Definition:
The correct value for the specific spatial object is not known to, and not computable by, the data provider. However, a correct value may exist.
Description:
EXAMPLE When the elevation of the water body above the sea level of a certain lake has not been measured, then the reason for a void value of this property would be 'Unknown'.
NOTE 'Unknown' is applied on an object-by-object basis in a spatial data set.
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