georectifying an image

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georectifying an image

Pete Flugstad
I would like to georectify a group of images I have (bmps).  I have
the lat/lon for each corner
of the images.  I have tried to use gdal_translate to do this and it
seems to work, however
the viewing utility I'm using (OpenMap), wants tiepoints in the
resulting geotiff to load it
properly.  Are tie points the same thing as GCPs?  It looks like I can
attach those by
adding gcp's to the image, possibly using gdal_translate  Is there a
better way to do that?
Do I need to use gdal_warp when I'm done?

One problem I'm seeing is that the -gcp flag to gdal_translate wants
northing/easting values,
but I have the corner lat/long's.  Is there a way to translate
lat/long to n/e values?

Thanks,
Pete
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Re: georectifying an image

Ted Florence
One very easy way to do this is by way of the Geographic Imager plugin
for Adobe Photoshop.
If you have the plugin (and Photoshop) you can do all kinds of spatial
things with imagery inside Photoshop including referencing and exporting
a geotiff.
Have a look at www.geographicimager.com

Pete Flugstad wrote:

> I would like to georectify a group of images I have (bmps).  I have
> the lat/lon for each corner
> of the images.  I have tried to use gdal_translate to do this and it
> seems to work, however
> the viewing utility I'm using (OpenMap), wants tiepoints in the
> resulting geotiff to load it
> properly.  Are tie points the same thing as GCPs?  It looks like I can
> attach those by
> adding gcp's to the image, possibly using gdal_translate  Is there a
> better way to do that?
> Do I need to use gdal_warp when I'm done?
>
> One problem I'm seeing is that the -gcp flag to gdal_translate wants
> northing/easting values,
> but I have the corner lat/long's.  Is there a way to translate
> lat/long to n/e values?
>
> Thanks,
> Pete
> _______________________________________________
> Geotiff mailing list
> [hidden email]
> http://lists.maptools.org/mailman/listinfo/geotiff
>
>  

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Re: georectifying an image

Frank Warmerdam
In reply to this post by Pete Flugstad
Pete Flugstad wrote:

> I would like to georectify a group of images I have (bmps).  I have
> the lat/lon for each corner
> of the images.  I have tried to use gdal_translate to do this and it
> seems to work, however
> the viewing utility I'm using (OpenMap), wants tiepoints in the
> resulting geotiff to load it
> properly.  Are tie points the same thing as GCPs?  It looks like I can
> attach those by
> adding gcp's to the image, possibly using gdal_translate  Is there a
> better way to do that?
> Do I need to use gdal_warp when I'm done?

Pete,

Yes, in order to produce a conventional "north up" rectified image you
should run gdalwarp on the image after attaching gcps.

eg.

   gdal_translate -gcp ... in.tif image_with_gcps.tif
   gdalwarp image_with_gcps.tif rectified_image.tif

You might want to consider a non-default resampling type for gdalwarp,
like bilinear - just add "-r bilinear"

   gdalwarp -r bilinear image_with_gcps.tif rectified_image.tif

> One problem I'm seeing is that the -gcp flag to gdal_translate wants
> northing/easting values,
> but I have the corner lat/long's.  Is there a way to translate
> lat/long to n/e values?

You can use lat and long, but keep in mind that the easting is the longitude
and the northing is the latitude.

Also, if you have lat/long values you can assign a coordinate system.

eg.

  gdal_translate -a_srs WGS84 -gcp .. -gcp .. in.tif out.tif

There are also mechanisms avalable to transform lat/long values to
projected coordinates if needed, but they are a bit involved and you
may not need them.

Best regards,
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