Adding Boundaries to Spatial Polygons Object

Question

I have the following SpatialPolygonsDataFrame.

require(raster)
usa <- getData('GADM', country='USA', level=2)
metro <- subset(usa, NAME_1=="Nebraska" & NAME_2 %in% c("Dodge","Douglas","Sarpy","Washington"))
plot(metro)

I would like to be able to replicate the following map boundaries (defined by the colors):

Does anyone know a good plan of attack? I realize this is a somewhat manual process. I have already downloaded all US Census files that are of a more detailed geography. I was hoping that a more detailed level of geography could be aggregated to answer the above question, but unfortunately the districts do not line up the same.

Is there a R function already out there that would be helpful in assisting this manual process? At the very minimum, I would like to be able to leverage the perimeter of the 4-county area.

Answer 1

Use writeOGR from the rgdal package to create a shapefile of your metro object. Then install QGIS (http://www.qgis.org/), a free and open-source GIS, and load the shapefile as a new layer.

Then you can edit the layer, add new polygons, edit lines etc, then save as a shapefile to read back into R.

Additionally, you may be able to "georeference" your image (by identifying known lat-long points on the image) and load that into QGIS as a raster layer. That makes it easier to digitise your new areas. All you need for that is a few lat-long coordinates of specific points, such as the corners of polygons or line intersections, and then QGIS has a georeferencing plugin that can do it.

I don't think you'll find any R code as suitable for digitising new geometries over an image as good as QGIS.

After half an hour (and twenty years experience, not all of which you'll need) I've got this:

I didn't precisely digitise your new boundaries though, just roughly for speed. That QGIS screen cap shows the five coloured areas under the four metro areas.

Step one was georeferencing. This screengrab shows how the PNG has been georeferenced - the red line is the metro area shapefile drawn with transparency over the PNG after the PNG has been converted to a GeoTIFF by matching control points.

Step two was then using QGIS editing tools to split, join, and create new polygons. Then I just coloured them and added labelling to pretty it up.

I could probably bundle these files all up for you to neaten, but it really doesn't take that long and you'll learn a lot from doing it. Also, this is probably a gis.stackexchange.com question...