[R-sig-Geo] Exporting Multipolygon geojson with rgdal::writeOGR

[Roger Bivand]

On Thu, 22 Oct 2015, Andy Teucher wrote:

> I’m finding that writeOGR isn’t exporting multipolygons properly using
> the GeoJSON driver. I have a simple test case (borrowed from here:
> http://gis.stackexchange.com/questions/137977/writeogr-alters-multipolygon-holes)
> with a geojson string with one multipolygon containing two polygons. I
> use readOGR to create a SpatialPolygonsDataFrame out of it, then write
> it with writeOGR:
>
> library(rgdal)
>
> js <- '{
> "type": "MultiPolygon",
> "coordinates": [[[[102.0, 2.0], [103.0, 2.0], [103.0, 3.0], [102.0, 3.0],
> [102.0, 2.0]]], [[[100.0, 0.0], [101.0, 0.0], [101.0, 1.0], [100.0, 1.0],
> [100.0, 0.0]]]]
> } '
>
> spdf <- readOGR(js, layer='OGRGeoJSON', verbose=FALSE) # Create a
> SpatialPoygonsDataFrame
>
> temp <- tempfile()
> writeOGR(spdf, dsn = temp, layer = "", driver="GeoJSON")
> cat(readLines(temp))
>
> # Output:
> { "type": "FeatureCollection", "crs": { "type": "name", "properties":
> { "name": "urn:ogc:def:crs:OGC:1.3:CRS84" } }, "features": [ { "type":
> "Feature", "id": 0, "properties": { "FID": 0 }, "geometry": { "type":
> "MultiPolygon", "coordinates": [ [ [ [ 102.0, 2.0 ], [ 102.0, 3.0 ], [
> 103.0, 3.0 ], [ 103.0, 2.0 ], [ 102.0, 2.0 ] ], [ [ 100.0, 0.0 ], [
> 100.0, 1.0 ], [ 101.0, 1.0 ], [ 101.0, 0.0 ], [ 100.0, 0.0 ] ] ] ] } }
> ] }

I can replicate this with GDAL 2.0.1 and rgdal 1.0-7 - reading temp in 
gives the orphaned hole. I'm surprised that passing the js object to 
readOGR() doesn't fail, but that isn't the source of the problem. spdf 
appears to be structured correctly. Please provide your GDAL and rgdal 
versions, and OS details from sessionInfo().

We are completely relying on the GDAL drivers here - we can't cherry pick 
for particular drivers (historically excepting ESRI Shapefile), so your 
debugging will need to examine writeOGR(), the C code it calls, and the 
interactions between the rgdal C code and called OGR functions. Please 
check which other drivers are affected (I think KML is, is GML?). Can you 
place debugging Rprintf(...); in the rgdal C code to see where this is 
coming from?

I can start looking at this sometime next week, so please make a start 
yourself; contributions from others are very welcome.

Roger

>
> If you look closely at the output, you can see that the 'coordinates'
> array now contains a single polygon array with two coordinate arrays:
> the boundary, and a second one which is now treated as a hole of the
> first (orphaned as it is outside the bounds of the polygon). The
> original 'coordinates' array consists of two polygon arrays, each
> consisting of a single coordinate array which defining a polygon (with
> no holes), which is correct according the GeoJSON spec:
> http://geojson.org/geojson-spec.html#polygon.
>
> I'm always hesitant to call things a bug, but this doesn't appear to
> happen using ogr2ogr on the command line:
>
> writeOGR(spdf, ".", "test", driver = "ESRI Shapefile") # Write a
> shapefile to convert using ogr2ogr
> system("ogr2ogr -f GeoJSON test_from_shp.geojson test.shp")
> cat(readLines("test_from_shp.geojson"))
>
> # Output:
> { "type": "FeatureCollection",  "features": [ { "type": "Feature",
> "properties": { "FID": 0 }, "geometry": { "type": "MultiPolygon",
> "coordinates": [ [ [ [ 102.0, 2.0 ], [ 102.0, 3.0 ], [ 103.0, 3.0 ], [
> 103.0, 2.0 ], [ 102.0, 2.0 ] ] ], [ [ [ 100.0, 0.0 ], [ 100.0, 1.0 ],
> [ 101.0, 1.0 ], [ 101.0, 0.0 ], [ 100.0, 0.0 ] ] ] ] } } ] }
>
> The resulting output is correct.
>
>
> Thanks in advance for any help on this.
>
> Andy
>
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-- 
Roger Bivand
Department of Economics, Norwegian School of Economics,
Helleveien 30, N-5045 Bergen, Norway.
voice: +47 55 95 93 55; fax +47 55 95 91 00
e-mail: Roger.Bivand at nhh.no