[GRASS5] Re: [Proj] Continuing confusion of datums vs. projections

Paul Kelly paul-grass at stjohnspoint.co.uk
Thu Jul 8 03:49:21 EDT 2004

Hello Michel

On Thu, 8 Jul 2004, Michel Wurtz wrote:

> I just wanted to say that you cannot propose a GIS without offering a
> *correct* function to transform a map from one coordinate system to
> another one, even if the Datum involved is not the same.  The problem
> is not "should we develop this as internal or external library" : that
> doesn't matter.  The problem is "I have many sources of data, with an
> accuracy of a few meters.  I want to display/compare/mix/analyse them
> together.  My GIS should manage that".

Gerald is talking about a projection library (PROJ), not a GIS (GRASS). Of 
course GRASS can do datum transformations but it is perfectly feasible to 
only use the PROJ library for projection calculations and to implement 
datum shifting separately. This is what Thierry is doing with KerGIS (another
GIS, based on GRASS).

I would also add that within GRASS there are examples of the utility of 
projection software that do not involve datums, e.g. in calculating the 
latitude and longitude angles of a position on the earth for use in sun 
angle calculations (r.sun etc.).

> So the coordinate transformation functions must take care of datums.
> If Grass can't do that, Gras is unusable for a vast majority of application,
> and we will not have any usable open-source GIS that can stand in front of
> ArcView, MapInfo, GeoConcept and other commercial GIS softwares...

Just to clarify for other people reading, GRASS *can* do that. The point 
of this discussion was more a low-level technical one about how this 
support is organised and programmed within the GIS and not a discussion of 
the high-level functionality visible to the user.


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