Re: [gdal-dev] Guidance sought on using gdal-translate
Hi Frank and friends, Just to wrap this one up - we actually ended up receiving a geocoded image and I used gdal_translate to assign EPSG:27700 (the image was missing the coord sys declaration - this seems to be the case with a few image providers I've noticed): gdal_translate -a_srs EPSG:27700 ... Anyhow all good. Thanks for the help. Cheers, -C On 16/08/2008, at 7:54 PM, Christopher Hunt wrote: Thanks for your reply Frank. On 16/08/2008, at 2:05 PM, Frank Warmerdam wrote: I believe EPSG:27700 is British National Grid which is a projected coordinate system based on the OSGB 1936 datum. So you should ensure the provider means the dataset is actually in BNG, in addition to being based on OSGB 1936. OK, will do. If you can get BNG coordinates then that is great. I've asked so fingers crossed. If you can only get WGS84 or OSGB1936 lat/long coordinates then you can convert these to BNG using a program like cs2cs though that is a bit tricky. Understood. Hopefully one last question on this though: the a_ullr parameters - given the a_srs spec of EPSG:27700, are the a_ullr parameters expected to be in BNG coordinates i.e. eastings and northings values? Asking this another way, what actually dictates the coordinate system used for the a_ullr parameters? Cheers, -C ___ gdal-dev mailing list gdal-dev@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/gdal-dev
Re: [gdal-dev] Guidance sought on using gdal-translate
Thanks for your reply Frank. On 16/08/2008, at 2:05 PM, Frank Warmerdam wrote: I believe EPSG:27700 is British National Grid which is a projected coordinate system based on the OSGB 1936 datum. So you should ensure the provider means the dataset is actually in BNG, in addition to being based on OSGB 1936. OK, will do. If you can get BNG coordinates then that is great. I've asked so fingers crossed. If you can only get WGS84 or OSGB1936 lat/long coordinates then you can convert these to BNG using a program like cs2cs though that is a bit tricky. Understood. Hopefully one last question on this though: the a_ullr parameters - given the a_srs spec of EPSG:27700, are the a_ullr parameters expected to be in BNG coordinates i.e. eastings and northings values? Asking this another way, what actually dictates the coordinate system used for the a_ullr parameters? Cheers, -C ___ gdal-dev mailing list gdal-dev@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/gdal-dev
Re: [gdal-dev] Guidance sought on using gdal-translate
On Fri, Aug 15, 2008 at 8:41 AM, Christopher Hunt [EMAIL PROTECTED] wrote: Hi again Frank et. al, I've made some progress. The aerial image provider now tells me that it is projected as OSGB 1936 which I believe is EPSG:27700. Thus if I use a command like: gdal_translate -a_srs EPSG:27700 -a_ullr then I should be able to create my GeoTIFF. Unfortunately the image provider hasn't provided the top left/bottom right coords so I'm going to Google Earth to approximate them (yes, ugly I know). I'm hopeful that the actual top left/bottom right will ultimately be provided to me, but meanwhile I want to ensure that the a_ullr parameters I provide will be in metres i.e. eastings and northings from the OSGB 1936 origin. Is this correct? Chris, I believe EPSG:27700 is British National Grid which is a projected coordinate system based on the OSGB 1936 datum. So you should ensure the provider means the dataset is actually in BNG, in addition to being based on OSGB 1936. If you can get BNG coordinates then that is great. If you can only get WGS84 or OSGB1936 lat/long coordinates then you can convert these to BNG using a program like cs2cs though that is a bit tricky. Best regards, -- ---+-- I set the clouds in motion - turn up | Frank Warmerdam, [EMAIL PROTECTED] light and sound - activate the windows | http://pobox.com/~warmerdam and watch the world go round - Rush | Geospatial Programmer for Rent ___ gdal-dev mailing list gdal-dev@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/gdal-dev
Re: [gdal-dev] Guidance sought on using gdal-translate
Christopher Hunt wrote: Hi Frank et. al, Sorry to be a pest. I've been trying tons of things, trawling tons of search results, and I'm really no better off than I was a few days ago. :-P In summary I have an aerial photo image of an airport with three well known points (expressed in lat/longs). I'm wanting the image to be projected using a equidistant cylindrical projection. On gdal_translate's gcps, I'm presuming that the origin of the image is assumed to be top-left for the gcp pixel and line values... and that the gcp easting and northing values are expressed in metres? Chris, Correct. How do I centre a projection? lat_0 and lon_0 values? Thanks again for your time - I appreciate that time is precious. I honestly believe that reprojection is the wrong tool to take a raw aerial photo and try to turn into an orthophoto. Normally this process is address by orthorectification software that takes into account the geometry of the camera lens, the orientation of the camera when the photo is taken, and the elevation model of the target region. If you are going to try and accomplish orthorectification by something as simple as reprojection then the fit is going to be relatively poor in all but the most ideal circumstances. So, sorry to say it, but I'm not going to wrack my brain to try and solve a problem that I think is likely insolvable. Best regards, -- ---+-- I set the clouds in motion - turn up | Frank Warmerdam, [EMAIL PROTECTED] light and sound - activate the windows | http://pobox.com/~warmerdam and watch the world go round - Rush| President OSGeo, http://osgeo.org ___ gdal-dev mailing list gdal-dev@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/gdal-dev
Re: [gdal-dev] Guidance sought on using gdal-translate
On 30/07/2008, at 9:07 AM, Frank Warmerdam wrote: Christopher Hunt wrote: Hi Frank, Thanks for the reply and for the confirmation that I'm on the right track. Dumb newbie question: wouldn't most aerial photos taken, say, from 12,000ft be ortho in their projection? Christopher, I don't think that is a good assumption at all. And if you were going to go with that assumption it seems to me you would want the projection center to be in the photo center. Ah yes, so if the projection centre were approximately 51.47177116, -0.46551585 then: proj +proj=ortho +datum=WGS84 +lat_0=51.47177116 +lon_0=-0.46551585 -0.49310374 51.47858961 -1912.68759.39 -0.46551585 51.47177116 0.000.00 -0.43439870 51.46490423 2158.02 -763.96 ^C ...to get my eastings and northings and then: gdal_translate -a_srs '+proj=ortho +datum=WGS84 +lat_0=51.47177116 +lon_0=-0.46551585' \ -gcp 61 115 -1912.68 759.39 \ -gcp 465 280 0.00 0.00 \ -gcp 899 435 2158.02 -763.96 \ infile outfile i.e. the +lat_0 and +lon_0 centres the projection right? Unfortunately the above still doesn't yield the result I'm expecting. But then, I'm in the habit of treating projections as black boxes, and I seldom try to physically interprete their characteristics. Understood. Unfortunately I don't have a great deal of projection information for the image that I have. Thanks for any further guidance. Cheers, -C ___ gdal-dev mailing list gdal-dev@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/gdal-dev
