Merge pull request #76 from crotwell/unit_naming_rules

add unit recommendation from iris validator

docs/appendices.rst

@@ -71,8 +71,24 @@ http://docs.fdsn.org/projects/source-identifiers
 
 Unit Naming Rules
 ===========================================
-The FDSN stronly recommends the use of SI unit names whenever possible, which are
-defined case sensitively.
+The FDSN strongly recommends the use of SI unit names whenever possible. For
+details see the SI brochure at:
+
+   `<https://www.bipm.org/en/publications/si-brochure>`_
+
+* Use SI units, base or derivative, and their standard symbols whenever possible. Spell out and avoid abbreviations of non-SI units.
+
+* Exponents for powers are specified with `**`, e.g. `s**2`.
+
+* Multiplication is specified using `*` while division is specified with `/`. For example `hit/(cm**2*hour)`.
+
+* Use parentheses sparingly, only when FORTRAN-like precedence would not be correct.
+
+* Unit definitions are singular, not plural, e.g. use `second` not `seconds`.
+
+* Unit names should not mix symbols and full names, use one or the other, e.g. use `m/s` or `meter/second` but not `meter/s` or `m/second`.
+
+* Units should be used case sensitively, using the case specified by SI rules.
 
 In the SEED format all unit names are recommended to be uppercase and
 SI when possible.  For this reason, some unit documentation and