation for a location at the equator with day interval = 1, use a sky size of 2,800 or above.
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 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 human-made structures incorporated in the DEMs, the number of directions needs to increase. Increasing the number of directions increases accuracy but 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 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, 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.