This callback can be used to generate elevation for a given OpenLayer
Feature (typically from its properties).
If a single number is returned, it will be used for all vertices in the geometry.
If an array is returned, each value will be used to determine the height of the corresponding vertex in the geometry.
Note that the cardinality of the array must be the same as the number of vertices in the geometry.
This callback can be used to generate elevation for a given OpenLayer Feature (typically from its properties).