[GRASS-dev] ascii export and import, large file problem

Paul Kelly paul-grass at stjohnspoint.co.uk
Thu Apr 19 18:48:59 EDT 2007

Hello Hamish
Thanks for bringing this up again. ISTR being too busy to reply straight 
away the last time and then forgetting...

On Wed, 18 Apr 2007, Hamish wrote:
> --relevant quotes--
> [1...]
> For a real y-axis label I think we need to add a new (optional) "units"
> element to the raster format (e.g. in cell_misc/).

OK this seems like a very good idea. Several times Helena has suggested 
the idea of adding "vertical datum" support to a location, but I was 
always quite negative about it in pointing out that there was not even 
any way of indicating that a particular raster map contained elevation 
data. But in fact, the obvious answer seems now to be that it should be 
done on a per-raster map basis rather than a per-location basis. I.e. if 
the metadata for a raster map indicates that it contains elevation data 
(i.e. units are a length measure) then there should also be a facility to 
specify the vertical datum the length measure is relative to.

> [2...]
>> The user should be able to add a title for the legend.
> For vlegends that's doable. For raster legends I hope to add
> G_set_raster_units() and G_get_raster_units() to write/read a simple
> string containing raster data units (set from "r.support units=") which
> will be stored in $MAPSET/cell_misc/$RASTERMAP/units. Raster legends and
> things like lat/lon NVIZ and r.shaded.relief could use the tag if it
> existed.
> --endquote--

I think it's worth putting enough thought in so that the feature is easily 
usable for more than just legend titles. Especially how to specify 
vertical datums. I suppose there are a few very standard ones, like WGS84 
ellipsoidal height and that kind of thing. Actually I think the EPSG 
co-ordinate system database also includes vertical datums. Maybe it has 
some kind of unique codes that we could re-use. Maybe we should have a 
vdatum.table file with descriptions. Or maybe we should just put a 
human-readable name in and leave it for manual interpretation.

I'm really not too sure.


> apparently cell_misc/ is evil, so that moves it to
> $MAPSET/units/$RASTERMAP until GRASS7  when it moves to
> $MAPSET/raster/$RASTERMAP/units
> I don't like to add another element dir to $MAPSET, but...
> Hamish
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