Updated: 02 Dec 2019


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:



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

VAR_UNITS = ppmv




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

Base unit prefix Comment
Y yotta
Z zetta
E exa
P peta
T tera
G giga
M mega
k kilo
h hecto
da deka
d deci
c centi
m milli
u micro
n nano
p pico
f femto
a atto
z zepto
y yocto

Base unit Comment SI Conversionent FLAG
% percent 0 0.01
1 X If dimensionless or no specific unit 0
A X ampere 0
C coulomb 0 1
cd X candela 0
d day 0 86400
deg angular degree 0 1.74533E-2
degC degree Celsius 273.15 1
h hour 0 3600
Hz hertz 0 1
J joule 0 1
K X kelvin 0
l liter 0 1E-3
lm lumen 0 1
lx lux 0 1
m X metre 0
min minute 0 60
MJD2K Modified Julian Day 2000 0 86400
mol X mole 0
Np neper 0 1
N newton 0 1E3
NONE X For text entries only, otherwise use 1 0
Pa pascal 0 1
photons X photons 0
psu X practical salinity unit 0
rad X radian 0
s X second 0
sr X steradian 0
V volt 0 1
W watt 0 1
kg X kilogram 0
Gal Galileo 0 1E-2
ppmv parts per million (volume) 0 1E-6
pptv parts per trillion (volume) 0 1E-12
ppbv parts per billion (volume) 0 1E-9
ppv X Parts per volume 0
molec X molecule 0
DU dobson unit 0 4.4614E-4