Remove polygons having less than 4 coordinates in R

Question

I have a multiPolygon shapefile of buildings in a particular city. I read it in with readOGR and and transform it to my desired planar projection with the following code:

buildings_data<-readOGR("buildings.shp")
buildings<- spTransform(buildings_data, CRS("+proj=utm +zone=45 +a=6377299.151
                                        +b=6356098.145120132 
                                        +towgs84=295,736,257,0,0,0,0 +units=m +no_defs "))

It gives me the following output along with transforming:

There were 19 warnings (use warnings() to see them)

All the warnings are:

In Polygon(coords = crds) : less than 4 coordinates in polygon

So how do I remove these polygons with less than 4 coordinates?

Answer 1

Your code is correct. But you shapefile (buildings) seems to be corrupted and have some topology errors and geometry validity issues, just as here.

You need to inspect and correct your original building data. It is difficult to tell you where the problem lies without having access to it.

An option in R to further explore this is the cleangeo package.

install.packages("cleangeo")
library(cleangeo)

buildings_data <- readOGR("buildings.shp")
buildings<- spTransform(buildings_data, CRS("+proj=utm +zone=45 +a=6377299.151 +b=6356098.145120132 +towgs84=295,736,257,0,0,0,0 +units=m +no_defs"))
clgeo_IsValid(buildings) # This tells you if the geometry is valid
clgeo_Clean(buildings, errors.only = NULL, strategy = "POLYGONATION",verbose = FALSE)
clgeo_IsValid(buildings) # check again...

Or you can also explore and clean the original dataset with QGIS, ArcGIS or similar and check topology and geometry issues.