Google Earth

A headache for a detailed study of anti-invasion defences are the methods of presenting the data; publishing costs and the need to appeal to as wide an audience as possible in order to sell copies can place restrictions on how the historian can present his/her findings on paper.

In order to interpret the relationships between defence works and the landscape they were designed to defend it is important to view them arranged in a representation of that landscape, whether it be on a sketch map or employing detailed professional mapping.

While some studies employ mapping at various scales where appropriate, those that result in a large number (i.e., hundreds) of sites being identifed are usually unable to show them all on detailed mapping, and so are reduced to compiling lengthy gazeteers that simply list grid references. This does not present the data in a way that helps to interpret the defences; printed gazeteers, although undeniably important, generally appeal only to those with the patience to sit down with a map and see where each site actually is. What is needed is a presentation format that requires the reader to undertake less interpretation of their own in order to gain maximum use of the data. I believe that Google Earth is an example of such a format.

What Google Earth can do

In a nutshell, the Google Earth software is a means of viewing satellite imagery and aerial photographs of the entire planet, allowing you to zoom in to any location, depending on the quality of imagery available for that area. The software is also free to download, although there are subscription versions.

It is a fantastic resource that is proving to be of immense value in trying to interpret the defences of 1940-41.

One of many strengths of Google Earth is that one single map can be used to see the wider picture (for example, the system of stop lines) as well as zooming in to see the detailed landscape.

Plotting features

Structures such as pillboxes and roadblocks can be plotted, as well as areas (for example, minefields) and features such as anti-tank ditches and stop lines can also be drawn onto the landscape.

The image at right shows East Sussex as seen in Google Earth with 55 Division's 1941 boundary in black, stop lines in red, and defence works in yellow.

One notable feature is the ability to plot fields of fire onto the landscape; the defences begin to make more sense when you do this. The image below shows the fields of fire of the machine guns at Galley Hill, Bexhill. The red zone is the arc of fire of a Vickers gun, blue for Bren guns and green for a Home Guard Browning machine gun. (Note: the majority of buildings seen in the image below are post-war; hence the need to study the landscape as it was, not as it is.)

Rotate and tilt the landscape to get alternative views

The technology allows you to view the known defences vertically or from different angles and directions, and, crucially, in relation to each other.

You can also zoom out to view a large region (such as the East Sussex area above), or zoom in close to view small features; in some cases individual pillboxes and anti-tank cubes can be seen. More features will become apparent as the Google imagery quality improves.

The image at right shows a tilted view of some of the defences near Winchelsea; note the red stop lines and coastal crust of defences.

Seeing the landscape from different angles allows you to see details and features that you may otherwise miss.

Use image overlays

This is a fantastic feature that allows you to import an image (for example a map) and lay it onto the Google landscape. This has proved of immense value in locating features such as buildings that have been destroyed since the war.

Notable success has also been gained in overlaying defence scheme traces in order to plot the defences.

The image at right shows an Ordnance Survey six-inch map of Bexhill superimposed on the modern landscape.

Google earth allows the transparency to be adjusted to let you see the terrain underneath.

[Map reproduced from 1937-1940 map with the kind permission of the Ordnance Survey]

Incorporate 3D models

Google Earth has its own in-built set of geometry tools that allow you to draw very basic boxes that can be used to represent features such as pillboxes or other buildings.

The image at right shows some buildings that were roughly drawn using basic geometry onto the image overlay map seen above.

These reconstructions were used to help place various defence works whose locations were described in relation to these buildings that were destroyed after the war.

Google Sketchup is another fantastic free tool that allows you to draw almost anything in 3D and place it into Google Earth and is far more versatile than Google Earth's simple geometry tools.

Work with GPS data

As my data input comes from written sources, direct GPS input is currently of little value, although it will become useful in the future.

A problem I have is that Google Earth will not otherwise accept data in the British National Grid format, which needs to first be converted to longitude and latitude coordinates. This is because map references are concerned with projecting points from a sphere onto a two-dimensional plane such as a map, whereas Google Earth is a three-dimensional representation that requires the 3D longitude and latitude coordinates. See the Maps page for details on converting Cassini Grid to National Grid.

GPS compatibility is not a feature of the free download version of Google Earth.

Create guided tours

With a set of points plotted, you can set Google Earth to visit them by zooming down on each in turn.

By drawing a path you can fly along it over areas of interest.

Distribute data

The ability to distribute data is also a boon; Google's KML (XML) mark-up language allows data plots to be emailed or downloaded for anyone to access.

The Defence of Britain Project have converted their database to KML and it can be downloaded for viewing from the Archaeology Data Service website (see below).

I have also compiled a tour that allows you view the DoB data along the GHQ stop line from Newhaven up to Penshurst; this is available below.

I will eventually make all of my data available for viewing in Google Earth.

Useful Links

• Google Earth: http://earth.google.com/index.html
• Google Sketchup: http://sketchup.google.com/
• Defence of Britain Project dataset: http://ads.ahds.ac.uk/catalogue/specColl/dob/download.cfm