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

[Christopher Hunt]

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