[GIS] Creating empty Polygons or SpatialPolygons in R

polygonpolygon-creationrsp

I'm working on a project where I'm looking at overlap in users circling an area in an image. Esentially, I make a SpatialPolygonsDataFrame where each user gets one row. Some users don't select anything. So, I'm trying to figure out how to include that information. When I attempt to create a "NULL" polygon, and then scale it up, either with something like

Polygon(matrix(rep(NA, 4), nrow=2)

Polygon(NULL)

or anysuch, none of these work.

1) Can I create an empty polygon?
2) or add a row to a SpatialPolygonsDataFrame with no matching SpatialPolygons object?

Best Answer

To create an empty SpatialPolygons object use SpatialPolygons(list())

credit to jebyrnes, but I thought it should be posted as an answer not a comment.

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[GIS] Add Polygons to SpatialPolygons via loop / iteration using R

Yes, using a lapply, e.g.:

x = c(0,1,1,0,0)
y = c(0,0,1,1,0)
bbox_matrix_sp = cbind(rep(x,13),rep(y,13))
require(sp)
sp_re_alle = SpatialPolygons(lapply(1:13, function(x) Polygons(list(Polygon(bbox_matrix_sp[((x-1)*5+1):(x*5),])), paste0("reh",x))))

in case you are converting grids into polygons, there are direct conversion methods in sp, as in

demo(meuse, ask = FALSE) 
m.p = as(meuse.grid, "SpatialPolygonsDataFrame")

R – How Spatial Polygon %over% Polygon Works When Aggregating Values

Thanks for the clear question and reproducible example.

Your understanding is correct, and this boils down to a bug in rgeos::over, which was fixed a month ago but has not made it into a CRAN release yet. The following is a work-around if you're only interested in the number of intersections:

world.map$val = sapply(over(geometry(world.map), pointbuff.spdf, returnList = TRUE), NROW)

I'm using NROW here instead of length so that it works with the wrong rgeos (0.3-8, from CRAN) as well as the corrected (0.3-10, from r-forge). The earlier suggestion of using

a = aggregate(pointbuff.spdf, world.map, sum)

also counts the number of intersections, but only with the fixed rgeos version installed. Its advantage, besides a more intuitive name, is that it directly returns a Spatial object, with the geometry of world.map.

To get rgeos 0.3-8 working, add

setMethod("over",
    signature(x = "SpatialPolygons", y = "SpatialPolygonsDataFrame"),
        rgeos:::overGeomGeomDF)

to your script, before you use over.

Best Answer

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