AW: [gdal-dev] An issue about using GDALWARP to dotheNTv2transformation

[Schmitz, Uwe]

Heng,

> Hi Uwe,
> 
> Hope you had a wonderful Christmas break.

Yes, thank you (but I have the impression that mine has 
been shorter than yours ;-))

> Regarding the issue about the NTv2 transformation, 
> the map data provider has confirmed that the raster 
> data do cover some areas which are outside the Germany 
> map, hence the area of the grid shift file.  I have 
> taken your second suggestion and ignore the coordinates 
> outside BeTA2007 grid.  However,  by running the suggested 
> command on some raster data, this time I have got the 
>  message "Inverse grid shift iteration failed, presumably 
> at grid edge. Using first approximation." Do you know 
> why I am having this message? Do I need to worry about it?
>
No! NTv2 is based on a grid of shift values from System A
to System B (DHDN -> ETRS89 in your case). For the other
direction (B->A, or ETRS89->DHDN) the shift value is 
calculated by an iterative approach. Near the grid edges it's
possible that during the iteartion the coordinates fall 
outside the grid. I would imagine that this is the case
for the message you see.

Assuming you don't need precise transformation results 
outside the BeTA2007 grid, you can safely ignore this 
message.

Regards
Uwe