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I used extract() function to get remote sensing image cell value.The same code to process tif file produce error message. Code as follows：

r1 <- raster("2000_2605_1.1_km_monthly_NDVI.tif")
polygon_mus <- extent(c(107, 111, 37, 40))
cell <- extract(r1, polygon_mus)

This *.tif is a output file from MODI hdf file with MRT. The process result is as follow:

Error in (function (classes, fdef, mtable)  : 
  unable to find an inherited method for function extent?for signature ?NULL"?

Where is the error? I seem to be missing it.

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  • Did you check if polygon_mus and r1 overlay? (e.g. by plotting both together)
    – Iris
    May 23, 2016 at 15:29
  • Unfortunatly, I can only guess without a reproducable example (stackoverflow.com/questions/5963269/…).
    – Iris
    May 23, 2016 at 15:29
  • Ha~It's amazing~Thanks for your help Iris
    – Iris
    May 24, 2016 at 1:32
  • The problem is overlay error, but now I have some problems to make them have same extent.I am a student ,so I have too much to learn.
    – Iris
    May 24, 2016 at 1:47

1 Answer 1

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Your raster and your polygon do not overlap. If they do:

> r1 = raster(extent(c(107,110,38,40)))
> polygon_mus <- extent(c(107, 111, 37, 40))
> cell <- extract(r1, polygon_mus)

Then that works. If they don't:

> r1 = raster(extent(c(112,114,38,40)))
> cell <- extract(r1, polygon_mus)
Error in (function (classes, fdef, mtable)  : 
  unable to find an inherited method for function ‘extent’ for signature ‘"NULL"’

I get your error message. Check the extent(r1) to see where it is. I'll bet it doesn't overlap the extent you defined in polygon_mus.

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  • The r1 is a rasterlayer with coordinate,but the polygon_mus is a custom extent class without any coordinate.I try to reproject polygon_mus to the same project with r1,but got error message : Error in rgdal::rawTransform(projfrom, projto, nrow(xy), xy[, 1], xy[, : no arguments in initialization list
    – Iris
    May 24, 2016 at 1:43
  • Thank you very much! I found the polygon_mus's extent as follow: > polygon_mus class : Extent xmin : 107 xmax : 111 ymin : 37 ymax : 40 **but, the extent of r1 is **> extent(r1) class : Extent xmin : 8895604 xmax : 10007555 ymin : 3335852 ymax : 4447802
    – Iris
    May 24, 2016 at 1:44

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