[gdal-dev] An issue about using GDALWARP todotheNTv2transformation

[Heng Feng]

Hi Uwe,

Thanks for your quick response as usual :) 

Yes, I had to use the "BIGTIFF=YES" option on some of these large data to get the re-projection done, but did not need to use this option on all the data which had this problem. The word "large" was actually mentioned due to the fact that all the data which do not have this "gap" problem after the re-projection are smaller than the ones which have. If you want, I can give you more details about the actual size of those data tomorrow as I do not have access to the data at the moment.

I actually can clearly see those "gaps" in FW OpenEV and in one of our own tools (only for viewing). The gaps are always there no matter I zoom in/out them in whichever way. Of course, the gaps will become bigger if I zoom in the picture.

The original data are actually provided in smaller tiles (GeoTiff) and all I need to do this time is re-projecting the data and displaying them as a whole dataset. However, after I did the re-projection on those tiles separately, there appear to be black lines around those tiles when displaying them in one view/window (even after merging them with GDALWARP command. Shall I use the option "skip_nosource=yes" to mosaic the tiles in the GDALWARP command?). And this was why I chose to merge the smaller tiles first and then re-project the merged large dataset. 

BR,
Heng
-----Original Message-----
From: Schmitz, Uwe [mailto:uwe.schmitz at bezreg-koeln.nrw.de] 
Sent: 16 February 2011 09:43
To: Heng Feng; gdal-dev at lists.osgeo.org
Subject: AW: [gdal-dev] An issue about using GDALWARP todotheNTv2transformation

Heng,
> 
> Hi Uwe,
> 
> Sorry to bother you again. I actually need your help about another 
> GDALWARP problem I am having. I just thought it might be better to 
> send you an email directly about this (a shortcutJ) although I did 
> just post a message regarding this problem on the GDAL DEV forum:
> http://osgeo-org.1803224.n2.nabble.com/gdalwarp-reprojection-problem-td6030749.html.
> 
> I am just wondering whether you have seen this kind of problem before. 
> Do you have idea about what could be the cause for this problem?
>
personally I try to avoid large data sets in one file. I tend to put them in many smaller manageable tiles. So may be I'm not the right one to ask for help (especially if you hit a real bug ;-)) in this case.
But there are many people here with deep knowledge of such things, that I'm pretty sure you will get further help from them.

So let me try to get some more informations regarding your above mentioned posting. What do you mean by "large" data. Are you beyond 65k-Pixel in width or height, so that you need BigTIFF? 
You wrote:
"... the output pictures seemed to contain the gaps, which appeared to be a few horizontal lines when zooming out."

I would assume that the gaps will disappear or even come up randomly if you zoom out (according to which interpolation algorithm your favorite viewer uses). But may be I haven't the right impression of what you mean with "gap".

Further, if I have to do gdalwarp, I will take two
steps:
   1. gdalwarp to an intermediate file with a fast and
      "easy" compression (uncompressed if you can afford
      the space)
   2. gdal_translate the intermediate file to the target file
      with the desired format and compression I don't know if these two steps are still necessary.


Uwe

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