GRTSmh_base4frac: the raster data source GRTSmaster_habitats converted to base 4 fractions

The data source file is a monolayered GeoTIFF in the FLT8S datatype. In GRTSmh_base4frac, the decimal (i.e. base 10) integer values from the raster data source GRTSmaster_habitats (link) have been converted into base 4 fractions, using a precision of 13 digits behind the decimal mark (as needed to cope with the range of values). For example, the integer 16 (= 4^2) has been converted into 0.0000000000100 and 4^12 has been converted into 0.1000000000000.

Long base 4 fractions seem to be handled and stored easier than long (base 4) integers. This approach follows the one of Stevens & Olsen (2004) to represent the reverse hierarchical order in a GRTS sample as base-4-fraction addresses.

See R-code in the GitHub repository 'n2khab-preprocessing' at commit ecadaf5 for the creation from the GRTSmaster_habitats data source.

A reading function to return the data source in a standardized way into the R environment is provided by the R-package n2khab.

Beware that not all GRTS ranking numbers are present in the data source, as the original GRTS raster has been clipped with the Flemish outer borders (i.e., not excluding the Brussels Capital Region).