This section contains functions to define a new vertical Z-axis
and to get information from an existing Z-axis.
A Z-axis object is necessary to define a variable.
The following different Z-axis types are available:

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\begin{deflist}{{\large\tt ZAXIS\_DEPTH\_BELOW\_LAND \ \ }}
\item[{\large\tt ZAXIS\_GENERIC           }]  Generic user defined level
\item[{\large\tt ZAXIS\_SURFACE           }]  Surface level
\item[{\large\tt ZAXIS\_HYBRID            }]  Hybrid level
\item[{\large\tt ZAXIS\_SIGMA             }]  Sigma level
\item[{\large\tt ZAXIS\_PRESSURE          }]  Isobaric pressure level in Pascal
\item[{\large\tt ZAXIS\_HEIGHT            }]  Height above ground in meters
\item[{\large\tt ZAXIS\_ALTITUDE          }]  Altitude above mean sea level in meters
\item[{\large\tt ZAXIS\_DEPTH\_BELOW\_SEA }]  Depth below sea level in meters
\item[{\large\tt ZAXIS\_DEPTH\_BELOW\_LAND}]  Depth below land surface in centimeters
\end{deflist}
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