Unit strings that are currently allowed in STC
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An XSD excerpt is appended that show the relevant sections of the STC
1.3 schema.
Note that the list below not only list the basic unit symbols but is
also explicit about which combinations with prefixes are allowed.
Time unit strings:
's' (second)
'h' (hour = 3600 s)
'd' (day = 86400 s)
'a' or 'yr' (Julian year = 365.25 d)
'cy' (Julian century = 36525 d)
'' (empty, i.e., dimensionless; for ISO-8601 format)
Spectral coordinate unit strings:
frequency
'Hz'
'kHz'
'MHz'
'GHz'
wavelength
'm'
'mm'
'um' (micrometer)
'nm'
'Angstrom'
energy
'eV'
'keV'
'MeV'
'GeV'
'TeV'
Spatial coordinate unit strings:
angular
'deg' (degree)
'rad' (radian)
'h' (hour)
'arcmin'
'arcsec'
linear
'm' (meter)
'km'
'mm'
'AU'
'pc' (parsec)
'kpc'
'Mpc'
'lyr' (lightyear)
'' (empty, i.e., dimensionless, for unit sphere)
Three-units strings are for special 3-D vectors where the components
do not share the same unit; one may prefer to give each component
its own unit, instead:
'deg deg m'
'deg deg Mpc'
Then there is the position angle definition (not strictly a unit):
Position angles may be counted from North (through East),
(positive) X (to positive Y),
or (positive) Y (to positive X) axis
'North'
'X'
'Y'
Velocity units are constructed as posUnitType / velTimeType
and the position and velocityTime units are specified separately
in order to reduce the size of the enumeration list
the latter may be second, day, hour, year (a or yr), century,
but not empty
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Relevant sections from the STC v1.3 schema:
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Time units may be second (s), hour (h: 3600 s), day (d: 86400 s), Julian year (a, yr: 365.25 d), Julian century (cy: 36525 d), or empty (i.e., dimensionless) for ISO-8601 format
Spatial coordinate units may be angular (degrees, radians, hours, arcmins, arcsecs), linear (meters, km, mm, AUs, parsecs, kpc, Mpc, lightyears), or empty (i.e., dimensionless). The three-units strings are for special 3-D vectors where the components do not share the same unit; one may prefer to give each component its own unit, instead.
Angular coordinate units may be degrees, radians, hours, arcmins, arcsecs
Position angles may be counted from North (through East), (positive) X (to positive Y), or (positive) Y (to positive X) axis
Velocity units are a posUnitType / velTimeType; the latter may be second, day, hour, year (a or yr), century, but not empty
Spectral units may be frequency (Hz, kHz, MHz, GHz), wavelength (meters, mm, micron, nm, Angstrom), or energy (eV, keV, MeV, GeV, TeV)
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