document the difference between HFA_USE_RRD and USE_RRD in the gdaladdo and HFA format manual

[msieczka]

The followiwng syntax:

gdaladdo --config HFA_USE_RRD YES --config USE_RRD YES some.tif 2 4 8

creates a small aux file and a big rrd file are created. The following:

gdaladdo --config USE_RRD YES some.tif 2 4 8

only a big aux file.

In the 1st case the sum of aux and rrd file sizes is equal the size of the aux file from the 2nd case.

What is the difference between the 2 methods, is any better?

[Jukka Rahkonen]

Background: Mail from gdal-dev ​http://lists.osgeo.org/pipermail/gdal-dev/2008-August/017853.html

Frank Warmerdam writes: "Maciek,

The HFA_USE_RRD was implemented at the request of a client to mimic the behavior of another application. It isn't really any better than just putting the overviews in the .aux file. If you file a ticket I'll try and document things somewhat better."

The config option HFA_USE_RRD is not documented in http://trac.osgeo.org/gdal/wiki/ConfigOptions. Perhaps it is only documented in the gdal-dev mail and somewhere in the code. This config option is probably not used much.

[Jukka Rahkonen]

If Frank will not make a try with the documentation soon perhaps I should do it. It may take a few weeks/months/years from me also.

[Jukka Rahkonen]

Improved documentation in HFA format web page frmt_hfa.html r28331. I did not edit the gdaladdo manual page because I considered that at least the use of --config USE_RRD and --config HFA_USE_RRD together is an unplanned feature that just happens to work. It affects only users of ERDAS and perhaps some old ArcGIS versions and documenting this special use case on general gdaladdo page makes more confusion than good. ERDAS users probably read the HFA page and for most ArcGIS users overviews in .aux file work as well so nearly everybody should be satisfied.