Finding location of specific value in spatial TIFF file using R?

Question

I want to find location of specific value in one spatial GIS TIFF file and then compare location to main file in order to find some information. for example consider this image which is my main image.

For example, I want to find location of values greater than 255 in following image.

I read both image using raster library.

Edit:
by location I mean longitude and latitude.

Answer 1

Always a good idea to make a simple test. Here's a 5x6 raster:

> r = raster(matrix(runif(30),5,6))

which has these values:

> r[]
 [1] 0.45825245 0.68935493 0.18918221 0.32181772 0.90459237 0.47537165
 [7] 0.38745306 0.57012344 0.21467928 0.24150341 0.54005789 0.09430709
[13] 0.52352994 0.01489950 0.54890148 0.04762656 0.83931822 0.14997015
[19] 0.02994982 0.89359828 0.22488212 0.55764499 0.30531830 0.13751166
[25] 0.90074801 0.94862034 0.88637761 0.13849934 0.64490219 0.67825152

and its in a (0,1) unit square:

> extent(r)
class      : Extent 
xmin       : 0 
xmax       : 1 
ymin       : 0 
ymax       : 1 

Now we use the coordinates function which gets the coordinates of the grid points, and subset those that match whatever condition we want. For example, all grid points where the raster is over 0.9:

> coordinates(r)[r[]>0.9,]
              x   y
[1,] 0.75000000 0.9
[2,] 0.08333333 0.1
[3,] 0.25000000 0.1

Alternatively, mask out the condition with NA and convert to Spatial Points:

> r = raster(matrix(runif(30),5,6))
> r[r[] < 0.9] = NA
> (pts = as(r, "SpatialPoints"))
class       : SpatialPoints 
features    : 3 
extent      : 0.25, 0.9166667, 0.5, 0.7  (xmin, xmax, ymin, ymax)
crs         : NA 

That's useful if you want to do further spatial work with the sp package. If you just want the coordinates from that then:

> coordinates(as(r, "SpatialPoints"))
             x   y
[1,] 0.2500000 0.7
[2,] 0.9166667 0.7
[3,] 0.5833333 0.5