1

I have a raster map (a photocopy I took from an Atlas), and a vector map of the UK. The raster map shows pre-1890 Scottish county borders.

I have geocoded the map so that the raster borders overlap the vector map, but here comes my question: how do I create a vector layer showing the separate counties? This may be especially difficult, since some of the counties are not contiguous.

1

Unless the borders are coloured in a unique colour, you have to digitize the borders as polygons manually.

There is no tool that can digitize monochrome line patterns if they lay over other map objects of the same colour like roads and rivers.

Make sure that your boundary polygons snap to each other. I use a snap radius of 10 pixels (not Map units!) for such purposes.

1

You need to digitize the boundaries (assuming a search online has drawn a blank for any existing datasets - that would be my first port of call! EDIT - On that note, I just found this link which may be worth a look).

To digitize the boundaries, there are several approaches you can use but I would start by creating a line-dataset that represents the boundaries. At the coastline, extend the boundaries into the sea so they cross the coast. Many boundaries will follow rivers and you may be able to use a suitable rivers dataset (eg. the Free OS Strategi or VectorMap data - allowing for changes in the course of the rivers over time - the river may form a starting point that you can edit to match your underlying map). These rivers need to be pulled in to your boundaries layer obviously. For other boundaries just digitize by making your boundaries layer editable, select the new feature tool and simple 'draw' the line.

Once you have all your boundary lines (all extending out to sea where they meet the edge of coastline). Active the Processing plugin (comes as standard in QGIS - if you can't find it go to Plugins and activate it there). Find the SAGA Polygon-line intersection tool and intersect your UK coastline with the boundaries. BTW you can get a Scotland only coastline from OS for free in their Boundaries data (OpenData).

Finally, you will need to supply attributes for your counties.

EDIT: afterthought
This method presupposes that your coastline is a polygon data set. If not you can still intersect the lines and then assemble polygons but a better approach is to get a free polygon data set from OS OpenData.

  • Thank you very much. Where can I find the SAGA Plygon-line tool? – Cornelius Aug 29 '14 at 15:22
  • It is in the Processing plugin... and it would have been more helpful to you if I had typed it correctly thus: Polygon-line intersection tool. Open the Processing toolbox panel and type 'Intersection' in the search box. It'll be easy to find that way. – MappaGnosis Aug 30 '14 at 7:17
  • You may have to switch to Advanced Interface at the bottom of the Processing toolbox. – AndreJ Aug 30 '14 at 7:39

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.