5.1.8.  VAR_SI_CONVERSION

The variable attribute VAR_SI_CONVERSION provides the formula used to convert the data specified in VAR_UNITS to the equivalent data in the corresponding "base units"; the latter are identified in Table  7.5. The units conversion is compliant to the Unidata UDUNITS package [13]and has been extended by GEOMS aliases where appropriate. This attribute aims at facilitating calculations and comparisons by automated tools, for example, for comparison of data of the same geophysical entity but reported in different units. 

The conversion formula is specified as follows: 

VAR_UNITS = Offset + Conversion factor ×Base unit

Divisions and multiplications of units should be factored out such as to have the shortest possible string of base units, in which any base unit appears at most once. For example, for the units [ nm m­2] the attribute VAR_SI_CONVERSION = 0;1.0E­9;m­1.

String data types must have a corresponding attribute value of an empty string:

VAR_DATA_TYPE = STRING

VAR_SI_CONVERSION = [empty]

Dimensionless variables use base unit 1, e.g.:

VAR_UNITS = ppmv

VAR_SI_CONVERSION = 0;1E­6;1

Type: STRING

Entry: Three semicolon-separated fields

Format: Offset;Conversion factor;Base unit

Example 1: VAR_SI_CONVERSION = 0;1E­3;kg m­1 s­2 for VAR_UNITS = mPa

Example 2: VAR_SI_CONVERSION = 273.15;1;K for VAR_UNITS = Celsius

Example 3: VAR_SI_CONVERSION = 0;1.0E­9;m­1 for VAR_UNITS = nm m­2