Outputs from the Solar Radiation Graphics tool are raster representations and are not maps that correspond to the outputs from the area or point solar radiation analysis. Rather, they are representations of directions in a hemisphere of directions looking upward from a given location. In a hemispherical projection, the center is the zenith, the edge of the circular map representation is the horizon, and the angle relative to the zenith is proportionate to the radius. Hemispherical projections do not have a geographic coordinate system and have a lower left corner of (0,0).
It would not be practical to store viewsheds for all locations in a DEM, so when input locations are not specified, a single viewshed is created for the center of the input surface raster. When input point features or locations files are specified, multiple viewshed rasters are created for each input location. When multiple locations are specified, the output will be a multiband raster, where each band corresponds to the viewshed for a specific location.
The input locations table can be a point feature class or a table of point coordinates. When inputting locations by table, a list of locations must be specified with an x,y coordinate. The table can be a geodatabase table, a .dbf file, an INFO table, or a text table file. If using an ASCII coordinate file, each line should contain an x,y pair separated by a comma, space, or tab.
Output graphic display rasters do not honor extent or cell size environment settings. The output extents are always respective of the sky size/resolution and have a cell size equal to one. However, the underlying analysis will use the environment settings and may affect the results of the viewshed.
One or two sun map rasters may be generated, depending on whether the time configuration includes overlapping sun positions throughout the year. When two sun maps are created, one represents the period between the winter and summer solstice, and the other represents the period between the summer solstice and the winter solstice. Depending on the year, the solistices typically fall on the 20th or 21st of December and June, but occasionally they may be on the 22nd. When multiple sun maps are created, the default output is a multiband raster.
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 input surface rasters containing a spatial reference, the mean latitude is automatically calculated; otherwise, the 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).
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.
See Analysis environments and Spatial Analyst for additional details on the geoprocessing environments that apply to this tool.