Use sf polygon object as window in spatstat

Question

Hello all potential helpers,

I have a SpatialPolygonDataFrame object obtained from the tigris package and I would like to use it as a polyg

Answer 1

Note: I have edited this answer to contain full details.

As @TimSalabim mentions this is under way in sf, but until then you have to go through the old sp classes such as SpatialPolygons. Use something like as_Spatial in sf and then load maptools and use as.owin or as(x, "owin") on the Spatial object.

Furthermore, you can only use coordinates in planar (projected) space with spatstat and not coordinates on the curved surface of the earth. You have to project to a relevant planar coordinates system. Maybe <epsg.io/6345> is usable in this case. To project to this coordinate system use sf::st_transform(county_one, crs = 6345). Afterwards you convert to Spatial and then owin. Note: Choosing the relevant projection is a science, and I don’t know much about it, so do a bit of research if you want to make sure you don’t get too distorted results.

Specifically with the original example you can do:

# Needed packages
library(spatstat)
#> Loading required package: spatstat.data
#> Loading required package: nlme
#> Loading required package: rpart
#> 
#> spatstat 1.62-2       (nickname: 'Shape-shifting lizard') 
#> For an introduction to spatstat, type 'beginner'
library(sf)
#> Linking to GEOS 3.8.0, GDAL 3.0.2, PROJ 6.2.1
library(maptools)
#> Loading required package: sp
#> Checking rgeos availability: TRUE
library(tigris)
#> To enable 
#> caching of data, set `options(tigris_use_cache = TRUE)` in your R script or .Rprofile.
#> 
#> Attaching package: 'tigris'
#> The following object is masked from 'package:graphics':
#> 
#>     plot

county <- county_subdivisions(state = "Alabama", county = "Lee", class = "sf", progress_bar = FALSE)
county_one <- st_union(county)
plot(county_one)

county_flat <- st_transform(county_one, crs = 6345)
plot(county_flat)

county_owin <- as.owin(as_Spatial(county_flat))

100 random points in the county:

p <- runifpoint(100, win = county_owin)
plot(p)

Answer 2

just want to note here that coercion methods for sf classes are now registered (if that's the right word) by the sf package. I don't fully understand the R magic that finds methods, but it does work.

library(sf)
library(spatstat)

> methods(as.owin)
 [1] as.owin.boxx                    as.owin.data.frame              as.owin.default                 as.owin.distfun                
 [5] as.owin.dppm                    as.owin.funxy                   as.owin.im                      as.owin.influence.ppm          
 [9] as.owin.kppm                    as.owin.layered                 as.owin.leverage.ppm            as.owin.linfun                 
[13] as.owin.linnet                  as.owin.lintess                 as.owin.lpp                     as.owin.lppm                   
[17] as.owin.msr                     as.owin.MULTIPOLYGON*           as.owin.nnfun                   as.owin.owin                   
[21] as.owin.POLYGON*                as.owin.ppm                     as.owin.ppp                     as.owin.psp                    
[25] as.owin.quad                    as.owin.quadratcount            as.owin.quadrattest             as.owin.rmhmodel               
[29] as.owin.sf*                     as.owin.sfc*                    as.owin.sfc_MULTIPOLYGON*       as.owin.sfc_POLYGON*           
[33] as.owin.SpatialGridDataFrame*   as.owin.SpatialPixelsDataFrame* as.owin.SpatialPolygons*        as.owin.tess                   
see '?methods' for accessing help and source code

So, assuming you have properly projected your data (as noted by @Ege Rubak), calling as.owin directly should work:

library(tigris)

county <- county_subdivisions(state = "Alabama", 
                              county = "Lee", 
                              class = "sf", 
                              progress_bar = FALSE)

county <- st_union(county)

county <- st_transform(county, crs = 6345)

window <- as.owin(county)

plot(window)