Hi Komine,


By coordinates of the image, do you mean coordinates of bottom left - top
right coordinates?
Or do you mean spatial locations within a certain extent that you call
image?

In the first case, you could use raster crop function to work on a subset of
the total image that covers all the country
Then, you could perform map algebra to compute cell value differences from
one year to the next one.

See http://cran.r-project.org/web/packages/raster/vignettes/Raster.pdf

hth,

Mathieu

2011/8/29 Komine <momadou@yahoo.fr>

> Hi,
> I want to do  maps of changes for NDVI for several days. When I use a map 1
> for day 1, map 2 for day 2, map 3 for day 3 etc… in a same page,
> differences
> between maps were not visible.  For this reason, I want to do  maps of
> changes using differences between pixels value (or NDVI value) . Thus I
> want
> to extract coordinates and NDVI value for each pixel of my image (the image
> covers all a country). However, I have not all the pixel coordinates, but I
> have the coordinates of the image.
> Please how to extract coordinates and NDVI value for each pixel of my image
> when I don’t know the coordinates of all pixels. I use R.
> Sorry for my bad English.
> Thank you in advance
>
> Komine
>
>
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