Reading in a single polygon from a shapefile

Question

I have a shapefile/GML file with many thousands of polygons insides a specific layer.

I know the set of IDs that reference the set of polygons that I want to get the coordinates data from. Is there a way to get just this information easily, without having to load the whole shapefile layer into memory first?

I am currently using R with the rgdal package to load the shapefile/GML file, but am happy to look into other tools that might be able to help.

Answer 1

Time to ditch the shapefile! Here's a reproducible example using a combination of packages sf, gdalUtils and dplyr:

library(sf)
library(dplyr)
library(mapview)
library(gdalUtils)

## as an example we take nc shapefile from package sf
shpfile = system.file("shape/nc.shp", package = "sf")

## set working directory (where to save the converted file)
setwd("test_folder")

## covert to gpkg
target_file = gsub(".shp", ".gpkg", basename(shpfile))
ogr2ogr(src_datasource_name = shpfile, 
        dst_datasource_name = target_file, 
        f = "GPKG")

## use dplyr to connect to gpkg file
conn = target_file
db = src_sqlite(conn)

## read fid 10 from gpkg file
pol = tbl(db, "nc") %>% 
  filter(fid == 10) %>% 
  collect %>% 
  st_sf(crs = 4267)

## view with mapview
mapview(pol)

## check with original
mapview(read_sf(shpfile), col.regions = "grey") + pol

This is possible because gpkg is essentially a self-contained database. Therefore we can read using dplyr database functionality and hence filter by fid before collecting the data into memory. Afterwards we convert to a spatial object (sf in this case).