GML profile for simple features. Copyright (c) 2005 OGC, All Rights Reserved. Some common reasons for a null value: innapplicable - the object does not have a value missing - The correct value is not readily available to the sender of this data. Furthermore, a correct value may not exist. template - the value will be available later unknown - The correct value is not known to, and not computable by, the sender of this data. However, a correct value probably exists. withheld - the value is not divulged other:reason - as indicated by "reason" string Specific communities may agree to assign more strict semantics when these terms are used in a particular context.