Category Archives: Technology

Mapping the recent past – electronic Legal Deposit

If you want to see what your town looked like 50 years ago, or even 150, the Map Room can find you a detailed Ordnance Survey map to answer the question. And if you want a really large scale plan showing the same site in the present day, you can buy one via an agent for OS Mastermap. But what about the period in between?

The OS map extract above shows Stratford-upon-Avon in 1889 at a scale of 1:2500. Maps at the same or larger scales continued to be published, updated at intervals, until the late twentieth century. In the 1990s, the OS stopped producing printed maps at the largest scales of 1:10,000, 1:2500 and 1:1250. Present day large scale mapping continued to be produced in digital format, and could still be purchased, but each time the data was updated the previous version was lost. There was a danger that recent history would disappear into a black hole. If a researcher in 2022 wanted, for example, an OS Mastermap of Kendal from 2012, what would they do?

The Legal Deposit Libraries – of which the Bodleian is one* – sprang forward to fill the breach. Working with partner organisation thinkWhere, they negotiated a scheme for the OS to deposit an annual digital “snapshot” of large scale mapping across the whole of Great Britain from 1998 onwards. Northern Ireland, which has its own mapping agency, soon followed suit. The dataset is updated annually. It was an early example of electronic Legal Deposit, preceding the official eLD which began in 2013.

What is electronic Legal Deposit? Under Legal Deposit legislation the Bodleian Libraries, and the other LDLs,  are entitled to a copy of every item published in the UK. Legal Deposit of printed materials has existed in some form since 1662, and thousands of the books, maps, serials, and printed music items in the library are here as a result. Electronic Legal Deposit was based on this; it came into force in 2013 and since then many published items have been deposited in electronic rather than print format. You can read more about it here on the Electronic Legal Deposit Libguide. In most cases, electronic Legal Deposit items are listed on SOLO and can be read on any Bodleian Library reading room computer.  Maps deposited on electronic Legal Deposit usually require specialist viewing software, and can be seen on a dedicated terminal in the Rare Books and Manuscripts Reading Room of the Weston Library. You can log in using your Bodleian Libraries username and password. Extracts can be printed using your PCAS account.

This has recently been updated with a much wider range of maps; as well as the large scale OS maps described above you can see a whole array of different maps of the UK here. There are detailed town plans by XYZ Maps and The Clever Little Mapping Company, large scale coastal charts by Antares, and a wealth of cultural information. There is information from Historic England showing locations of all the listed buildings geographically plotted with links to the website, descriptions and images; Historic Environment Scotland and Welsh preservation organisation CADW show similar information for Scotland and Wales. Also included are World Heritage sites, protected monuments, battlefields and shipwrecks.  The map below shows the locations of listed buildings in Portsmouth.

The maps so far are almost exclusively for areas within the British Isles, but the system is set up to give access to maps from anywhere in the world via a map interface. As an increasing amount of publication is now digital rather than printed, this can only grow.

 

*The other LDLs are the British Library, National Library of Scotland, National Library of Wales, Cambridge University Library and Trinity College Dublin, in case you were wondering.

The shortest distance between two points

We came across this map, on a very unusual projection, while processing a previously uncatalogued set of nineteenth century French sea charts produced by the Dépôt des cartes et plans de la marine. Most are standard nautical charts, but this one – part of a set of three – is extraordinary.  The world appears to have been turned inside out; the chart is centred on the central Atlantic, and the land masses are progressively larger and more distorted the further they are from this point. The other two charts represent the Pacific and Indian Oceans in the same way.

The title makes the chart’s purpose clear: ‘Carte pour la navigation par l’arc de grand circle’. A great circle is, technically, the point at which the surface of a sphere intersects with a plane passing through its centre. In practical terms, a great circle drawn on the surface of the Earth between 2 points will be the shortest distance between those points (the Earth is not, of course, a perfect sphere, but it is close enough for this to be of use).

Navigational charts are traditionally drawn on the Mercator projection. This has the great advantage of showing a line of constant bearing (rhumb line) on the Earth’s surface as a straight line on the map. This is the simplest course to navigate, as mariners have known for many hundreds of years, but it is not the shortest. The shortest route is a great circle, and this requires constant adjustment of direction to stay on course. Sailing ships were limited by the challenges of winds and currents, and early steam ships by the need to refuel, but from the 1870s this principle began to have more practical applications. A straight line drawn on this orthodromic chart is a great circle course between the two points it connects, enabling navigators to plan their great circle journeys relatively easily. These charts were published in 1879. 

Charts of this sort do not appear to have passed into common use, and there could be several reasons for this. For one thing, the difficulties of plotting a great circle course are sufficient to outweigh the advantages for all but the longest ocean crossing journeys. Mariners continued to use rhumb line navigation well into the late twentieth century, by which time GPS systems had come into use. When a great circle course was followed, for sea or air travel, it was calculated in advance, sometimes using a chart of this sort. The course would then be plotted onto a Mercator projection chart where it was easier to follow. 

The usefulness of great circles can be seen most clearly on a modern map of long distance air travel. This is why aeroplane routes from, say, London to San Francisco always appear oddly curved when viewed on a map, with the route going much much north than you would expect. This is a great circle course, and the shortest way to connect two distant cities. A demonstration can be seen on this useful site http://demonstrations.wolfram.com/GreatCirclesOnMercatorsChart/.

The charts were created by Gustave Hilleret, a naval lieutenant and teacher at the École supérieure de guerre navale, who also published books on navigation. The projection is the Gnomonic projection with Equatorial aspect; the charts’ Bodleian shelfmark is B1 a.61/1 [39-41].