R – Getting a New Column with Distance to the Nearest Feature

rsf

Having two sf data frames (a is points and b is polygons),
I can get columns from the nearest feature by for instance:

c <- st_join(a, b, st_nearest_feature),

But how is the best way to get an additional column with the distance as well?

Best Answer

Don't do the join. st_nearest_feature(a,b) will get you the index (row number) of the nearest feature in b to each feature in a.

EG using data p and l from ?st_nearest_feature made into sf data frame:

> (nearest = st_nearest_feature(p,l))
[1] 1 2 2 3

Then use st_distance to get the element-wise distances between each element of p and the corresponding element of l:

> (dist = st_distance(p, l[nearest,], by_element=TRUE))
[1] 0.10 0.01 0.01 0.10

You could use the nearest index to do the join like this:

> (pljoin = cbind(p, st_drop_geometry(l)[nearest,]))
Simple feature collection with 4 features and 2 fields
geometry type:  POINT
dimension:      XY
bbox:           xmin: 0.1 ymin: -0.1 xmax: 0.1 ymax: 0.9
epsg (SRID):    NA
proj4string:    NA
  id st_drop_geometry.l..nearest...         geometry
1  a                              A POINT (0.1 -0.1)
2  b                              B POINT (0.1 0.11)
3  c                              B POINT (0.1 0.09)
4  d                              C  POINT (0.1 0.9)

(Column names are a bit mashed up but maybe you can work with that)

Then add the distance:

> pljoin$dist = dist
>

Related Solutions

R – Calculate Distance Between Points and Nearest Polygon

Let's set up some sample data:

library(sp)
library(spdep)
example(columbus)
plot(columbus)

We'll now get some points. Click 20 times on the map, some inside and some outside polygons:

pts = locator(20,type="p")

Convert to Spatial data type:

spts = SpatialPoints(pts)

Now use rgeos to compute point-polygon distances, and take the minimum for each point:

library(rgeos)
> apply(gDistance(spts, columbus,byid=TRUE),2,min)
         1          2          3          4          5          6          7 
0.25947439 0.03898401 1.27156515 1.50490316 0.00000000 0.00000000 0.00000000 
         8          9         10         11         12         13         14 
0.00000000 0.00000000 0.31312329 0.26742466 0.53934325 0.07764322 0.03909773 
        15         16         17         18         19         20 
0.11156343 0.29243322 0.08872334 0.00000000 0.00000000 0.00000000

There you go. That's the minimum distance from each of the 20 points to any of the polygons. The zeroes are for points inside one of the polygons.

Note you should use data in a projected spatial coordinate system and not lat-long.

If you have 17,000 points then you might want to do this in smaller subsets to avoid creating a 17,000 x 5,000 matrix if you have 5,000 polygons. You didn't say.

R SF – How to Append a Geometry Column to a Data Frame Using the R Package sf

I will assume that you can use dplyr if not you can adapt to base R. Here's a solution

library(sf)
library(dplyr)

RST_WIDTH  <- 123

tab_ds <- data.frame(ux = c( 1,  2, -3),
                     uy = c(-1, -2, -3)) %>%
  mutate(w = RST_WIDTH, h = RST_WIDTH)

tab_ds %>%
  rowwise() %>%
  mutate(geometry = list(sf.rect.xywh(ux, uy, w, h))) %>%
  st_as_sf(sf_column_name = "geometry")

## Simple feature collection with 3 features and 4 fields
## geometry type:  POLYGON
## dimension:      XY
## bbox:           xmin: -64.5 ymin: -64.5 xmax: 63.5 ymax: 60.5
## CRS:            NA
## # A tibble: 3 x 5
## # Rowwise: 
##      ux    uy     w     h                                geometry
##   <dbl> <dbl> <dbl> <dbl>                               <POLYGON>
## 1     1    -1   123   123 ((-60.5 60.5, 62.5 60.5, 62.5 -62.5, -…
## 2     2    -2   123   123 ((-59.5 59.5, 63.5 59.5, 63.5 -63.5, -…
## 3    -3    -3   123   123 ((-64.5 58.5, 58.5 58.5, 58.5 -64.5, -…

Hope it helps

Best Answer

Related Solutions

R – Calculate Distance Between Points and Nearest Polygon

R SF – How to Append a Geometry Column to a Data Frame Using the R Package sf

Related Question