[gdal-dev] geotransform rotation and gdal_merge

[David Hoese]

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 
> <mailto: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

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