Thursday, 4 September 2008

Publication dates for historic maps

Question: How do I find out what 'Publish Year Start' and 'Publish Year End' dates means for the maps in Historic Digimap?

Answer: The publishing date range of a map in the Map Details tab from the Main Map View in Historic Digimap. This is achieved by using the View Map Details tool (an i symbol) and clicking on the historic map at the point of interest.

The maps shown on the screen are created from the National Grid "cookie cut" files - images which have been rectified and clipped to create a national grid based "tiled" dataset akin to those available for the contemporary Ordnance Survey data. However, the National Grid does not match up with the County Series maps, because the County Series were created on a per County, rather than a national, basis. It is therefore likely that one "cookie cut" National Grid tile comprises parts of more than one County Series map sheet, each of which could have a different publishing date.

The metadata for the National Grid "cookie cuts" often does not contain one publishing date but the range (from, to) of publishing dates of the Orginal mapsheets which make up that national grid cookie cut tile. It is this range which is reported under the Map Details tab.

Unfortunately, EDINA does not have any survey dates for the maps in the Historic Digimap. Landmark Information Group who provided the maps did not capture the survey dates when the maps were digitised (scanned). This is because the maps were originally digitsed for a specific purpose which did not require the information held in the marginalia of each map sheet. During the digitisation process the marginalia was trimmed from the resultant files, meaning that it was never captured in digital form.

A solution to this is to visit a library which holds historic maps; many university libraries have historic map collections with specialised cartographers and support staff. The map details taken from Historic Digimap can be used to search for and locate the map(s) corresponding to your area of interest. Once found, it is in a map's marginalia where you can find the recorded survey dates.

More detailed information about historic maps and Historic Digimap can be found in the Help pages.


Anonymous said...

I would like to add that the historic digimap raster tiles are at a scale of 1:10,000 and not 1:10,560. I don't believe this is stated anywhere. Whilst for most people it is not a problem. it does become a problem if you are trying to figure out how one creates a world coordinate file and you attempt to use the maps original scale of 1:10,506, in your calculations.

EDINA Digimap Support said...

There may be some confusion here. There are several scales of maps which have been scanned from paper originals and are now provided through Historic Digimap. The two primary scales are 1:2500 and 1:10,560 for the pre-WWII County Series mapping. Post-WWII there are 1:1250, 1:2500, 1:10,000 and 1:10,560. For the smaller scale, this overlap in scales and dates represents the period during which the national surveys were undertaken. In the Historic Digimap Help pages there is a lengthy extract about map scales and characteristics from Dr Richard Oliver's book, "Ordnance Survey Maps: a concise guide for historians" which gives greater detail. The scale of each Series is also given in the drop-down lists of maps in the Historic Digimap mapping interface. Each "Edition" has its scale clearly quoted in the heading.

Since all of these are delivered as digital images, they can be displayed at any scale the user chooses if appropriate software is available. Clearly the quality of the image will vary significantly depending on the display scale. Some of the "cookie cut" versions (those which have been rectified to match the National Grid) have .tfw files delivered with them already, so you shouldn't need to create your own for these data files. A world file is a plain ASCII text file consisting of six values separated by new lines. Wikipedia has a detailed explanation of the file format: