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

[heng.feng]

Hi Uwe,

I have been trying to use gdal_merge.py to merge a few 8 bit colored .tif tiles together. Having tried all sorts of options for this command, I always ended up with a merged 8-bit grayscale picture. I did a search on the internet and found a couple of posts related to this, however it seemed to me that the gdal_merge.py just does not have the ability to preserve the color. Did I get the wrong idea about this? Do you have any suggestion to solve this problem?

Not sure if I should post a separate message for this gdal_merge.py issue.  I just thought this would be the quickest way to get the help :)

Thanks,
Heng
________________________________
From: Schmitz, Uwe [via OSGeo.org] [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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