Extract location of raster pixel along a transect

Question

I have the following raster

library(raster)
r <- raster(ncol=2421, nrow=5005)
r:
     class      : RasterLayer 
     dimensions : 2421, 5005, 12117105  (nrow, ncol, ncell)
     resolution : 1, 1  (x, y)
     extent     : 501121, 506126, 2809088, 2811509  (xmin, xmax, ymin, 
     ymax)
      crs        : +proj=utm +zone=17 +datum=WGS84 +units=m +no_defs 
     +ellps=WGS84 +towgs84=0,0,0 
     source     : E:/Datat Layers/Clip/Harney_XMerge.tif 
     names      : Harney_XMerge 
    values     : -3.126388e-13, 57.14  (min, max)

and the following transect in the form of polylines

Line:
class       : SpatialLinesDataFrame 
features    : 7 
extent      : 500864.6, 505506.2, 2809553, 2811079  (xmin, xmax, ymin, 
ymax)
crs         : +proj=utm +zone=17 +datum=WGS84 +units=m +no_defs 
+ellps=WGS84 +towgs84=0,0,0 
variables   : 3
names       : OBJECTID, Id,    Shape_Leng 
min values  :        1,  0, 2716.24783826 
max values  :        7,  0, 3188.64130203

I want to extract the coordinates of each pixel that falls along the transect but haven't been able to find any function that does so on my own.

I have used the following extract function but it only extracts the info from the raster in this case tree height data. Is there a way for me to use the extract function or another function to extract the UTM coordinates of each pixel that fall along my established transects?

 extract(r,line )

Robert Hijmans Robert Hijmans · Accepted Answer · 2019-08-07T05:40:35

When asking a question, please provide a minimal reproducible example. Have a look at the R help files for examples. For example (copied from the obvious place, ?raster::extract:

library(raster)
r <- raster(ncol=36, nrow=18, vals=1)
cds1 <- rbind(c(-50,0), c(0,60), c(40,5), c(15,-45), c(-10,-25))
cds2 <- rbind(c(80,20), c(140,60), c(160,0), c(140,-55))
lines <- spLines(cds1, cds2)

extract(r, lines)

In the documentation of extract (?extract) you can see that there is an argument cellnumbers. You can do

e <- extract(r, lines, cellnumbers = TRUE)

That returns a list with, for each polyline, a matrix with cellnumbers and values. From the cellnumbers you can get the coordinates.

f <- lapply(e, function(x) xyFromCell(r, x[,1]))

Or if you prefer a data.frame

e <- extract(r, lines, cellnumbers = TRUE, df=T)
d <- data.frame(ID=e[,1], xyFromCell(r, e[,2]))

head(d)
#     ID   x  y
#[1,]  1  -5 55
#[2,]  1   5 55
#[3,]  1 -15 45
#[4,]  1  -5 45
#[5,]  1   5 45
#[6,]  1  15 45

If you want need the points to be ordered along the line, use the extract argument along=TRUE

Extract location of raster pixel along a transect

1 Answers