[gdal-dev] geotransform rotation and gdal_merge

[Chaitanya kumar CH]

Dave,

The reason you have lat/lon values for each pixel is because they may not
be aligned exactly.
Even though it is not very 'efficient', you have to define the GCPs for
each pixel with the lat/lon values of that pixel. Then you use gdalwarp
while forcing use of these GCPs to combine all the images.

How are the lat/lon values stored?

On Sun, Jul 22, 2012 at 7:44 PM, David Hoese <dhoese at gmail.com> wrote:

>  Sure, pixels were probably not the best word to use. I have 15
> points(elements) per scan line per "variable" directly from an aircraft
> instrument.  So I might have a 15 element array of brightness temperatures,
> a 15 element array of latitudes corresponding to those points in the BT
> array, and another 15 element array of longitudes corresponding to those
> points in the BT array.  So the first element in the brightness
> temperature(BT[0]) array represents an area of the earth located at
> lon[0],lat[0] (I'm actually not sure if its the center of the area or the
> corner, but at the moment that doesn't matter).
>
> Does that make sense?
>
> -Dave
>
>
> On 7/21/12 9:46 AM, Chaitanya kumar CH wrote:
>
> Dave,
>
> You said that you have lat/lon values for each pixel. Can you explain?
>
> On Fri, Jul 20, 2012 at 8:44 PM, David Hoese <dhoese at gmail.com> wrote:
>
>>  I'm attempting to put aircraft scan data into geotiffs (1-3 scanlines
>> each) and then use gdal_merge.py to combine them into one large geotiff
>> that has the entire aircraft's path.  The scan lines are 15 pixels wide and
>> taken every 10 seconds, the geotiffs are wgs84 lat/lon, and I have lat/lon
>> values for each pixel.  To handle the case when the aircraft isn't flying
>> straight north I think I have to use the 2 rotation parameters in the
>> affine geotransform, is that right?  I don't have any test cases, but I
>> think if I don't use rotation anything that reads the geotiff will think
>> that the image is square(aligned) in lat/lon space.
>>
>> Whether or not I need to use this, can someone explain to me how to use
>> the rotation coefficients?  What are the actual values of the coefficients
>> supposed to be?  I couldn't find a good example and I couldn't get any
>> basic situations to make sense, like a 2x3 array turned 45 degrees.  I used
>> these equations:
>>
>>     Xgeo = GT(0) + Xpixel*GT(1) + Yline*GT(2)
>>     Ygeo = GT(3) + Xpixel*GT(4) + Yline*GT(5)
>>
>>
>> And lastly, does gdal_merge.py handle rotation?  I checked the source and
>> it doesn't ever seem to use elements 2 and 4 in its calculations.
>>
>> Thanks for any help.
>>
>> -Dave
>>
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>>
>
>
>
> --
> Best regards,
> Chaitanya kumar CH.
>
> +91-9494447584
> 17.2416N 80.1426E
>
>
>


-- 
Best regards,
Chaitanya kumar CH.

+91-9494447584
17.2416N 80.1426E
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