# [R] Centre of gravity of a mountain

Ab Hu master.rstat at yahoo.com
Wed Nov 10 07:06:50 CET 2010

Peter Langfelder wrote:
>
> Sorry, I'm not sure what you want to do in points 2-4. Shrink the
> mountain vertically or horizontally? You can for example look up image
> resizing algorithms if you want to shrink the area under the mountain
> but keep  the shape of the mountain (approximately) the same. For
> cross correlation, simply do cor( c(mountainMatrix1),
> c(mountainMatrix2)). The c() function takes values in a matrix and
> "straightens" them out into a single vector.
>
> Peter
>
Sorry, I was not clear.

Step2: I want to shrink (by half) the mountain along XY (i.e. ignore height)
based on the centroid value i find in step 1. Yes, I am looking for
algorithms similar to image resizing, more specifically i want to use
bilinear interpolation while resizing where neighboring 4 pixels are
averaged.
So, based on that concept, every value should be averaged with neighboring 4
values in the matrix.
Is there any package in R which uses such algorithm?

Step3: After step2, the relevant values of the mountain will be constrained
to 11x11, however i want the final matrix size to remain 21x21 with zeros
coerced where necessary.

Step4: Yes, since i will be generating several such maps I want to use cross
correlation to compare them for similarity. More specifically i need
Pearson's correlation, seems like cor will do it.

Thanks for the corrected z-coordinate formula. I needed just the X and Y
coordinates to get the centroid because I want to shrink my mountain only
along those dimensions.

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