How to extract the values from a raster in R

3
votes

I want to use R to extract values from a raster. Basically, my raster has values from 0-6 and I want to extract for every single pixel the corresponding value. So that I have at the end a data table containing those two variables.

Thank you for your help, I hope my explanations are precisely enough.

3
rextractraster
have you tried this?Extract data from a Raster object for the locations of other spatial data – seralouk
Did you try things? Try vignette('Raster') for example – jbaums
@sera, I already found this tutorial but I am quite confused about what you actually have to use for the extraction. – JCra
maybe see here that has examples Example 1 Example 2 – seralouk
@sera, I understand the steps in the Example 1 tutorial. I loaded my raster in R and then I tried to extract my values with the extract() function. Due to the reason that I am not using an AOI, like in the tutorial I am confused now with the x and y values. I tried this with the function: values_raster <- extract(x = values, df=TRUE) but this is not working. – JCra

3 Answers

6
votes

Example data

library(raster)
r <- raster(ncol=5, nrow=5, vals=1:25)

To get all values, you can do

values(r)
# [1]  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
#as.matrix(r)
#     [,1] [,2] [,3] [,4] [,5]
#[1,]    1    2    3    4    5
#[2,]    6    7    8    9   10
#[3,]   11   12   13   14   15
#[4,]   16   17   18   19   20
#[5,]   21   22   23   24   25

Also see ?getValues

You can also use indexing

r[2,2] 
#7 
r[7:8]
#[1] 7 8

For more complex extractions using points, lines or polygons, see ?extract

1
votes

x is the raster object you are trying to extract values from; y is may be a SpatialPoints, SpatialPolygons,SpatialLines, Extent or a vector representing cell numbers (take a look at ?extract). Your code values_raster <- extract(x = values, df=TRUE) will not work because you're feeding the function with any y object/vector. You could try to build a vector with all cell numbers of your raster. Imagine your raster have 200 cells. If your do values_raster <- extract(x = values,y=seq(1,200,1), df=TRUE) you'll get a dataframe with values for each cell.

0
votes

How about simply doing

as.data.frame(s, xy=TRUE) # s is your raster file