Tuesday, 31 July 2012

Making a county base map

Cumbria Relief Map
Since starting work at CBDC I have found one of the most useful layers is a simple raster layer - a relief map of the county giving some of the main geographic features that is useful for putting underneath species distribution maps.

Common Hawker from Virtual Fauna of Lakeland

I wanted to recreate the map, so that I had the underlying information to use and also so I could change the colour scheme whenever I wanted or do other tweaks. I was working in ArcMap which is a paid product, but all the data is available for free so anyone could give it a go with a free GIS like Quantum.

I found everything I needed in the OpenData from the Ordnance Survey. I used:

 Land Form PANORAMA®
  • The Digital Terrain Model ascii data (the subject of another blog) from  using "classified" symbology and manual intervals for each 100m and an extra interval for 0-50m (and below 0m, although this gets covered by other layers later).
OS VectorMap™ District 

This is split into Ordnance Survey "myriads" (the 100km squares that are denoted by the first two letters in the grid reference) so I needed SD, NX and NY to cover Cumbria.
  • For the sea I used TidalWater.shp. This didn't cover the bottom left corner so I drew a little dummy polygon for that corner (as an aside - this was harder than I thought it would be in ArcMap 10 with new editing "templates" to get my head around).
  • I wanted to add mudflats which weren't in the original map and these are in Foreshore.shp.
  • For just the big rivers in the county I needed to select by attributes from rivers_polyline.shp and use  "LEGEND" LIKE 'Main%' OR "LEGEND" LIKE 'Secondary%'
  • Lakes can be found in lakes.shp but this created a bit of a speckled effect from the smaller waterbodies. I calculated the area of each (in Arcmap this can be done by opening the attribute table, adding a field and then using calculate geometry). I used a bit of trial and error and decided in the end to keep waterbodies with a calculated area over 60Ha (select by attributes "area" > 600000).
  • Urban areas weren't on the original map but I thought it might be good to include them sometimes, so used urban_region.shp and "LEGEND" LIKE 'Large Urban Area polygon'.
  • I already had the Cumbria county boundary, although you could extract it from county_region.shp in OpenData Meridian™2 using "NAME" LIKE 'CUMBRIA COUNTY'.
  • However as I wanted to use the admin border for the inland stretch but not the coastline (so you can see the high and low water) I had to mess around with it for a bit. This was another point at which my GIS inexperience definitely was in evidence! Anyway in the end I worked out a method by converting the polygon into a polyline and then using the split tool in an ArcMap edit session to split at the two places the boundary reached the coast. I then had to remerge two of the sections to leave me with a polyline of two segments. I only used the inland segment in my final map.
  • I wanted to create a box around the county. I could have drawn this but instead I used the Feature Envelope to Polygon tool (only available in ArcInfo) which made a shapefile of the bounding box of the county. I then used the buffer tool on this with a 3 km buffer.
Each layer was clipped, mostly to the Cumbria boundary but to the bounding box in the case of the sea layers. This is the result:

(c) Cumbria Biodiversity Data Centre. Contains Ordnance Survey data © Crown copyright and database right 2011
Now this contains quite a bit more information than the original and is hence busier, but now I have a map .mxd file with everything in I can always simply it further if needed for a specific purpose.

By exporting the map as a tiff and ticking "write world file" I can circulate this base map for anyone to use in lots of different programs. I can also create a monochrome version. If anyone wants the georeferenced tiff let me know - it's my intention a selection will go on once the website is up and running.

Common Hawker (c) Cumbria Biodiversity Data Centre. Contains Ordnance Survey data © Crown copyright and database right 2011

Common Hawker (c) Cumbria Biodiversity Data Centre. Contains Ordnance Survey data © Crown copyright and database right 2011

Biodiversity needs Geodiversity

When I moved to Cumbria from Kent it was apparent that the useful base maps for the counties were quite different - in Kent the geology is almost always the most useful base layer and tells you where the upland areas (chalk) are anyway, and there is a lot less water to worry about. Of course helpfully geology is also freely available at a suitable scale for county maps from the British Geological Survey so you can have many happy hours devising the most useful base maps for your taxa group from a combination of all these layers. Many thanks to BGS and Ordnance Survey for making this possible for everyone.

The geology of Cumbria. Reproduced with the permission of the British Geological Survey ©NERC. All rights Reserved

1 comment:

  1. Thanks for this! It inspired me to have a go in QGIS, see here for my first attempt: