Monthly composites

Sometimes, it is required to calculate monthly value composites from the half-monthly GIMMS datasets, for example if the user needs to ensure temporal overlap with some other ecological or eco-climatological time series. gimms features a function called monthlyComposite which works both on vectors of filenames and 'RasterStack' objects (ideally returned by rasterizeGimms) and calculates monthly values based on a user-defined function. For instance, fun = max creates monthly maximum value composites (MVC).

Needless to say, the function is heavily based on stackApply from the fabulous raster package and assumes numeric vectors of monthly indices (or text substrings from pos1 to pos2 from which to deduce such indices, see ?monthlyIndices) as input variable. The actual code work is relatively straightforward.

## create monthly maximum value composites
gimms_mvc_v1 <- monthlyComposite(gimms_raster_v1, 
                                 indices = monthlyIndices(gimms_files_v1))
gimms_mvc_v1[[4]] # april 2000

class       : RasterLayer 
dimensions  : 2160, 4320, 9331200  (nrow, ncol, ncell)
resolution  : 0.08333333, 0.08333333  (x, y)
extent      : -180, 180, -90, 90  (xmin, xmax, ymin, ymax)
coord. ref. : +proj=longlat +datum=WGS84 +no_defs +ellps=WGS84 +towgs84=0,0,0 
data source : in memory
names       : ndvi3g_geo_v1_2000_04 
values      : -0.2937, 0.9996  (min, max)

Figure 3. 1-d Kernel density distribution of GIMMS NDVI3g.v1 values during the first (green) and second half (turquoise) of April 2000 and resulting value distribution of the maximum value composite layer (MVC; black).