[GRASSLIST:3557] Re: How to import LAEA into lat/lon?

[Eric G. Miller]

On Mon, Apr 22, 2002 at 02:28:34PM +1000, Terry Duell wrote:
> Hullo,
> I have been trying to figure out how to import some data which is in Lambert
> Azimuthal Equal Area projection, and then reproject into lat/lon.
> I thought I had it figured out!
> I set up an 'other' location defined with the laea projection, the correct
> spheroid, and data extents, etc, but couldn't find a way of defining the
> origin. I should point out that the data is in metres and has an origin at
> 135 00 00E and 15 00 00S. I didn't think any of the standard datums applied
> so chose 'datum', thinking that I might then be able to define the datum
> lat/lon. Not so.

When you created the location, were you not prompted for Latitude of
Origin[lat_0] and Central Meridian[lon_0]?  You still need at least an
ellipsoid in order to do the inverse projection. Note: lat/lon in itself
is somewhat meaningless without the datum/ellipsoid model.  Differences
can be several hundred meters...

> Anyway, the data was imported into the laea location ok, but when I try to
> run r.proj to reproject into a lat/lon location, it fails because there is no
> PROJ_INFO file in the laea location.

Rerun g.setproj, Select projection other (99) and you'll be prompted for
the projection.  Type "list" at the prompt, to see the list of
projections (you want "laea").  If at first you don't succeed ...

> I guess I need to choose one of the standard datums, but it needs to have the
> correct values, ie the origin used to define the laea data I imported. 
> Are the data associated with the standard datums documented somewhere within
> GRASS?...or am I on the wrong track here?

The datum doesn't care about your projection parameters (provided the
area covered is valid for the datum).  Nonetheless, you can scan through
grass5/etc/datum.table and/or grass5/etc/ellipse.table if you like.

> How do I define the necessary data for the PROJ_INFO file, to take into
> account the origin of the imported data, and also account for the units of
> measurement used?

Don't just pick any datum or ellipsoid, find out what the source data
is in. Most common datums and ellipsoids are known by GRASS. Even though
datum transformations aren't really supported yet, the ellipsoid
parameters are used.

-- 
Eric G. Miller <egm2 at jps.net>