High-level methods

Several ‘high level’ methods (functions) have been implemented for RasterLayer objects. ‘High level’ refers to methods that you would normally find in a GIS program that supports raster data. Here we briefly discuss some of these. See the help files for more detailed descriptions.

The high-level methods have some arguments in common. The first argument is typically ‘x’ or ‘object’ and can be a RasterLayer, or, in most cases, a RasterStack or RasterBrick. It is followed by one or more arguments specific to the method (either additional RasterLayer objects or other arguments), followed by a filename=”” and “…” arguments.

The default filename is an empty character “”. If you do not specify a filename, the default action for the method is to return a raster object that only exists in memory. However, if the method deems that the raster object to be created would be too large to hold memory it is written to a temporary file instead.

The “…” argument allows for setting additional arguments that are relevant when writing values to a file: the file format, datatype (e.g. integer or real values), and a to indicate whether existing files should be overwritten.

Modifying a Raster* object

There are several methods that deal with modifying the spatial extent of Raster* objects. The crop method lets you take a geographic subset of a larger raster object. You can crop a Raster* by providing an extent object or another spatial object from which an extent can be extracted (objects from classes deriving from Raster and from Spatial in the sp package). An easy way to get an extent object is to plot a RasterLayer and then use drawExtent to visually determine the new extent (bounding box) to provide to the crop method.

trim crops a RasterLayer by removing the outer rows and columns that only contain NA values. In contrast, extend adds new rows and/or columns with NA values. The purpose of this could be to create a new RasterLayer with the same Extent of another larger RasterLayer such that the can be used together in other methods.

The merge method lets you merge 2 or more Raster* objects into a single new object. The input objects must have the same resolution and origin (such that their cells neatly fit into a single larger raster). If this is not the case you can first adjust one of the Raster* objects with use (dis)aggregate or resample.

aggregate and disaggregate} allow for changing the resolution (cell size) of a ```Raster*`` object. In the case of`aggregate, you need to specify a function determining what to do with the grouped cell values (e.g.mean). It is possible to specify different (dis)aggregation factors in the x and y direction.aggregateanddisaggregate` are the best methods when adjusting cells size only, with an integer step (e.g. each side 2 times smaller or larger), but in some cases that is not possible.

For example, you may need nearly the same cell size, while shifting the cell centers. In those cases, the resample method can be used. It can do either nearest neighbor assignments (for categorical data) or bilinear interpolation (for numerical data). Simple linear shifts of a Raster object can be accomplished with the shift method or with the extent method. resample should not be used to create a Raster* object with much larger resolution. If such adjustments need to be made then you can first use aggregate.

With the projectRaster method you can transform values of Raster* object to a new object with a different coordinate reference system.

Here are some simple examples:

library(raster)
r <- raster()
r[] <- 1:ncell(r)
ra <- aggregate(r, 10)
r1 <- crop(r, extent(-180,0,0,30))
r2 <- crop(r, extent(-10,180,-20,10))
m <- merge(r1, r2, filename='test.grd', overwrite=TRUE)
plot(m)

image0

bf lets you flip the data (reverse order) in horizontal or vertical direction – typically to correct for a ‘communication problem’ between different R packages or a misinterpreted file. rotate lets you rotate longitude/latitude rasters that have longitudes from 0 to 360 degrees (often used by climatologists) to the standard -180 to 180 degrees system. With t you can rotate a Raster* object 90 degrees.

Overlay

The overlay method can be used as an alternative to the raster algebra discussed above. Overlay, like the methods discussed in the following subsections provide either easy to use short-hand, or more efficient computation for large (file based) objects.

With overlay you can combine multiple Raster objects (e.g. multiply them). The related method mask removes all values from one layer that are NA in another layer, and cover combines two layers by taking the values of the first layer except where these are NA.

Calc

The calc method allows you to do a computation for a single raster object by providing a function. If you supply a RasterLayer, another RasterLayer is returned. If you provide a multi-layer object you get a (single layer) RasterLayer if you use a summary type function (e.g. sum) but a RasterBrick if multiple layers are returned. stackApply computes summary type layers for subsets of a RasterStack or RasterBrick.

Reclassify

You can use cut or reclassify to replace ranges of values with single values, or subs to substitute (replace) single values with other values.

r <- raster(ncol=3, nrow=2)
values(r) <- 1:ncell(r)
getValues(r)
## [1] 1 2 3 4 5 6
s <- calc(r, fun=function(x){ x[x < 4] <- NA; return(x)} )
as.matrix(s)
##      [,1] [,2] [,3]
## [1,]   NA   NA   NA
## [2,]    4    5    6
t <- overlay(r, s, fun=function(x, y){ x / (2 * sqrt(y)) + 5 } )
as.matrix(t)
##      [,1]     [,2]     [,3]
## [1,]   NA       NA       NA
## [2,]    6 6.118034 6.224745
u <- mask(r, t)
as.matrix(u)
##      [,1] [,2] [,3]
## [1,]   NA   NA   NA
## [2,]    4    5    6
v <- u==s
as.matrix(v)
##      [,1] [,2] [,3]
## [1,]   NA   NA   NA
## [2,] TRUE TRUE TRUE
w <- cover(t, r)
as.matrix(w)
##      [,1]     [,2]     [,3]
## [1,]    1 2.000000 3.000000
## [2,]    6 6.118034 6.224745
x <- reclassify(w, c(0,2,1,  2,5,2, 4,10,3))
as.matrix(x)
##      [,1] [,2] [,3]
## [1,]    1    1    2
## [2,]    3    3    3
y <- subs(x, data.frame(id=c(2,3), v=c(40,50)))
as.matrix(y)
##      [,1] [,2] [,3]
## [1,]   NA   NA   40
## [2,]   50   50   50

Focal

The focal method currently only works for (single layer) RasterLayer objects. It uses values in a neighborhood of cells around a focal cell, and computes a value that is stored in the focal cell of the output RasterLayer. The neighborhood is a user-defined a matrix of weights and could approximate any shape by giving some cells zero weight. It is possible to only compute new values for cells that are NA in the input RasterLayer.

Distance

There are a number of distance related methods. distance computes the shortest distance to cells that are not NA. pointDistance computes the shortest distance to any point in a set of points. gridDistance computes the distance when following grid cells that can be traversed (e.g. excluding water bodies). direction computes the direction towards (or from) the nearest cell that is not NA. adjacency determines which cells are adjacent to other cells, and pointDistance computes distance between points. See the gdistance package for more advanced distance calculations (cost distance, resistance distance)

Spatial configuration

The clump method identifies groups of cells that are connected. boundaries identifies edges, that is, transitions between cell values. area computes the size of each grid cell (for unprojected rasters), this may be useful to, e.g. compute the area covered by a certain class on a longitude/latitude raster.

r <- raster(nrow=45, ncol=90)
r[] <- round(runif(ncell(r))*3)
a <- area(r)
zonal(a, r, 'sum')
##      zone       sum
## [1,]    0  93604336
## [2,]    1 168894837
## [3,]    2 158110025
## [4,]    3  87822040

Predictions

The package has two methods to make model predictions to (potentially very large) rasters. predict takes a multilayer raster and a fitted model as arguments. Fitted models can be of various classes, including glm, gam, randomforest, and brt. method interpolate is similar but is for models that use coordinates as predictor variables, for example in kriging and spline interpolation.

Vector to raster conversion

The raster packages supports point, line, and polygon to raster conversion with the rasterize method. For vector type data (points, lines, polygons), objects of Spatial* classes defined in the sp package are used; but points can also be represented by a two-column matrix (x and y).

Point to raster conversion is often done with the purpose to analyze the point data. For example to count the number of distinct species (represented by point observations) that occur in each raster cell. rasterize takes a Raster* object to set the spatial extent and resolution, and a function to determine how to summarize the points (or an attribute of each point) by cell.

Polygon to raster conversion is typically done to create a RasterLayer that can act as a mask, i.e. to set to NA a set of cells of a raster object, or to summarize values on a raster by zone. For example a country polygon is transferred to a raster that is then used to set all the cells outside that country to NA; whereas polygons representing administrative regions such as states can be transferred to a raster to summarize raster values by region.

It is also possible to convert the values of a RasterLayer to points or polygons, using rasterToPoints and rasterToPolygons. Both methods only return values for cells that are not NA. Unlike rasterToPolygons, rasterToPoints is reasonably efficient and allows you to provide a function to subset the output before it is produced (which can be necessary for very large rasters as the point object is created in memory).

Summarize

When used with a Raster* object as first argument, normal summary statistics functions such as min, max and mean return a RasterLayer. You can use cellStats if, instead, you want to obtain a summary for all cells of a single Raster* object. You can use freq to make a frequency table, or to count the number of cells with a specified value. Use zonal to summarize a Raster* object using zones (areas with the same integer number) defined in a RasterLayer and crosstab to cross-tabulate two RasterLayer objects.

r <- raster(ncol=36, nrow=18)
r[] <- runif(ncell(r))
cellStats(r, mean)
## [1] 0.5179682
s = r
s[] <- round(runif(ncell(r)) * 5)
zonal(r, s, 'mean')
##      zone      mean
## [1,]    0 0.5144431
## [2,]    1 0.5480089
## [3,]    2 0.5249257
## [4,]    3 0.5194031
## [5,]    4 0.4853966
## [6,]    5 0.5218401
freq(s)
##      value count
## [1,]     0    54
## [2,]     1   102
## [3,]     2   139
## [4,]     3   148
## [5,]     4   133
## [6,]     5    72
freq(s, value=3)
## [1] 148
crosstab(r*3, s)
##        layer.2
## layer.1  0  1  2  3  4  5
##       0  8 13 21 16 24 10
##       1 17 31 42 56 45 24
##       2 19 31 52 54 37 27
##       3 10 27 24 22 27 11