Karthik wrote:
Frank,
Thanks for your suggestion. I tried it out and I get the exact value
as expected. If I understand correctly, you are referring to the
intracell-reference-location field. It's used to denote whether the top left
coordinates are off by half a pixel or not. I have taken care of this
situation also. I think the variation is due to reading the wrong record. I
think I am somehow reading the one record below or above the required
record. I am also not sure whether Global mapper is using other
interpolations like Bicubic or Trilinear interpolation which can give more
accuracy.
Also, I want a reassurance on the math I used. I have attached a
JPEG image that explains the math I used to get the four surrounding points.
Karthik,
The image skips some of the core math you use to do the interpolation
but it seems generally reasonable.
One more basic question I have here. How can one be sure of the
values they are getting after interpolation? Is there some other
verification tool available other than Global Mapper to cross verify?
When I write interpolators I do test points and then manually compute the
results I would expect. In particular I do some test at pixel center (should
get the exact value of the original pixel) and on a grid corner (should get
a simple average of the four surrounding pixels).
I'm sorry I don't have more time to dig into this.
Best regards,
--
---------------------------------------+--------------------------------------
I set the clouds in motion - turn up | Frank Warmerdam, [EMAIL PROTECTED]
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush | President OSGeo, http://osgeo.org
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