[GRASSLIST:7007] Re: calculating center of intensity
Charles Ehlschlaeger
CR-Ehlschlaeger2 at wiu.edu
Fri Jun 3 15:06:16 EDT 2005
I don't have GRASS working on my computer to check this out, however,
here is a procedure to calculating the center of a shape:
Assume you have a raster map "shape" where the value of 0 is at all
places w/out the shape and a positive value where the shape is located at.
1) Find the total volume of the shape w/:
1a) r.mapcalc command function:
one = "1"
1b) r.volume d=shape clump=one
(There should be a better way of doing this.)
2) Calculate volumeOnAxis of shape in North-South dimension with the
r.mapcalc command:
NSvolumeOn = shape * y()
3) Calculate massOnAxis of shape in East-West dimension with the
r.mapcalc command:
EWvolumeOn = shape * x()
4) Find the total volume of NSvolumeOn with:
r.volume d= NSvolumeOn clump=one
5) Find the total volume of EWvolumeOn with:
r.volume d= NSvolumeOn clump=one
6) The coordinates of the center will be
east = total volume of EWvolumeOn / volume of shape
north = total volume of NSvolumeOn / volume of shape
A little csh and awk programming could replicate Idrisi's center program.
<INSERT DISCLAIMERS ABOUT USING AT OWN RISK...>
sincerely, chuck
Markus Neteler wrote:
>On Thu, Jun 02, 2005 at 03:42:36PM -0400, Jason Horn wrote:
>
>
>>Does anyone know of a way to calculate the center of a shape in a
>>raster file based on shape and pixel values? For example, imagine a
>>NEXRAD RADAR image showing a storm cloud. Could GRASS be used to
>>calculate the center of the cloud? I'm not having any luck finding a
>>grass raster function to do this.
>>
>>
>
>I did a quick google search for
> nexrad storm cell identification
>or
> nexrad SCIT
>which a couple of hits. Some programming might
>be needed. I don't know if these algorithms are freely
>available.
>
>Markus
>
>
>
--
Chuck Ehlschlaeger, Associate Professor & GIS Center Director
Department of Geography, Western Illinois University
306C Tillman Hall, 1 University Circle, Macomb, IL 61455
cre111 at wiu.edu, phone: 309-298-1841, fax: 309-298-3003
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