Trench Maps
The British Trench Map ontology models the WW1 British Trench Map Coordinate system and its relationship to modern latitude and longitude coordinates. The basis of the system is a grid of 1:40,000 scale map sheets. The ontology includes information about each sheet, and how they link up to form the grid; so we can transform the ontology OWL file to make a Scalable Vector Graphics (SVG) diagram of the grid, applying btmaps.xsl to produce grid.svg.
Each map sheet rectangle in grid.svg is 720 x 480, the same dimensions used when making an overlay for a 1:40,000 scale trench map. Also, each map sheet rectangle in the grid diagram is represented by an SVG group element that is translated to the correct position in the grid. This makes it possible to simply copy an SVG overlay into the SVG group that represents the relevant map sheet. A picture across multiple sheets can be built up in this way.
KML
The British Trench Maps ontology can model the geometry of a map sheet, but the OWL file doesn’t actually include the geometry data needed to display the sheet in a GIS. You can get the geometry of map sheets from tMapper though, which lets you locate the boundaries of a map sheet in the WGS84 coordinate system used in KML.
KML will draw polygons and overlay images on a map. You can take an SVG Overlay, convert it to a raster image, and position it using the GroundOverlay element. For example, try importing 51b-overlay.kmz into Google Earth. This shows the red and blue dots from the 11th Brigade Intelligence reports (see the overlay example) and the boundary of the 51b map sheet. See also sheet-51b.kmz, which has a 720 x 480 blue rectangle as the image, making it easier to see how the image lines up with the polygon drawn using the sheet geometry from tMapper. In these examples, the image was lined up with the sheet geometry through manual tinkering. A more thorough understanding of the coordinate systems and the available geometry data (and some maths) would increase the accuracy of results; and it would be nice to have the geometry for all the map sheets linked into the British Trench Map ontology.
The tMapper application lets you convert Trench Maps coordinates directly to latitude and longitude. Overlaying the raster image as above is therefore just a visualization choice. An alternative would be to add simple simple KML PlaceMark elements at each lat/long corresponding to a trench map reference. It depends what you want to show and how you want to show it. Mixing and matching display options is, of course, also possible.
Linked Data
The British Trench Map ontology gives each map an identity that can be used to relate places and events. It includes a currentLocationOf property that can relate a current place to a map sheet, and a wasLocationOf property that relates a map to an event that happened within the area it covers. The OWL file for the ontology includes examples of both. In particular, it has each major action of World War I (from DBpedia) linked to the relevant 1:40,000 map sheet.