DISCRETE is a string that contains "value1-value2=value3" pairs.
The individual pairs are separated by semicolons:
All value1, value2 and value3 are considered floating point
values. If 'a' is missing in the first pair, use -(FLT_MAX), if
'f' is missing in the last pair, use FLT_MAX.
If the 'left of line' is set to 'YES' in DLINES, the value of
value3 will be that value to the left (in order) of the contour
line value in the DISCRETE parameter. For above example, if the
contour line value is '3', then left of the line value will be
'2'. If the contour line value is '1', then left of the line
value will be RMISSD.