[Gdal-dev] adding 1 GCP pointless?

[Maciek Sieczka]

Hi,

I have just had the same issue which was left unanswered here
http://lists.maptools.org/pipermail/gdal-dev/2005-March/005352.html



Adding a single GCP doesn't yield an error:

$ gdal_translate -gcp 100 100 2000 2000 lzn.tif lzn2.tif

But the GCP is not added:

$ gdalinfo lzn2.tif
Driver: GTiff/GeoTIFF
Size is 720, 360
Coordinate System is `'
Metadata:
  AREA_OR_POINT=Area
  TIFFTAG_DOCUMENTNAME=/home/fishoo/tmp/lzn.tif
  TIFFTAG_XRESOLUTION=98
  TIFFTAG_YRESOLUTION=96
  TIFFTAG_RESOLUTIONUNIT=2 (pixels/inch)
Corner Coordinates:
Upper Left  (    0.0,    0.0)
Lower Left  (    0.0,  360.0)
Upper Right (  720.0,    0.0)
Lower Right (  720.0,  360.0)
Center      (  360.0,  180.0)
Band 1 Block=720x11 Type=Byte, ColorInterp=Red
Band 2 Block=720x11 Type=Byte, ColorInterp=Green
Band 3 Block=720x11 Type=Byte, ColorInterp=Blue


I noticed 2 GCPs is the minimum number to be effectively added. What is
the reason? Is it that less than 2 is not enough for warping the image
after? But is 2 enough? I though 1st order affine requires at least 3...

What i was trying to accomplish by adding 1 GCP is the following thing:
I've lost a worldfile for my tif. But I know the coordinates of one
characterstic object in the image. I thought I could just add it as
GCP using gdal_translate and then "move" the whole image to it's
destination with gdalwarp. Is this impossible?

Could gdal_translate issue a warning if adding 1 GCP is really
pointless?

Best,
Maciek

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