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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.

  • Can you just subset the layer first? For example go into QGIS and select just the ids that you want and export a new version? – jbchurchill May 11 '17 at 16:53
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    I don't know if it's possible to read just one polygon from a Shapefile. However, I'd suggest you to have a look at the sf package, which at least will grant you a great improvement in reading speed. – lbusett May 11 '17 at 18:44
  • This can be done using OGRSQL on FID in recent versions of GDAL. It's long been possible, just not efficient until recently. You need to craft a VRT and R can read that. I'm also working on this kind of flexibility via Rcpp/GDAL here: github.com/hypertidy/vapour I'd be happy to target a very specific application if you are interested. – mdsumner Aug 5 '17 at 5:46
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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).

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