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My goal is to modify an existing shapefile by merging certain polygons.
After importing the shapefile and using the UnionSpatialPolygons command, I get the polygon outline that I want.
However, this is now a SpatialPolygons object and not a SpatialPolygonsDataFrame, so I'm unable to export it to a shapefile using writeOGR.
How can I get around this problem?
As the name says, a SpatialPolygonsDataFrame is basically just a SpatialPolygons object with data attached (the attribute table). The data must have at least as many rows as there are features
library(rgdal)
ob <- SpatialPolygons(..)# Your SpatialPolygons Object
spp <- SpatialPolygonsDataFrame(ob,data=as.data.frame("yourData"),proj4string=CRS("+proj= aea > +ellps=GRS80 +datum=WGS84"))
writeOGR(spp,"shapes","testShape",driver="ESRI Shapefile",)
----EDIT----
If you want to convert your SpatialPolygonsDataFrame back to a SpatialPolygons object you just need to address the object structure within R
ob <- SpatialPolygons(spp@polygons,proj4string=spp@proj4string)
IDs argument correctly when executing UnionSpatialPolygons. Otherwise you might get trouble specifying data argument when converting back to SpatialPolygonsDataFrame.
data=as.data.frame("yourData")? After dissolving inner polygons using unionSpatialPolygons(...), I would like to write result as new shapefile
Nov 8, 2014 at 2:54
union issue..
Problems:
1: the outcome of UnionSpatialPolygons is a spatial polygon
2: converting the result back into a spatial polygon data frame is a real pain
-a. you need a very exact data frame to attach to a spatial polygon
-b. data you used for UnionSpatialPolygons has more rows than the output and is not formatted in the way that is needed.
My (ugly) solution:
### Coerce into spatial polygon data frame with id and row name of spatial polygon
# Make a data frame that meets the requirements above:
df<- data.frame(id = getSpPPolygonsIDSlots(your.spatialpolygon))
row.names(df) <- getSpPPolygonsIDSlots(your.spatialpolygon)
# Make spatial polygon data frame
spdf <- SpatialPolygonsDataFrame(your.spatialpolygon, data =df)
# Then don't forget to make sure the projection is correct
# XXXX is your SRID
proj4string(spdf) <- CRS("+init=epsg:XXXX");
spdf <- spTransform(spdf , CRS("+init=epsg:XXXX"));
