Historical OS Mapping (Landmark)
Ordnance Survey has one of the largest collections of historical mapping in the United Kingdom and until recently this was only held as a paper archive. As the result of a joint venture between Ordnance Survey and Landmark Information Group Ltd the paper archive has been converted into digital format. The digital archive consists of 1:10 560 or 1:10 000 and 1:2500 scale County Series and National Grid mapping.
Introduction
In October 1840 the Ordnance Survey decided to adopt the six-inch mapping scale (1:10,560) previously used in Ireland, and in September 1841 work began to survey Lancashire. In July 1854 rural 1:2,500 scale maps were commissioned and in 1880 this was accelerated to cover all areas of the country. England, Wales and Scotland were surveyed in the following years, and each county was subsequently revised three to five times. These sheets have become known as County Series, named as such because individual counties were surveyed separately from their neighbours, often on different origins. In 1944-5 the mapping was transferred to the National Grid. Counties were now no longer surveyed on different origins: instead the whole country was mapped as a whole and a new map naming convention was used. The standard scales became 1:2,500, or 1:1,250, or 1:10,000 in moorland areas. OS now has one of the largest collections of this historical mapping in the UK, until recently held on paper. The 1:2500 County Series maps, also known as 25 inch, measure 1.5 miles x 1.0 miles or 3.84 sq km. 1:10560 scale measure 6 miles x 4 miles or 61.44 sq km for a full sheet and 3 miles x 2 miles or 15.36 km for a quarter sheet. The 1:2500 scale National Grid maps measure 1 km x 1 km and 1:10000 scale National Grid maps measure 5 km x 5 km or 25 sq km. A joint venture between Ordnance Survey and Landmark Information Group has undertaken the task of creating copies in digital form, both in its original (raw) format, and in processed format for use for example in a GIS.
Source Material
The source mapping in the scanning process was the original mapping published at the time of survey and held in the OS archives.
These maps are 'working editions' and have therefore been in constant use over the last century. The maps are stored on racking
and were only bagged within the last decade; this, coupled with constant handling, has resulted in some maps being torn, folded,
and affected by a certain amount of dust. Maps used in the daily business of OS have also been drawn / written on by those who have used them,
often obscuring the data originally published (i.e. boundary changes).
Finally, because storage has not been in a controlled environment, the paper of some maps have become slightly warped,
obviously affecting the accuracy of that map; this is especially the case with Full Sheet 1:10,560 maps.
A number of factors affect the quality of the scanned image. In particular dust on the maps has resulted in dark areas -
other imperfections result from folds and tears which make it impossible to scan an even image; in some extreme cases a section
of the map is missing resulting in a loss of that data. It is important to note, however, that damaged maps are the exception and
that most of the Historical Mapping is in very good condition especially considering its age.
Although the collection of paper maps in the OS archive is approximately 93% complete there are maps missing.
It is the intention of the joint venture (OS & Landmark Information Group) to locate and scan the missing maps and include them in the archive.
The Methodology
The production process is composed of a number of stages which are described below:-
Scanning
Scanning was commenced by Landmark staff during May 1995. Industrial roller scanners were chosen to meet operational needs,
as over 400,000 maps had to be scanned. The resolution chosen, 300 dpi, matched scanning speed, with file size and line detail.
The results are clear images and manageable file sizes, although on poorer quality maps a higher dpi would have resulted in cleaner,
crisper images this would have meant considerably larger files, making them less manageable, costing more to store, and would have
slowed the scanning team considerably. The data format chosen for the raster images was Binary (black and white pixels) tiled TIFF
with group 4 compression.
Due to the use of multicamera roller scanners a minor distortion can be introduced into the raster image. This is due in part to the
camera optics not being 100% linear and the constant stretch or compression error caused by the rollers as the map is fed into the scanner.
The acid test for the scanning process was to ensure that the images were of an acceptable standard for inclusion the range of products
developed by Landmark Information Group and OS.
Raw Audit
Before processing could commence in Exeter, each raster image, i.e. scanned images in their original format, had to be opened, and its date and map name checked to ensure that the scanning was successful. An Excel file was created listing all maps available and was run through a database comparing it back to an actual DOS listing of the raster images. Any mistakes were corrected and the database check was re-run until no errors occurred. The raster images and Excel file were then backed up to CD-ROM creating an accurate raw record of these Historical Maps.
Processing
In order to tile the data and create a mosaic of Historical Maps covering the whole country, each scanned image had to be processed, removing all information outside the neat line leaving just the map tile itself. Image Centre (supplied by S.S.I.) was chosen as the processing software as it was the best available at that time which could perform the specialised tasks required. The process involved two phases. The first ensured that each image was orthogonal by placing two points along one edge of the map (top or bottom), Image Centre levelled out this line leaving the map horizontal. The second process ensured that the image was cropped from one corner to the diagonally opposite corner removing all data outside the crop marks in order to obtain a 'tile'. This process produces accurate historical tiles. Unfortunately a small number of the maps have become warped due to age; limitations within Image Centre meant that it could deskew to only 0.004 of a degree, and scanning, no matter how accurate, will not always produce perfect copies. As a result the maps were not perfect rectangles and once deskewed, although they were flat on the edge chosen, the opposite edge was still at an angle so that when cropped by selecting diagonally opposite corners, areas outside of the detail were left. These had to be removed which meant a second crop using the other two corners as reference points. Because of this limitation some data could be lost, resulting in a less than perfect join when the maps are lined up.
Processed Audit
The QA process involves opening every processed map and ensuring that the following two phases had been completed correctly. One checking that there were no black / white lines. Two making sure that the maps were readable. Please note that the actual 'paper size' of each map varies and so the number of Pixels and the .Tif images will vary in Pixel extent in X and Y. The cropping process (see above) merely involves removing unwanted data, it does not involve re-sizing the image. A DOS listing of the processed maps was then compared against the raw Excel file to ensure no errors occurred. Once audited, the data was backed up to CD-ROM.
Summary
This process results in an Excel file listing all maps (including dates) and two CD-ROMs, one with raw files and one with processed files which all match. Once a county revision was completed the task was then repeated with the next county revision.Index
The shape of each county grid was derived from the Ordnance Survey book of indexes for England and Wales and Ordnance Survey book of indexes for Scotland. Each county index was then transferred into a GIS and projected on the original county origin. Each rectangle was named with a unique identifier made up of the county identifier and the map sheet name.
Production of Co-ordinates
The County Series index maps have been used to calculate the national grid co-ordinates for each map sheet. The unique identifier (as mentioned above) is listed with the four pairs of co-ordinates that define the corners of the map sheet.
Conversion of Historical County Series Maps into National Grid tiles
The County Series Mapping was surveyed using the Casini Projection, which was replaced in the 1950's when Ordnance Survey brought in the National Grid system (see Contents3.doc for details of how scanning, processing and ngp creation was performed. The National Grid Projection differs to the one used for the County Series maps. In order to overlay National Grid on to County Series one set of data needs to be re-projected. Many GIS Software products cannot perform this task, so RMSI India with the assistance of Landmark Information Group Ltd and the Ordnance Survey have developed a process where by 1:10,560 or 1:2,500 scale County Series maps can be converted into actual National Grid tiles.
Each Raster image (Banded Tiff) is reprojected to fit the National Grid Projection and cut out into its National Grid blocks. The white background data is then made transparent so that other Raster images can be imported, warped and joined to this file. This creates a single National Grid file which can form a mosaic of up to eight Cassini Maps.
Each file is named using Ordnance Survey's naming conventions with a two character prefix relating to which Historical County the data originated from. For example: 05SJ3965 05 = County Name in this case Cheshire SJ3965 = OS National Grid naming Convention PCX files do not use the '05' prefix so they can be registered in GGP software. Files are stored in directories relating to the Historical County the data came from. For example: 05CHES21 = National Grid Tiles taken from 1:2,500 scale Cheshire County Series 1st Edition Maps. Please note that because the resulting National Grid Tiles are cut from a mosaic of County Series Data, white areas will occur in the tiles either due to County Boundaries or the Historical Data not being available.
Historical Data Technical Information
Ordnance Survey has one of the largest collections of historical mapping in the United Kingdom and until recently this was only held as a paper archive. As the result of a joint venture between Ordnance Survey and Landmark Information Group Ltd an extensive archive has been created in digital form from Ordnance Survey's historical archive of 1:10 560 or 1:10 000 and 1:2500 scale County Series and National Grid mapping.
Historical Data is offered as black and white raster data in a range of formats on CD-ROM. Raster data provides a map image where the map information has been converted to a grid of pixels that can be displayed on a computer screen. Historical Data produces a good quality map background which can be plotted using a range of suitable plotters.
The CD-ROM contains:
- the raster images;
- a file containing National Grid coordinates for the map (tile) corners;
- a file containing the map sheet publication dates; and
- text files containing general information about Historical Data.
Scale and source
The data is available for the following scales:- 1:10 560 scale pre-war County Series mapping
- 1:10 560 or 1:10 000 scale post-war National Grid
- :2500 scale pre-war County Series mapping
- 1:2500 and 1:1,250 scale post-war National Grid A B C D & E editions
Scanning and processing
Each of the paper maps is scanned at a resolution of 300 dots per inch (dpi). This resolution optimises file size with quality of line detail.
The results are clear images and manageable file sizes. The 1:2500 scale post-war maps are scanned at a resolution of 200 dpi since
these are scanned from aperture cards.
The scanned raster images have had all the information outside the 'neat line' (boundary of the mapping) removed, and have been
de-skewed as much as possible to align the maps north and south. A grid for each county has been created from Ordnance Survey
material based on the original county origin. Each map (tile) is uniquely identified and the four pairs of National Grid coordinates
that define the corners of the map sheet are provided.
Quality
The majority of the historical mapping scanned is in excellent condition resulting in clean line work and background. However, a number of factors affect the quality of the scanned image. In particular ingrained dust on the maps has, in a small proportion of cases, resulted in dark areas as does colour on the paper maps. Other imperfections can occur as a result of folds and tears in the mapping, which make it impossible to scan an even image. The paper of some maps has become slightly distorted, affecting the overall geometry of the map, this is especially the case with the full sheet 1:10 560 scale County Series maps. As a result, these maps are not perfect rectangles and once de-skewed there may be some loss of data and a less than perfect join when the maps are lined up. However, this category of maps is a small proportion of the total.Epochs
Historical Data is available for different time periods, referred to as epochs. These epochs roughly equate to the first County Series survey and subsequent revisions, and the first National Grid resurvey. Not all areas will have mapping available for each epoch as the number of revisions for each county varies. Generally, epochs 3 and 4 are only available for urban areas.- Epoch 1 generally refers to first County Series survey published between 1846 and 1901
- Epoch 2 generally refers to first revision County Series survey published between 1888 and 1915
- Epoch 3 generally refers to second revision County Series survey published between 1900 and 1949
- Epoch 4 generally refers to third revision County Series survey published between 1922 and 1969
- Epoch 5 refers to first National Grid re-survey published dates from 1945
Coverage
Data is supplied in tiles as County Series sheets or National Grid sheets depending on the publication date. Over 400 000 tiles are available covering Great Britain at 1:2500 and 1:10 560 or 1:10 000 scales. Tile sizes are as follows:- 1:2500 County Series maps (also known as 25 inch),
- 11/2 miles by 1 mile or 3.84 km2
- 1:10 560 County Series (also known as six-inch) Full sheets, 6 miles by 4 miles or 61.44 km2
- 1:10 560 County Series (also known as six-inch) Quarter sheets, 3 miles by 2 miles or 15.36 km2
- 1:2500 scale National Grid maps, 1 km by 1 km
- 1:10 000 scale National Grid maps, 5 km by 5 km or 25 km2
Raster formats
Historical Data is supplied on CD-ROM and is available in the following formats:- TIFF tiled Group 4 compression.
- TIFF banded Group 4 compression.
- PCX.
File sizes
File sizes will vary according to the format and compression techniques used. They will also vary from tile to tile depending upon the amount of detail. A sparse rural tile will compress further than a dense urban area tile. Typical file sizes for 1:2500 scale tile (at 300 dpi) are:- TIFF tiled Group 4 compression 0.5 - 4.0 Mb
- TIFF banded Group 4 compression 0.5 - 4.0 Mb
- PCX 4 - 15 Mb
- TIFF tiled Group 4 compression 0.5 - 3.0 Mb
- TIFF banded Group 4 compression 0.5 - 3.0 Mb
- PCX 2 - 12 Mb
Availability
Historical Data can be ordered from Landmark Information Group Ltd (details given below). A simple viewing package is available from Landmark Information Group Ltd or alternatively the data can be loaded into a geographical information system (GIS) or most desktop mapping packages.Copyright
Historical Data is supplied under a joint Ordnance Survey and Landmark Information Group Ltd perpetual licence agreement, for agreed internal business use of the data on up to 50 PCs or workstations. The licence gives the right to make unlimited hard copies for internal use. (Subject to the terms of the licence agreement). The licence fee is included in the price of the data. For further information about Historical Data contact the OS Sales Desk at:- Ordnance Survey
- Romsey Road
- SOUTHAMPTON
- United Kingdom
- SO16 4GU
- Phone: 023 80 792773
- Fax: 023 80 792324
- E-mail: sales@ordsvy.gov.uk Web site www.ordsvy.gov.uk
- Landmark Information Group Ltd
- 7 Abbey Court
- Eagle Way
- EXETER
- EX2 7HY
- Phone 01392 441738
- Fax 01392 441709
- E-mail mailbox@landmark-information.co.uk Web site www.landmark-information.co.uk