问题
I have a Spatialline that I converted from a polygon shapefile (digitized manually based on features in "imagebrick" - this means that spatially the "polyline" and "imagebrick" are overlapped as I wanted)
polyline <- as(shapefiles_data[1,],"SpatialLines")
> polyline
class : SpatialLines
features : 1
extent : 357714.3, 357719, 4076030, 4076035 (xmin, xmax, ymin, ymax)
coord. ref. : +proj=utm +zone=11 +datum=NAD27 +units=m +no_defs +ellps=clrk66 +nadgrids=@conus,@alaska,@ntv2_0.gsb,@ntv1_can.dat
And a Rasterbrick
> imagebrick
class : RasterBrick
dimensions : 29180, 14940, 435949200, 4 (nrow, ncol, ncell, nlayers)
resolution : 0.6, 0.6 (x, y)
extent : 354038.4, 363002.4, 4066992, 4084500 (xmin, xmax, ymin, ymax)
coord. ref. : +proj=utm +zone=11 +datum=NAD27 +units=m +no_defs +ellps=clrk66 +nadgrids=@conus,@alaska,@ntv2_0.gsb,@ntv1_can.dat
names : band1, band2, band3, band4
min values : 0, 0, 0, 0
max values : 65535, 65535, 65535, 65535
I was trying to extract pixels in the Rasterbrick that touched by the Line(SpatialLine). But I also wanted to pull out xy coordinates associated with those extracted pixels and store them together in a SpatialPointsDataFrame. I've tried:
extract_polyline_test <- extract(imagebrick, polyline)
extract_polyline_test <- as.data.frame(extract_polyline_test)
> extract_polyline_test
band1 band2 band3 band4
1 125 156 73 392
2 129 161 80 366
3 156 208 122 374
4 142 175 97 330
5 102 117 31 389
6 117 143 57 381
7 162 221 127 413
8 213 302 204 405
.. ... ... ... ...
Then, I was trying to use RasterToPoints function to pull out the xy coordinates. This requires a raster layer that contains all the pixels that I want xy from. I've tried to use "crop" to get that raster "line" but it does not work.
rastercrop_polyline <-crop(imagebrick,polyline)
rastercrop_polyline
> rastercrop_polyline
class : RasterBrick
dimensions : 8, 7, 56, 4 (nrow, ncol, ncell, nlayers)
resolution : 0.6, 0.6 (x, y)
extent : 357714.6, 357718.8, 4076030, 4076035 (xmin, xmax, ymin, ymax)
coord. ref. : +proj=utm +zone=11 +datum=NAD27 +units=m +no_defs +ellps=clrk66 +nadgrids=@conus,@alaska,@ntv2_0.gsb,@ntv1_can.dat
data source : in memory
names : band1, band2, band3, band4
min values : 72, 63, 0, 156
max values : 274, 415, 299, 497
s.polyline <- rasterToPoints(rastercrop_polyline)
> s.polyline
x y band1 band2 band3 band4
[1,] 357714.9 4076035 93 96 17 342
[2,] 357715.5 4076035 72 63 0 377
[3,] 357716.1 4076035 90 93 17 371
[4,] 357716.7 4076035 125 156 73 392
[5,] 357717.3 4076035 129 161 80 366
[6,] 357717.9 4076035 156 208 122 374
[7,] 357718.5 4076035 142 175 97 330
[8,] 357714.9 4076034 100 108 25 326
.. ........ ....... ... .. .. ...
It seems that the "crop" function does not crop a "line" of raster but a whole "polygon". Is there any other way to get the xy coordiantes of the pixels that I extracted?
p.s. I was also trying to extract surrounding pixels of each extracted pixels. How should I do this? I notice that there is a function parameter in extract function. Should I work on writing a small function within the extraction function or there are any other ways to do this?
p.p.s I was also thinking to use Rasterize function to rasterize the "SpatialLine" and then use the RasterToPoints to pull out the xy coordinates. But I can not control each pixel of the "rasterized SpatialLine" equals to the pixels that touched by the "line" in the original imagebrick.
Thanks everyone for your time viewing my question and thanks in advance for your help.
回答1:
You say that you were "thinking to use Rasterize function to rasterize the "SpatialLine" and then use the RasterToPoints to pull out the xy coordinates."
That seems correct for what you want
But then you say that you "can not control each pixel of the "rasterized SpatialLine" equals to the pixels that touched by the "line" in the original imagebrick."
But they are, except that cells with multiple lines are only represented once.
Another approach:
# set up some example data
r <- raster()
values(r) <- runif(ncell(r))
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 <- SpatialLines(list(Lines(list(Line(cds1)), "1"), Lines(list(Line(cds2)), "2") ))
# values for lines
v <- extract(r, lines)
# create raster with cell numbers
ids <- init(r, v='cell')
# extract these
cells <- extract(ids, lines)
# compute xy
xy = lapply(cells, function(x) xyFromCell(r, x))
来源：https://stackoverflow.com/questions/22469879/extract-raster-pixel-value-as-well-as-pixel-xy-coordinates-using-a-spatialline-o