• Illum:

This program calculates, from a Digital Elevation Model (DEM), the illumination received by a point in the terrain when the Sun occupies a certain position in the celestial sphere. This position can be defined:

• From the Sun elevation and azimuth, in degrees (for example 74.82 and 170.34); the Astres program will give you this information if it is unknown, which are constant throughout the extent of the DEM. This corresponds to option 1 of the program.
• From the elevation values (or zenith angle) and solar azimuth in degrees or in multiple degree units (defined by the corresponding syntax) as calculated for MODIS products, for example MOD09 (for more information see the Surface Reflectance User's Guide). It is related to program options 2 and 3.

The program have two options:

Superfice: This program calculates, from a Digital Elevation Model (DEM), the illumination received by a point in the terrain when the Sun occupies a certain position in the celestial sphere.

Veïns: This option uses neighbors to calculate the normal field.

The output file contains values being the cosine of the angle of incidence, i.e., the cosine of the angle between the vector normal to the maximum slope plane and the solar vector (given by the solar azimuth and elevation). From the two possible angles (i<180º and i'>180º, i+i'=360º) the cosine of the smaller is given.

The DEM may have byte, (short) integer or real format, compressed or uncompressed. By default, the DEM extent and spatial resolution will be carried over to the illumination model output, but it can also be calculated at a finer scale and in that case the model will approximate a small surface of local tendency to the combined elevation pixels that contribute toeach output pixel. The normal vector of this suface will provide the calculations related to solar position.

The output file is always in byte format and it contains values 'v' of the cosine of the incidence angle scaled as: v*100+100. This way, cosine values, that should be in the interval [-1,1], go to the interval [0,200]. This is suitable for byte format and takes in three significant figures for the cosine values, enough for most applications. So, an output value of 173 should be interpreted as a cosine value of 0.73, corresponding of an angle of incidence of 43º.

If the DEM has flag values and they are correctly documented, pixels that are affected by a flag value in the input DEM will get the value 255, documented as NODATA in the output file. This will cause a little reduction of the useful area in the borders between flag and elevation values. This is due to the fact that flag values do not allow to perform a rigorous calculation of the illumination. A way to mitigate this problem is to iterate once the program FagoVal before executing ILLUM. This allows the enlargement the area with "reasonable" height values. In this execution use the flag value as the value to be eliminated and use the mean criterion to substitute this value.

The program allow for calculations taking 4 (up-down and left-right) or 8 (up-down, left-right and diagonals) nearest neighbours at each point. In the last case, the weight of the diagonal neighbours is smaller in proportion to the distance to the center of the point to evaluate.

If the DEM is a multiband raster with pixels of different size for each band, the /MULTIBANDA parameter must be used and a resulting multiband raster with the corresponding illumination models must be generated. It should be noted that with Option 1 solar position is considered unique for all bands. If the geographical extends were excessively separated between them a unique execution for all bands with the solar position at the center of the extend ought to be done. For the multiband case, it is possible to add a prefix to the original file names.