[gdal-dev] Correct gdalwarp usage from UTM to Equidistant Cylindrical Projection

[Christopher Hunt]

Hi there,

First off, wow, gdal is great.

I'm obviously new to gdal so I'm hoping that you'll put up with what  
I'm sure will be regarded as a "newbie" style of question.

I have an image from Terraserver that is in a UTM 30 projection. I  
need to convert it to a Equidistant Cylindrical Projection. I would  
like to verify that I'm using the gdalwarp command properly so I'm  
helping that you can help me.

Here's the image info I have;

> Image Information:
> Date Taken: 1/1/1999
> Native Resolution: 15
> File Size: 3000 x 3000 (9.0 megapixels)
> Image Area: 18,225.00 sq km
>
> Coordinate Information:
> Center point Latitude: 51.46660 (51∞ 27' 60")
> Center point Longitude: -0.44990 (0∞ 26' 60")
>
> Upper left corner Latitude: 52.09038 (52∞ 5' 25")
> Upper left corner Longitude: -1.39985 (-1∞ 23' 59")
> Lower right corner Latitude: 50.83562 (50∞ 50' 8")
> Lower right corner Longitude: 0.47438 (0∞ 28' 28")
>
> Geo-reference Information:
> Projection: UTM Zone 30
> Datum: WGS 84
> Image Units: Meters

Furthermore, here's the associated tfw file contents:

> 45.000000
> 0.000000
> 0.000000
> -45.000000
> 609626.000120
> 5772299.000249


FInally here's the command I think I should be using:

> gdalwarp -s_srs '+proj=utm +zone=30 +datum=WGS84 +x_0=609626.000120  
> +y_0=5772299.000249' -t_srs '+proj=eqc' -dstalpha -multi  
> 169460-65786.tif out.tif

I certainly get a resultant image but I'm not experienced enough with  
cartography to understand whether the image is correct.

One thing that does confuse me slightly is that the centre lat/long in  
the resultant image is not the same as in the source image... I would  
have expected that they would be, or at least that the top left corner  
(which is what I thinking the northerly and easterly parameters point  
to) should be the same.

Another thing: I would expect to specify either a scale, a resolution  
or the other corner of the image... do I need to include something  
more in the command for this?

Thanks for any enlightenment. I'm fascinated by cartography and relish  
the prospect of learning gdal more.

Kind regards,
Christopher
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