[GRASS5] datum shifts

[rgrmill at rt66.com]

> Shouldn't this be done at GRASS compile time, so to have the
> right binary tables for the platform?
> Or will the format change with proj library versions
> that have the same binary interface?

Yes, it does need to be done when GRASS is compiled.


> > I think the data tables 
> > have to go under $GISBASE somewhere.  
> 
> That is not hardcoded, though...
> It providing an option to specify a non-default path a good idea?

That part that has to be hard coded is the part after $GISBASE, probably 
"/etc/nad/".  No more than I know now, it looks easier for me to code 
this if there is no user option.

> > Wherever they do go has to be 
> > coded into the proj library.
> 
> This confuses me a bit, is it an addition to proj then?

I guess I could be more specific.  A program that uses the nad<->nad 
conversion in the proj library has to call a function called "nad_init". 
"nad_init" takes one argument, and that is the fully qualified file name 
for the data table that it will use for the conversions.  "nad_init" is 
part of the proj distribution.  It looks now like the name should be 
'getenv("GISBASE")/etc/nad/<table name>'.  GISBASE is set during GRASS 
installation and hopefully is available during execution.  "/etc/nad/" 
will probably be hard coded; I suppose there are options.  <table name> 
is currently hardcoded to name the table that covers most of the US and 
the more populated areas of Canada, but it actually needs to be set at 
run time to any of 8 (13 including high precision data) different 
tables.

Roger


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