Sunday, 31 May 2015

Some more work in progress re: Cornish GIS maps

Here is a little more about my work in progress in generating maps of Cornwall in QGIS.

I have downloaded various Ordnance Survey VectorMap data, along with some layers which come from OpenStreetMap. The OS VectorMap has layers including woodland, tidal water, surface water polygons (e.g. lakes and reservoirs), foreshore and surface water lines (rivers etc.) and others.

These is more comprehensive than OpenStreetMap, however the OpenStreetMap vector layers come with names associated with them in the attribute table in some cases. Certain of the OS VectorMap layer objects will exist in the VectorMap 'Named Places' shapefile, but as points rather than as names in the attribute tables of the polygons themselves.

In the map below I have used the woodland polygons from the OS VectorMap and the ones from OpenStreetMap, but only the OpenStreetMap ones are labelled, since only they have names in the table.

I have also generated contours based on the Shuttle Radar Topography Mission, 1 arcsecond data, which is now freely available for most of the world. It is also possible to use OS Terrain50 contours, which generally appear smoother than these ones, however given these are released in individual squares, I found myself having so many vector layers in the rendering that QGIS crashed.

The Falmouth area, using contours from SRTM 1 arcsec
There is a fairly large amount of data entry to be done to put the names in Cornish, and beyond the list that has already been published by MAGA I would expect it to be slower work.

The OS VectorMap Named Places shapefile for the SW grid square has 2759 entries, of which 679 are various geographic features and the remainder are populated places.

The OpenStreetMap files have approximately 122 named woodlands in Cornwall (of 810 polygons, of over 9000 in the SW grid square alone for OS VectorMap), 56 named water polygons (of 327), 21 parks (of 45), 242 named waterways (of 2923) as lines (some duplicate names for different parts of a feature), and 1337 named places (of which I have already entered 421 in Cornish based on the MAGA Kernow list). There are 10342 named roads in Cornwall from the Open Street Map data.

Wednesday, 13 May 2015

Georeferencing a tithe map and overlaying modern mapping in QGIS

The National Library of Wales recently released a tithe map of St Woolos, Newport to Wikimedia Commons. There is a crowdsourced project to digitise, georeference, and transcribe the field names from these maps, however I wanted to try out geoferencing in QGIS with its georeferencing plugin, partly because it may be useful in the Taves an Tir project that Kowethas an Yeth Kernewek are doing, since scans have also been made of the tithe maps of Cornwall.

After downloading the full resolution (16,298 × 22,877 pixels) tithe map of St. Woolos, I used the QGIS georeferencer plugin which works by matching manually chosen points between the scan, and a map. In this case I used a map of the UK road network ultimately from OpenStreetMap via, as well as some of the OS VectorMap data. This includes railways, woodland areas and parks shown below. I styled the road map in QGIS depending on the category of road.

I used 8 points, finding road junctions that I could match between the two maps and a Helmert transformation (more information from QGIS web documentation). It was clear when I did this that one of my points was incorrectly placed, which isn't too surprising given the road network has changed since 1845, but discarding this gave a good result.

I've also downloaded some of the 1 arcsec resolution Shuttle Radar Topography Mission data, which has recently been made available for most of the world, including the British Isles, and transformed this to a contour map in QGIS.

Below is a map of a portion of the St. Woolas tithe map area, with the modern road network overlaid along with OS VectorMap data for railways, woodland, surface water and parks.

There are many options in QGIS for feature labelling, for instance it is possible to suppress labelling of small features to avoid label overcrowding, which makes it label street names selectively. 

Link to higher resolution version (8MB png file)

Aside - Welsh language names in OpenStreetMap

OpenStreetMap does have some Welsh language names, if the name:cy tag is used. It is possible to see these using the Multilingual Map Test site by Jochen Topf.
It is slightly involved to extract this from a download from because it is in an 'other_tags' column along with various other things and a bit of string slicing is needed to get it out.
The name:cy tag is only used where the Welsh language form of the place differs from the version commonly used on English maps.

Tuesday, 12 May 2015

If nothing is done on Climate Change

If all the ice caps melt completely, the eventual increase in sea level would be about 80 metres. This is what the British Isles would look like: