[gdal-dev] Guidance sought on using gdal-translate

[Christopher Hunt]

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?

How do I centre a projection? lat_0 and lon_0 values?

Thanks again for your time - I appreciate that time is precious.

Cheers,
-C

On 30/07/2008, at 5:38 PM, Christopher Hunt wrote:

> 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
> -1912.68	759.39
> > -0.46551585 51.47177116
> 0.00	0.00
> > -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