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

[heng.feng]

Brilliant. Thanks a lot, Uwe. This is all I want to know.

Yes, you are right that I only recently started working on these data again although I have only got one day off for Christmas (in China) :)

From: Schmitz, Uwe [via OSGeo.org] [mailto:ml-node+5931200-375580391-308251 at n2.nabble.com]
Sent: 17 January 2011 09:22
To: Heng Feng
Subject: AW: [gdal-dev] An issue about using GDALWARP to dotheNTv2transformation

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
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