The flag -a determines the average computation of the input raster maps. In case the flag is not set, the average calculation is:
average = (min + max) / 2
average = sum(input maps) / (number of input maps)
GDD Growing Degree Days are calculated as
gdd = average - lower
In case the -a is set, the Winkler indices are calculated instead of GDD, usually accumulated for the period April 1st to October 31st (northern hemisphere) or the period October 1st to April 30th (southern hemisphere).
BEDDs Biologically Effective Degree Days are calculated as
bedd = average - lower
The Huglin heliothermal index is calculated as
huglin = (average + max) / 2 - lower
Mean raster values are calculated as
mean = average
For all formulas is min the minimum value, max the maximum value and average the average value. The min, max and average values are automatically calculated from the input maps.
The shift and scale values are applied directly to the input values. The lower and upper maps, as well as the range options are applied to constrain the accumulation. In case the lower and upper maps are not provided the limits option with default values will be applied.
If an existing map is provided with the basemap option, the values of this map are added to the output.
new = old * scale + shift
With the -n flag, any cell for which any of the corresponding input cells are NULL is automatically set to NULL (NULL propagation) and the accumulated value is not calculated.
Negative results are set to 0 (zero).
Without the -n flag, all non-NULL cells are used for calculation.
If the range= option is given, any values which fall outside that range will be treated as if they were NULL. Note that the range is applied to the scaled and shifted input data. The range parameter can be set to low,high thresholds: values outside of this range are treated as NULL (i.e., they will be ignored by most aggregates, or will cause the result to be NULL if -n is given). The low,high thresholds are floating point, so use -inf or inf for a single threshold (e.g., range=0,inf to ignore negative values, or range=-inf,-200.4 to ignore values above -200.4).
The number of input raster maps to be processed is given by the limit of the operating system. For example, both the hard and soft limits are typically 1024. The soft limit can be changed with e.g. ulimit -n 1500 (UNIX-based operating systems) but not higher than the hard limit. If it is too low, you can as superuser add an entry in
/etc/security/limits.conf # <domain> <type> <item> <value> your_username hard nofile 1500
Use the file option to analyze large amount of raster maps without hitting open files limit and the size limit of command line arguments. The computation is slower than the input option method. For every sinlge row in the output map(s) all input maps are opened and closed. The amount of RAM will rise linear with the number of specified input maps. The input and file options are mutually exclusive. Input is a text file with a new line separated list of raster map names and optional weights. As separator between the map name and the weight the character | must be used.
r.series.accumulate in=MOD11A1.Day,MOD11A1.Night,MYD11A1.Day,MYD11A1.Night out=MCD11A1.GDD \
scale=0.02 shift=-273.15 limits=10,30
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