Vertical datums

[gwaldron]

Add support for vertical datums to SpatialReference?.

Vertical datums define the reference against which you express height values. So far all osgEarth height data is HAE (height above ellipsoid).

Some data sets (like DTED) use MSL (height above mean sea level) or other vertical reference grids. We need to support that for accurate correlation and visual representation.

GDAL does not yet support vertical datums, but Frank W might be working on it (but I cannot find the link now)

This is in support of the vert_cs support in LIBLAS:
 http://trac.liblas.org/wiki/VerticalCS

Here's another link "recommendations for vertcs support in gdal":
 http://www.daylongraphics.com/download/vunits_in_gdal.pdf