Extract random coordinate points using classes (ranges) of values from a raster image

Question

I want to use a raster to stratify an image using different texture. I have a raster image (ndvi_2019) for which I want to extract random coordinates as points from each classes. First, I want to form these create these classes I want to extract these coordinates by classing the values in the ndvi raster object as below;

class 1= <0
class 2= 0:0.1
class 3= 0.1:0.2
class 4 = >0.2

The assumption is that the classification forms the different strata which can then indicate different strata.

ndvi_2019<- (choma_clip[[4]]- choma_clip[[3]])/(choma_clip[[4]]+choma_clip[[3]]) 
r<- ndvi_2019

Attempt:

r<- setValues(r, rnorm(ncell(r)))
pts<- spsample(as(extent(r@extent, "SpatialPolygons"), 30, type="random")) class1<- r/r <0 
class2<- r/r[r== 0:0.1]]
class3<- r/r[[r== 0.1:0.2]]
class4 <- r/r>0.2

Answer 1

I don't think you need to create new objects if all you want to do is sample points in a raster according to a criteria. You can simply filter the cells you want using the which() function.

Using the raster library, you can call the values(r) function to get a list of values, or just use r[] (where r is the raster object.)

The which() function will tell you which cells fit each criteria. Once you have that list, you can sample() on it to get a sample. Lastly, the coordinates() function will return the coordinates for each cell of the raster object you've selected. The coordinates(r) function will return coordinates of all cells, so you have to use the square brackets to define which rows you want (the sampled rows), as well as both columns (x and y).

library(raster)
numPts <- 30 # Or however many points you want

r <- raster("path/to/your/raster")
sampledCoords1 <- coordinates(r)[sample(which(r[]<0), numPts),1:2]
sampledCoords2 <- coordinates(r)[sample(which((r[]>=0)&(r[]<0.1)), numPts),1:2]
sampledCoords3 <- coordinates(r)[sample(which((r[]>=0.1)&(r[]<0.2)), numPts),1:2]
sampledCoords4 <- coordinates(r)[sample(which(r[]>=0.2), numPts),1:2]