Move points to nearest raster cell • rSDM

library(rSDM)

rSDM includes a handy function to move points to the nearest raster cell with data (i.e. not NA).

Example data

Let’s generate example point coordinates:

locs <- data.frame(lon = c(1, 2, 1, 2, 2.2), lat = c(1.2, 1, 2.3, 3, 2))
locs.sf <- locs2sf(locs)
locs.sf
#> Simple feature collection with 5 features and 0 fields
#> Geometry type: POINT
#> Dimension:     XY
#> Bounding box:  xmin: 1 ymin: 1 xmax: 2.2 ymax: 3
#> Geodetic CRS:  WGS 84
#>        geometry
#> 1 POINT (1 1.2)
#> 2   POINT (2 1)
#> 3 POINT (1 2.3)
#> 4   POINT (2 3)
#> 5 POINT (2.2 2)

Now let’s generate an example raster:

library(terra)
#> terra 1.7.79

ras <- rast(nrows = 2, ncols = 2, xmin = 0.5, xmax = 3.5, ymin = 0.5, ymax = 3.5,
 resolution = 1, vals = c(NA, 1, 1, NA, NA, 1, NA, 1, 1))

As we can see in the map below, some points fall outside raster cells with data:

occmap(locs.sf, ras, pcol = "black", psize = 3)

Move point coordinates to nearest raster cell

moved <- points2nearestcell(locs.sf, ras)
#> 3 out of 5 points have been moved.
#>     X   Y X_new Y_new      distance moved
#> 1 1.0 1.2     2     1 113377.05 [m]  TRUE
#> 2 1.0 2.3     2     3 135632.95 [m]  TRUE
#> 3 2.2 2.0     3     2  88901.89 [m]  TRUE

Note you can choose the format of the output map, e.g. now using ggplot (interactive leaflet maps are available too):

points2nearestcell(locs.sf, ras, map = "ggplot")
#> 3 out of 5 points have been moved.
#>     X   Y X_new Y_new      distance moved
#> 1 1.0 1.2     2     1 113377.05 [m]  TRUE
#> 2 1.0 2.3     2     3 135632.95 [m]  TRUE
#> 3 2.2 2.0     3     2  88901.89 [m]  TRUE

#> Simple feature collection with 5 features and 0 fields
#> Geometry type: POINT
#> Dimension:     XY
#> Bounding box:  xmin: 2 ymin: 1 xmax: 3 ymax: 3
#> Geodetic CRS:  WGS 84
#>      geometry
#> 1 POINT (2 1)
#> 2 POINT (2 1)
#> 3 POINT (2 3)
#> 4 POINT (2 3)
#> 5 POINT (3 2)

The function returns a spatial object where the coordinates of the points falling outside the raster have been moved:

moved
#> Simple feature collection with 5 features and 0 fields
#> Geometry type: POINT
#> Dimension:     XY
#> Bounding box:  xmin: 2 ymin: 1 xmax: 3 ymax: 3
#> Geodetic CRS:  WGS 84
#>      geometry
#> 1 POINT (2 1)
#> 2 POINT (2 1)
#> 3 POINT (2 3)
#> 4 POINT (2 3)
#> 5 POINT (3 2)

Let’s compare the original and new coordinates:

sf::st_coordinates(locs.sf)
#>        X   Y
#> [1,] 1.0 1.2
#> [2,] 2.0 1.0
#> [3,] 1.0 2.3
#> [4,] 2.0 3.0
#> [5,] 2.2 2.0

sf::st_coordinates(moved)
#>      X Y
#> [1,] 2 1
#> [2,] 2 1
#> [3,] 2 3
#> [4,] 2 3
#> [5,] 3 2

If you don’t want to change coordinates but only check which would the nearest raster cells for each point, use move = FALSE.

Move points to nearest raster cell within a given distance

In case you want to move points only if the nearest raster cell is within a given distance, you could use the distance argument to set a threshold:

moved <- points2nearestcell(locs.sf, ras, distance = 100000)
#> 1 out of 5 points have been moved.
#>     X   Y X_new Y_new     distance moved
#> 1 1.0 1.2     1   1.2     0.00 [m] FALSE
#> 2 1.0 2.3     1   2.3     0.00 [m] FALSE
#> 3 2.2 2.0     3   2.0 88901.89 [m]  TRUE