[Proj] grid-shift issues

[Frank Warmerdam]

Craig Bruce wrote:
> This makes the call for the point properly use "ntv2_0".  However, our US
> customers may not be happy with this string since "ntv1_can" (lat 40 to 84)
> and "conus" (lat 20 to 50.25) overlap.  Is there some way that PROJ could
> be made more intelligent about selecting the best grid-shift file to use?

Craig,

I understand your problem, but I don't have a solution.  In theory we could
provide (optional) area of use polygons for each grid file, but that would
be alot of work and evaluating each point to see which area of use polygon
it was in would be very expensive for those transforming alot of points.


> Secondly, the following web page:
> 
>   http://www.remotesensing.org:16080/proj/gen_parms.html
> 
> indicates that a special grid-shift file called "null" is distributed
> with PROJ releases after 4.4.6 that can be used to avoid fatal errors when
> transforming a point not covered by any of the grid-shift files, but the
> "null" grid-shift file does not seem to be present in PROJ release 4.4.8.
> 
> I reverse-engineered my own "null.lla" as:
> 
> World zero grid shift
>    2   2   1  -180.00000   360.000000000000   -90.00000   180.000000000000
> 0: 0 0 0 0
> 1: 0 0 0 0
> 
> This file builds through nad2bin but it doesn't seem to work.  Where can
> I get the official "null.lla"?

I have attached null.lla.  It is the PROJ CVS tree.  I don't think any of the
grid shift files are in the PROJ.4 source tarball are they?  It may be that the
build version of null is also missing from the last grid shift file snapshot.
If so, let me know and I will put up a new one.

Best regards,
-- 
---------------------------------------+--------------------------------------
I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | Geospatial Programmer for Rent

-------------- next part --------------
Null transform for whole world
   3   3   1  -180.00000   180.00000   -90.00000   90.00000
0: 0 0 0 0 0 0 
1: 0 0 0 0 0 0 
2: 0 0 0 0 0 0