I have two raster layer of dimension (7801, 7651). I want to compare each pixel of one raster layer with the other and create a new raster which has the minimum pixel value among the initial two raster. That is, if any i,j pixel of raster 1 has value 25 and same i,j pixel of raster 2 has value 20, thus in the output raster the i,j pixel should be 20.
You can just use min with two raster layers.
Let's start with a reproducible example:
library(raster)
r1 <- raster(ncol = 5, nrow = 5)
r1[] <- 1:ncell(r1)
plot(r1)
r2 <- raster(ncol = 5, nrow = 5)
r2[] <- ncell(r2):1
par(mfrow = c(1,3))
plot(r1)
plot(r2)
Now we calculate the min of each overlapping cell within the two raster layers very easily with the implemented cell statistics:
r3 <- min(r2, r1)
plot(r3)
Furthermore, you can also apply statistics like mean, max, etc.
If the implemented statistics somehow fail, or you want to use your own statistics, you can also directly access the data per pixel. That is, you first copy one of the raster layers.
r3 <- r1
Afterwards, you can apply a function over the values.
r3[] <- apply(cbind(r1[], r2[]), 1, min)
Using #loki's example, you have three more options to calculate minimum value for both layers:
library(raster)
calc(stack(r1,r2),fun=min,na.rm=T)
stackApply(stack(r1,r2),indices = c(1,1),fun='min',na.rm=T)
overlay(r1,r2,fun=min,na.rm=T)
Related
I'm trying to calculate the majority value in a categorical raster data set in R, for example using land cover data. This would be similar to the focal statistics tool in ArcGIS using the majority statistic. I am able to calculate the majority land cover type using a rectangular moving window and the modal function:
library(raster)
# create data
r <- raster(nrows = 120, ncol = 120, xmn=0)
r[] <- sample(3, ncell(r), replace=TRUE)
a<-focal(r, w=matrix(1,3,3), fun=modal) # 3x3 moving window
plot(a)
However, when I apply a focal weight to define my circular moving window, the output values appear to be treated as continuous numbers and not discrete values, even if I convert the raster to a factor:
#convert to a factor factor
r.f<-as.factor(r)
#set up window
fw <- focalWeight(r.f, 4.5, type='circle')
#apply focal fxn
r.f.focal<-focal(r.f, w=fw, fun=modal, na.rm=TRUE)
It appears zeros might be added into the analysis and that is creating the problem. Can anyone steer me in the right direction?
I think it will be fixed if d (which is now 4.5) is an integer. Your defined radius should be based on cell counts (1,2,3,...,n). However, still, the output will be in floating format as each xij in the kernel is a floating point with sum of 1. To achieve an integer output there is also a third option.
library(raster)
set.seed(070319)
# create data
r <- raster(nrows = 120, ncol = 120, xmn=0)
r[] <- sample(3, ncell(r), replace=TRUE)
a<-focal(r, w=matrix(1,3,3), fun=modal) # 3x3 moving window
par(mfrow=c(1,2))
plot(r)
plot(a)
#set up window
fw <- focalWeight(r, 4, type='circle')
#apply focal fxn
r.f.focal<-focal(r, w=fw, fun=modal, na.rm=TRUE)
par(mfrow=c(1,2))
plot(r)
plot(r.f.focal)
for integer output you can also do this:
#set up window
fw <- ceiling(focalWeight(r, 4, type='circle'))#for integer output
#apply focal fxn
r.f.focal<-focal(r, w=fw, fun=modal, na.rm=TRUE)
par(mfrow=c(1,2), oma=c(0,0,0,1))
plot(r)
plot(r.f.focal)
I have a grid that contains gaps (NAs) that I want to fill using interpolation. My grid shows autocorrelation in the x and y dimensions, so I would like to try bilinear interpolation. Most of the solutions I have found are focused on 'upsampling' (interpolation for the purpose of increasing number of samples/size of grid), but I do not want/need to change the grid size. I just want to fill NAs using interpolation. Other potential solutions do not seem to handle NAs for the input grid of values (the 'z matrix'), or are neighborhood-based solutions rather than bilinear interpoloation, or simply have no answer.
I found that with the raster package, I can input a grid (as a raster) that contains NAs, and use the 'resample' command to output a grid of the same size. However, the results look like nearest neighbor interpolation rather than bilinear interpolation.
Am I missing something such that there is a way to do bilinear interpolation with the raster package? Or is there a better way to do bilinear interpolation simply to fill NAs?
library(raster)
# raster containing gap
r <- raster(nrow=10, ncol=10)
r[] <- 1:ncell(r)
r[25] <- NA
# The s raster is the same size as the r raster
s <- raster(nrow=10, ncol=10)
s <- resample(r, s, method='bilinear')
plot(r)
plot(s)
s[25]
s[35]
# s[25] appears to have been filled with neighbor s[35]
UPDATE
The Akima package seems like a promising alternative to the raster approach above, but I'm having trouble if there are NAs in the input grid of values (the Z matrix). Here's an example parallel to the example above to demonstrate. (Again, I'm interpolating to a grid the same size as the original).
library(akima)
# Use bilinear interpolation (no NAs in input)
rmat<-matrix(seq(1,100,1), nrow = 10, ncol = 10, byrow = T)
x <- seq(1,10,1)
y <- seq(1,10,1)
smat <- bilinear.grid(x, y, rmat, nx = 10, ny = 10) # works
plot(raster(rmat), main = "original")
plot(raster(smat$z), main = "interpolated")
# Try using bilinear interpolation but with an NA
rmat<-matrix(seq(1,100,1), nrow = 10, ncol = 10, byrow = T)
rmat[3,5] <- NA
x <- seq(1,10,1)
y <- seq(1,10,1)
smat <- bilinear.grid(x, y, rmat, nx = 10, ny = 10) # Error about NAs
UPDATE2
There was a great question from #Robert Hijmans about why not use a moving window average with the focal() command in the raster package. The reason is that I want to try bilinear interpolation, and I don't think a moving window average always gives the same answer as bilinear interpolation. However, this was not clear in the example I posted (in that example moving window and bilinear interp do give the same answer), so I'll demonstrate in a new example below. Note that the bilinear interpolation solution should be 8 for the example below (here is a handy calculator for tests).
library(raster)
r <- raster(nrow=10, ncol=10)
# Different grid values than earlier examples
values(r) <- c(rep(1:5, 4), rep(4:8, 4), rep(1:5, 4), rep(4:8, 4), rep(1:5, 4))
r[25] <- NA
plot(r)
# See what the mean of the moving window produces
f <- focal(r, w=matrix(1,nrow=3, ncol=3), fun=mean, NAonly=TRUE, na.rm=TRUE)
f[25] # Moving window gives 5 but bilinear interp gives 8
# Note that this seems to be how the moving window works with equal weights
window_test <- c(r[14:16], r[24:26], r[34:36])
mean(window_test, na.rm = T)
Am I missing something here? Maybe there is something clever with the weights argument of focal() that can produce a bilinear interpolation solution?
Let's use equal distance cells to avoid differences because of cell size variation with lon/lat data
library(raster)
r <- raster(nrow=10, ncol=10, crs='+proj=utm +zone=1 +datum=WGS84', xmn=0, xmx=1, ymn=0, ymx=1)
For this example, you might use focal
values(r) <- 1:ncell(r)
r[25] <- NA
f <- focal(r, w=matrix(1,nrow=3, ncol=3), fun=mean, NAonly=TRUE, na.rm=TRUE)
I see that you dismiss "neighborhood-based solutions rather than bilinear interpoloation". But the question is why. In this case, you may want a neighborhood-based solution.
Update. Then again, in case of cells that are not approximately square, bilinear would be preferable.
values(r) <- c(rep(1:5, 4), rep(4:8, 4), rep(1:5, 4), rep(4:8, 4), rep(1:5, 4))
r[25] <- NA
The problem with bilinear interpolation normally uses 4 contiguous cells, but in this case, where you want the value for the center of a cell, the appropriate cell would be the value of the cell itself, because the distance to that cell is zero, and thus that is where the interpolation ends up. For example, for cell 23
extract(r, xyFromCell(r, 23))
#6
extract(r, xyFromCell(r, 23), method='bilinear')
#[1] 6
In this case the focal cell is NA, so you get the average of the focal cell and 3 more cells. The question is which three? It is arbitrary, but to make it work, the NA cell must get a value. The raster algorithm assigns the value below the NA cell to that cell (also 8 here). This works well, I think, to deal with NA values at edges (e.g. land/ocean), but perhaps not in this case.
`
extract(r, xyFromCell(r, 25))
#NA
extract(r, xyFromCell(r, 25), method='bilinear')
#[1] 8
That is also what resample gives
resample(r, r)[25]
# 8
Is this what the on-line calculator suggests too?
This is very sensitive to small changes
extract(r, xyFromCell(r, 25)+0.0001, method='bilinear')
#[1] 4.998997
What I would really want in this case is the mean of the rook-neighbors
mean(r[adjacent(r, 25, pairs=FALSE)])
[1] 6
Or, more generally, the local inverse distance weighted average. You can compute
that by setting up a weights matrix with focal
# compute weights matrix
a <- sort(adjacent(r, 25, 8, pairs=F, include=TRUE))
axy <- xyFromCell(r, a)
d <- pointDistance(axy, xyFromCell(r, 25), lonlat=F)
w <- matrix(d, 3, 3)
w[2,2] <- 0
w <- w / sum(w)
# A simpler approach could be:
# w <- matrix(c(0,.25,0,.25,0,.25,0,.25,0), 3, 3)
foc <- focal(r, w, na.rm=TRUE, NAonly=TRUE)
foc[25]
In this example this is fine; but it would not be correct if there were multiple NA values in the focal area (as the sum of weights would no longer be 1). We can correct for that by computing the sum of weights
x <- as.integer(r/r)
sum_weights <- focal(x, w, na.rm=TRUE, NAonly=TRUE)
fw <- foc/sum_weights
done <- cover(r, fw)
done[25]
I would like to know if there is way to create zonal statistics for RasterLayerObjects, specifically the count of a given cell value (e.g. a land-use class) in R without having to reclassify the whole raster. The solution should be memory efficient in order to work on large raster files i.e. no extraction of the values into a matrix in R is desired.
Below an example of how I handle it until now. In this case I reclassify the original raster to hold only 1 for the value of interest and missings for all other values.
My proposed solution creates both, redundant data and additional processing steps to get me to my initial goal. I thought something like zonal(r1[r1==6],r2,"count") would work but obviously it does not (see below).
# generate reproducible Raster
library("raster")
## RASTER 1 (e.g. land-use classes)
r1 <- raster( crs="+proj=utm +zone=31")
extent(r1) <- extent(0, 100, 0, 100)
res(r1) <- c(5, 5)
values(r1) <- sample(10, ncell(r1), replace=TRUE)
plot(r1)
## RASTER 2 (containing zones of interest)
r2 <- raster( crs="+proj=utm +zone=31")
extent(r2) <- extent(0, 100, 0, 100)
res(r2) <- c(5, 5)
values(r2) <- c(rep(1,100),rep(2,100),rep(3,100),rep(4,100))
plot(r2)
# (1) ZONAL STATISTICS
# a. how many cells per zone (independent of specific cell value)
zonal(r1,r2,"count")
# b. how many cells per zone of specific value 6
zonal(r1[r1==6],r2,"count")
# -> fails
# with reclassification
r1.reclass<-
reclassify(r1,
matrix(c(1,5,NA,
5.5,6.5,1, #class of interest
6.5,10,NA),
ncol=3,
byrow = T),
include.lowest=T # include the lowest value from the table.
)
zonal(r1.reclass,r2,"count")
you can use raster::match.
zonal(match(r1, 6),r2, "count")
As you can see from plot(match(r1, 6)), it only returns raster cells which hold the desired value(s). All other cells are NA.
r1==6 as used in your try unfortunately returns a vector and therefore cannot be used in focal anymore.
I have two thematic raster layers r1 and r2 for same area each following same classification scheme and has 16 classes. I need to find minimum distance between cell of r1 and cell of r2 but with same value. E.g. nth cell in r1 has value 10 and coordinates x1,y1. And in r2, there are 2 cells with value 10 and coordinates x1+2,y1+2 and x1-0.5,y1-0.5. Thus the value that I need for this cell would be 0.5,0.5.
I tried distance from raster package but it gives distance, for all cells that are NA, to the nearest cell that is not NA. I am confused as to how can I include second raster layer into this.
You can use knn from class package so that for each cell of r1 find index of nearest cell of r2 with the same category:
library(class)
library(raster)
#example of two rasters
r1 <- raster(ncol = 600, nrow = 300)
r2 <- raster(ncol = 600, nrow = 300)
#fill each with categories that rabge from 1 to 16
r1[] <- sample(1:16, ncell(r1), T)
r2[] <- sample(1:16, ncell(r2), T)
# coordinates of cells extracted
xy = xyFromCell(r1, 1:ncell(r1))
#multiply values of raster with a relatively large number so cells thet belong
#to each category have smaller distance with reagrd to other categories.
v1 = values(r1) * 1000000
v2 = values(r2) * 1000000
# the function returns indices of nearest cells
out = knn(cbind(v2, xy) ,cbind(v1, xy) ,1:ncell(r1), k=1)
So, use rasterToPoints to extract SpatialPoints object for unique thematic class. Then use the sp::spDists function to find the distance between your points.
library(raster)
r1 <- raster( nrow=10,ncol=10)
r2 <- raster( nrow=10,ncol=10)
set.seed(1)
r1[] <- ceiling(runif(100,0,10))
r2[] <- ceiling(runif(100,0,10))
dist.class <- NULL
for(i in unique(values(r1))){
p1 <- rasterToPoints(r1, fun=function(xx) xx==i, spatial=T)
p2 <- rasterToPoints(r2, fun=function(xx) xx==i, spatial=T)
dist.class[i] <- min(spDists(p1,p2))
}
cbind(class = unique(values(r1)),dist.class)
The loop may not be efficient for you. If it's a problem, wrap it into a function and lapply it. Also, be carefull with your class, if they aren't 1:10, my loop won't work. If your projection is in degree, you will probably need the geosphere package to get accurate results. But the best in that case I think is to use a projection in meters.
A memory safe approach using the raster-package would be to use the layerize() function to split up your raster value into a stack of binary rasters (16 in your case) and then use the distance() function to compute distances in the layers of r2, masking them with the respective layers of r1. Something like this:
layers1 <- layerize(r1, falseNA=TRUE)
layers2 <- layerize(r2, falseNA=TRUE)
# now you can loop over the layers (use foreach loop if you want
# to speed things up using parallel processing)
dist.stack <- layers1
for (i in 1:nlayers(r1)) {
dist.i <- distance(layers2[[i]])
dist.mask.i <- mask(dist, layers1[[i]])
dist.stack[[i]] <- dist.mask.i
}
# if you want pairwise distances for all classes in one layer, simply
# combine them using sum()
dist.combine <- sum(dist.stack, na.rm=TRUE)
I want to get a pairwise distance matrix between the centroids of every cell with every other cell in a raster layer. As well, I'm trying to figure out how to get the pairwise matrix of differences between values within the raster layer.
I tried looking at the {gdistance} and {spatstat} packages but it doesn't seem like there is a function for this.
Here is an example:
df <- data.frame(x = rnorm(100, 5, 3), y = rnorm(100, 10, 6))
rast <- raster()
ncol <- 20
nrow <- 20
test <- rasterize(df, rast, FUN=mean)
plot(test, xlim=c(min(df$x), max(df$x)), ylim=c(min(df$y), max(df$x)))
Any ideas?