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

[Christopher Hunt]

Anyone? :-)

On 13/07/2008, at 6:35 PM, Christopher Hunt wrote:

> 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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