convert matrix to raster in R

Question

I have a matrix data with spatial coordinates and one variable. The spatial resolution is 1000 meters.

> str(dat1)
> List of 3
> $ x: num [1:710] 302340 303340 304340 305340 306340 ...
> $ y: num [1:1241] 5431470 5432470 5433470 5434470 5435470 ...
> $ z: num [1:710, 1:1241] 225 225 225 225 225 ...

I want to convert it into raster format.

> dat1$x[1:10]
> [1] 302339.6 303339.6 304339.6 305339.6 306339.6 307339.6 308339.6 309339.6 310339.6 311339.6
> dat1$y[1:10]
>  [1] 5431470 5432470 5433470 5434470 5435470 5436470 5437470 5438470 5439470 5440470

I used the following code to do it. But the resolution I get is not the same with that I have. Any better way to get the same resolution with my real data?

> r <-raster(
             dat1$z,
             xmn=range(dat1$x)[1], xmx=range(dat1$x)[2],
             ymn=range(dat1$y)[1], ymx=range(dat1$y)[2], 
             crs=CRS("+proj=utm +zone=11 +datum=NAD83")
            )
> r

class       : RasterLayer 
dimensions  : 710, 1241, 881110  (nrow, ncol, ncell)
resolution  : 571.3135, 1746.479  (x, y)
extent      : 302339.6, 1011340, 5431470, 6671470  (xmin, xmax, ymin, ymax)
coord. ref. : +proj=utm +zone=11 +datum=NAD83 
data source : in memory
names       : layer 
values      : 13.65059, 248.6229  (min, max)

Solution

Try reading the help for raster. When creating a raster from a matrix, the sense of rows and columns isn't what you think it is. You were feeding it a 1241x710 matrix but taking the max and min from the wrong vectors.

Try the following:

> # small version of your test set
> dat1=list()
> dat1$x=seq(302339.6,by=1000,len=71)
> dat1$y=seq(5431470,by=1000,len=124)
> dat1$z=matrix(runif(71*124),71,124)
> str(dat1)
List of 3
 $ x: num [1:71] 302340 303340 304340 305340 306340 ...
 $ y: num [1:124] 5431470 5432470 5433470 5434470 5435470 ...
 $ z: num [1:71, 1:124] 0.765 0.79 0.185 0.461 0.421 ...
> image(dat1,asp=1)

Nice square pixels. Now create your raster:

r <-raster(
             dat1$z,
             xmn=range(dat1$x)[1], xmx=range(dat1$x)[2],
             ymn=range(dat1$y)[1], ymx=range(dat1$y)[2], 
             crs=CRS("+proj=utm +zone=11 +datum=NAD83")
            )
plot(r)

Totally NON-square pixels. And if you look carefully, the matrix is rotated 90 degrees from the image plot. Or transposed or something.

Solution: just create the raster from the x,y,z list:

 > r=raster(dat1);plot(r)

Square pixels, same way round as image plot, and resolution is now what you expect:

> r
class       : RasterLayer 
dimensions  : 124, 71, 8804  (nrow, ncol, ncell)
resolution  : 1000, 1000  (x, y)
extent      : 301839.6, 372839.6, 5430970, 5554970  (xmin, xmax, ymin, ymax)
coord. ref. : NA 
data source : in memory
names       : layer 
values      : 7.738103e-05, 0.9995497  (min, max)

Comments

[Jian Zhang]

The resolution should be calculated like this: '> (1011340-302339.6)/710 [1] 998.5921 > (6671470 - 5431470)/1241 [1] 999.1942

[Dominik]

Since I just spent 4 hours on this, I'll add to this answer. What Spacedman means when he says "the sense of rows and columns isn't what you think it is" he means that a rasterlayer is organized by row, i.e. left-right, top-bottom, while a matrix is organized by column, i.e. top-bottom, left-right. so in this case, dat1[15] will give you a different result than r[15]. I personally don't think this is spelled out clearly enough in the raster manual.

[Mikko]

You can also transpose the matrix (m): m <- t(m); raster(m[nrow(m):1,]). Source: gis.stackexchange.com/questions/60387/…