SABRE Maps/Calibrating

Georeferencing scanned maps for SABRE Maps can seem very daunting at first, as there's an awful lot of background information that's useful for understanding the full picture, but not essential to know for casual assistance with the simplest maps to complete.

The easiest way by far to get involved in georeferencing scanned map sheets is by using the SABRE Maps Online Grid Calibrator, which provides all the information needed in a simple-to-use format. The Online Grid Calibrator should be used for most cases for Ordnance Survey GB, Ordnance Survey of Ireland and Ordnance Survey Northern Ireland sheets.

For maps that are not supported by the Online Grid Calibrator, or to understand the in-depth processes behind Georeferencing, then read on here.

Calibration in depth

Note that when using the Online Grid Calibrator, these steps are done for you

The simplest maps are those that are based completely on the OSGB National Grid, or OSNI/OSI Irish Grid, which is most maps post-World War 2 in Great Britain, and starting in the 1960s in Ireland.

To georeference these simplest type of maps, you basically need to know three things:

• The top left Grid Reference on the map
• The bottom right Grid Reference on the map
• The distance between thick gridlines printed on the map

Then, all you need is to grab a copy of the SABRE Maps Offline Grid Calibrator, download it to your computer and put it somewhere sensible; then ask for a map scan, save it in the same location at the Grid Calibrator; load the map image into the Grid Calibrator by typing in the filename and pressing Load map image; type in the values mentioned above; press Set Calibration; and then start clicking the thick gridline joins - start at the top left, and work your way along the top, clicking on each thick gridline, with the final point being on the right hand side; then go down to the next thick gridline and repeat. When you've finished, press Save Calibration, and the tool will save a small CSV to your "Downloads" location on your computer. Then just provide that CSV back to us. You can also see the map in Google Earth with a small additional process, so please get in touch if you'd like to do that as well.

There's a straighforward OSGB One Inch Seventh Series map scan that you can try this all out with located here.

Calibrator tools

A number of tools are available to help create calibration files.

Map borders can safely be left intact during the calibration process, as the tile generator tools will slice off anything outside the calibration area.

You must use the correct tool designed for the type of map you have, especially if that map covers more than a small area. If you do not, then poor results will be obtained. Tools should be used in the order listed below, with the OpenStreetMap calibration tool (for lat/lon) being used only for maps that cannot use the Grid Calibrator tool. For more information about which maps use which type of projection, then see the relevant section below.

Again, bear in mind that those tempting lat/lon values on maps older than 1984 definitely do not use the WGS84 co-ordinate system, and so simply copying the lat/lon values printed on the map will give errors. The SABRE Maps Co-ordinate Finder shows the difference between the two types of lat/lon values.

There is a long-standing bug in Google Chrome where sometimes very large images cause severe performance issues, and scrolling around an image the size of a map is extremely painful. If you use Google Chrome as your primary browser, and this issue occurs within one of the SABRE Maps Calibrator tools, then switch to an alternate browser and performance will be as expected. Note that the Chromium-based Microsoft Edge browser does not appear to suffer from this issue.

Online Grid Calibrator

Main Article: SABRE Maps/Online Grid Calibrator

The Online Grid Calibrator is the primary method for georeferencing map scans, and should be used in all cases where possible.

Offline Grid Calibrator

There is a small bug in the Grid Calibrator that occasionally means that the map image is not displayed at the correct size, especially when completing multiple maps in a single session. To prevent this from happening, click twice on Load map image and following the second click the map image will be displayed correctly. If you can help resolve this, please contact us!

Using the Grid Calibrator on an OS One Inch map of the Shetland Islands

When using the Grid Calibrator, complete each row in turn, clicking on the left edge, each thick gridline, and the right edge each time. For the average OS One Inch Seventh Series map, as here, 30 points (represented by the blue dots) are generally required for a good result. Other map types have a different number, depending on the map size.

Because an OS 1:25k Provisional Edition sheet is physically much smaller than a One Inch Seventh Series sheet, only 9 points are generally required for a good result.

The Grid Calibrator is used to quickly create points that refer to various projection formats.

It supports the following projections and grid systems:

• OSGB National Grid
• Irish Grid
• OSGB National Yard Grid
• Cassini (Delamere)
• Bonne (Scotland)
• Bonne (Ireland)
• Cassini (War Office)

The Grid Calibrator can be found at this link.

To load in a file, copy and paste a URL for it (eg: file:///Users/sabre/maps/7th/185.jpg) into the "File path" box and click "Change". If you have trouble, then save the calibrator tool to the same location on your computer as the map you're trying to georeference, then you can simply type in the filename into the box.

Don't worry if the instructions look complicated - all items within the calibrator tool have tooltips to help you out.

Firstly, you need to select the exact projection of the map from the options given, with the most common being OS National Grid and Irish Grid.

The calibrator then wants to know six values :

1. Eastings from: The most western (or leftmost) grid reference value on the map or taken from the SABRE Maps Co-ordinate Finder.
2. Eastings to: The most eastern (or rightmost) grid reference value on the map.
3. Eastings every: The distance between every thick grid reference line across the map in metres (or yards for National Yard Grid maps). For OS One Inch Seventh Series maps this is 10000. For ten mile maps, this is 100000. For 1:25k Provisional Edition, use 5000. For Cassini (Delamere) or Bonne maps, put the distance in miles between gridlines here, for example, 2 for One Inch Popular Edition maps.
4. Northings from: The most northern (or topmost) grid reference value on the map
5. Northings to: The most southern (or bottommost) grid reference value on the map
6. Northings every: This is usually the same value as "Horizontal every" but for thick grid reference lines down the map.

When you have set up the values, click "Set calibration". You are now ready to start calibrating.

To georeference OSGB National Grid, Irish Grid and National Yard Grid maps

Begin by clicking in the top left hand corner of the map. Then click at the top edge where the first thick north-south grid reference line meets it. Then click on the second, third etc. and finally on the top right hand corner.

Then click at the left edge where the first thick west-east grid reference line meets it. Then click at the intersection of that and the first north-south grid reference line meets it. Then click on the second, third etc. and finally on the right hand edge.

Then repeat for each west-east grid reference line, and finally for the bottom left hand corner and edges through to the bottom right hand corner.

As you click on points, the calibration points will appear just above the map image on the right hand side of the screen. Bear in mind that for some grid types (such as the National Yard Grid) conversion calculations are performed, so the numbers you see may not be those you are expecting.

Note that some maps (for example, the 1:25k Provisional Edition) are actually too small to have thick gridlines across them. For these sheets, it is recommended to use the 5000m "Every" value, which equates to every fifth gridline.

To georeference Cassini (Delamere) and Bonne maps

Complete as for the above maps, but each gridline is set at a particular distance from the top left corner, with each map series having a different distance. For these maps, the value of "Every" should be set to the gridline value appropriate to the map.

All of OS, OSNI and OSI used the common values beneath for maps with gridlines.

Gridline examples

Map Scale	Gridline distance (miles)
One Inch	2
Half Inch	4
Quarter Inch	8

For maps without gridlines, then they will need to be calibrated from the four corners only, with "Every" set to a large value such as 10000.

Saving the calibration files

Once you have finished, there are then two options.

"Save calibration" will save a CSV file with the same name as the original JPG file to your default download location. If you have issues with this, then there is also a "Copy calibration" button that will copy all of the calibration points to the clipboard to be pasted into a text editor such as Notepad++ in order to save.

In addition, there is space for an optional Sheet number and a "Copy bounds info" button. This is used exclusively for putting together the full layer bounds file, so is not generally required; and if in doubt it should be left blank.

The Grid Calibrator tool uses the Co-ordinate converter developed by Ed Fielden for Fielden Maps.

OpenStreetMap Calibrator

Using the OpenStreetMap calibrator to align points on a 1923 MOT map of West London

The OpenStreetMap calibrator is used as a calibrator of last resort, and simply maps lat/lon pairs to pixels on a map image, and can be found here. To load in a file, copy and paste a URL for it (eg: file:///Users/sabre/maps/7th/185.jpg) into the "file" box and click "Load map image". Note, some browsers such as Google Chrome complain unless you put the map file on a website and copy and paste the address into the "File path" box instead.

You will then see a standard OpenStreetMap map at the top, and your map image at the bottom. Find the same point on both maps, then click on the point in the modern map, followed by the point in the map you are calibrating. As you click on points, the calibration file will appear in a box above the maps.

Again, once you have finished, there are then two options. "Save calibration" will save a CSV file with the same name as the original JPG file to your default download location. If you have issues with this, then there is also a "Copy calibration" button that will copy all of the calibration points to the clipboard to be pasted into a text editor such as Notepad++ in order to save.

Bear in mind that the final map output will be to the leftmost, topmost, bottommost and rightmost points created, so try to get as far into the corners of the map as possible, or failing that, try to get as many points near to the edges as possible. This will then minimise the "lost" areas of the map.

MapRect

MapRect is a tool for Windows that is similar to the Grid Calibrator. It is useful for lower-spec PCs that have difficulty loading large map images on modern web browsers.

On running MapRect, you can load in a map image and start clicking on points. If you want to set up a particular set of points, there is a "Config" button that presents the start, every and end point for the horizontal and vertical axes, in the same way as the Grid Calibrator.

After clicking on a point, it appears as a red marker over the image, and the relevant line in the output calibration file will be generated.

Note that MapRect only supports the OSGB National (metric) Grid at present. For all other map types, you should use the Grid Calibrator.

You can download MapRect here.

Generating a Google Earth compatible KML file

For the next stage of the georeferencing process, we create a Google Earth-compatible KML file. This is used for two purposes: firstly, so that you can see that your georeferencing is of a good quality, and the map is where it's meant to be shown; and secondly as the tiling process requires a KML file in order to complete.

The program used to generate the KML file needed for the process is called warp-sabre, created by Ritchie333. It is derived off a suite of OpenStreetMap tools called warp-gbos, with some extra bells and whistles to handle a whole load of other projections relevant to SABRE Maps.

It currently supports the following projections:

• OSGB National Grid
• Irish Grid
• WGS84 Lat/Lon
• Cassini (Delamere)
• Bonne (Scotland)
• Bonne (Ireland)

It does not currently support:

• OSGB National Yard Grid (hence the calibration tool converting to OSGB National Grid)
• OSGB36 Lat/Lon
• Irish Transverse Mercator Grid
• Cassini (War Office) Grid (hence the calibration tool converting to OSGB National Grid)
• War Office Irish Grid
• UTM

Before starting on this section, make sure you have both the original stitched map scan, and the matching calibration file as created in earlier steps. You also need to download the warp-sabre programs from this location on the SABRE webspace. Unzip the files within, and place them in the same folder as your map image and CSV file that you generated with the Calibrator tool.

The basic command line needed to generate the KML file for an OSGB National Grid map is as follows, replacing the items in italics with the names in each case:

warp-sabre.exe -i mapfilename.jpg -p calibrationfilename.csv -o KMLname --fit 1

Note that KMLname does not have a file extension. Each KML file has a (mostly!) standardised name in order to make the tiling requirements simpler. Some examples are given below, though if an example isn't given, please confirm with the SABRE Maps Co-ordinator what the KML filename should be.

Map series	KML file example name	Notes
OSGB One Inch 7th Series/ New Popular	78.kml	Combined sheet 138/151 is 138151.kml. Note for those five sheets where NPE sheetlines are different, a different format is used.
OSGB One Inch New Popular	106npe.kml	Only used for sheets 106, 160, 161, 185 and 189 which differ from the equivalent Seventh Series sheets.
OSGB One Inch Fifth Edition	5th-112.kml	Sheet 113 comes in two sizes, the later large sheet version is 5th-113l.kml; whilst the later district version of Sheet 95 is 5th-95d.kml
OSGB Scottish Popular	spe01.kml
OSGB England and Wales Popular	pop01.kml
OSNI One Inch Third Edition	osni_01.kml
OSNI Popular Edition	osni_pop_01.kml
OSI One Inch	i01.kml
OSGB Landranger	001.kml	Reserved for future use
OSI/OSNI Discoverer/Discovery	dis001.kml	Reserved for future use
OSGB Quarter Inch Fifth Series	05.kml
OSGB Quarter Inch Fourth Edition	qi-4-01.kml	Scottish sheets have their sheet number prefixed with "s", for example, qi-4-s04.kml
OSGB Quarter Inch Third Edition	qi-3-01.kml	Scottish sheets have their sheet number prefixed with "s", for example, qi-3-s04.kml
OSGB 1:25k Provisional Edition	TL21.kml	Whilst early maps have a sheet number that uses the original numbered National Grid squares, we standardise on the modern lettered squares - so Sheet 21/58 would be SS58.kml.
OSI Half Inch	i05.kml
OSGB Half Inch Second Series	28.kml

If the map is not an OSGB National Grid map, then the correct item from the below table needs to be added to the end of the warp-sabre command line:

Projection	additional command argument
Irish Grid	--inproj osi
WGS84 Lat / lon	--inproj mercator
Cassini (Delamere)	--inproj cassini
Bonne (Scotland)	--inproj bonnes
Bonne (Ireland)	--inproj bonnei

This will then generate a KML file (and associated JPG with the same name). Opening this KML file within Google Earth will show the quality of the calibration as features on the map such as roads, rivers, railways and so on can be compared to their actual location. Note that a perfect match won't always be possible, but if the KML file map and reality are too far apart, then a poor quality final product will be achieved. In this case, the map should be recalibrated before re-entering this process.

In addition, don't be put off if your KML file appears skew within Google Earth, with part margins showing. This happens because the various Grids are skewed when compared to Lat/Lon, which can be easily seen if a map has Lat/Lon values also printed within the margins. These part-margins will be sorted out during the tiling process later on.

Note that the --inproj mercator parameter is special, as it can take a mixture of points and co-ordinates in any other projection, that are automatically converted to WGS84. For example, the post-war revisions of Scottish Popular Edition maps can be calibrated using a mix of the corners in Cassini (Delamere) and other points using the National Grid.

Map Projections and Co-ordinates

This seemingly innocuous spot in Cheshire is actually the centre of the known universe as far as MOT maps are concerned. However, these days, we use the National Grid.

To georeference a map, you need to know what map projection and co-ordinate system that map uses. Co-ordinates and projections take a number of formats, and whilst a number of these are now considered archaic, as they are relevant for the purposes of producing mapping for SABRE Maps, we need to be aware of them.

The basic co-ordinate system that most people are familiar with is latitude / longitude (often shortened to lat / lon), which defines any point on earth as a pair of angles. 0 degrees latitude is the equator, 90 is the North Pole, 0 degrees longitude is the Prime Meridian that runs through the Royal Observatory at Greenwich.

However, just describing a point as lat / lon isn't enough. The Earth is not a perfect sphere, and is squashed a bit at the poles, so to make an accurate translation between a three-dimensional planet and a two-dimensional map, we need to specify a projection. On SABRE Maps, a lat / lon that is not otherwise qualified is always in reference to the World Geodetic System 1984 (WGS84), which is the same system your satnav uses, so it's become a de facto standard in the world. For example, the SABRE AGM is held near lat 52.587135 lon -2.117393.

Older maps do feature lat / lon, but as our favourite historic maps that are out of copyright were all produced before 1984, you should be extremely careful, as the values on map won't be using WGS84 and hence won't correspond to a typical satnav! For example, the Ordnance Survey uses a slightly different projection called OSGB1936 which is similar to, but not the same as WGS84; and so Lat/Lon values printed on the map are not suitable for use within SABRE Maps and using them will give errors.

OSGB National Grid

The eastern edge of a Seventh Series map shows distances north in metres from the origin of the National Grid (in this case, 302,000 metres north).

The Ordnance Survey National Grid is probably the most familiar projection and co-ordinate systems to people in Great Britain.

It is a system which defines a point as a metric distance from an origin a little way southwest of the Scilly Isles, called a grid reference. Grid references take two forms; the first is a combination of two letters identifying 500kmx500km squares followed by three numbers each for east and north (as seen when you click on a square in SABRE Maps). However, for calibration in SABRE Maps, we use the full grid reference of metres east and metres north. For example, the AGM is held near SO920988 or 392000:298800.

Almost all Ordnance Survey maps produced from 1945 use the National Grid, and maps are generally aligned at grid references that are multiples of 100m (very often also multiples of 500m).

As such, these are one of the simplest sets of maps to georeference for SABRE Maps - everything is already present on the map for the information needed, and the various tools used in SABRE Maps all support the National Grid.

Irish Grid and Irish Transverse Mercator Grid

This OSNI map uses the Irish Grid projection

For Ordnance Survey of Ireland (OSI) and Ordnance Survey Northern Ireland (OSNI) maps, there is the Irish Grid, in which the island of Ireland is divided into 25 squares, each identified by a single letter. The squares are numbered A to Z with I being omitted. They use a slightly different system to the OSGB National Grid, and the two grids are slightly skew to one another.

Similar to the GB National Grid, the Irish Grid also has full co-ordinates in metres that don't use the grid squares, and we use that reference for georeferencing.

The Irish Grid was replaced by OSI and OSNI in 2001 by the Irish Transverse Mercator Grid, though at present the Irish Grid is still shown on mapping. The Irish TM Grid isn't supported within the SABRE Maps toolset at present, mostly because no map using that system is due to drop out of copyright until the 2050s, so we have plenty of time to add it!

Again, Irish Grid maps are extremely simple to georeference for SABRE Maps, as everything is present on the printed map that is needed.

OSGB National Yard Grid

Whilst many references, both online and in reference books, will state that the OSGB National Grid was created for post-war mapping, this is incorrect. The National Grid was first used in the 1930s for map series including the OS One Inch Fifth Edition, the Quarter Inch Fourth Edition and the Ten Mile Road Maps. The big difference between these and later maps is that grid references are measured in yards instead of metres, hence the later retcon of National Yard Grid, which is a term that was not used at the time.

National Yard Grid maps are again easy to georeference as all the information needed is present on the vast majority of maps. However, the tools that generate the map tiles themselves don't directly support the National Yard Grid, so the Grid Calibrator tool converts the numbers for you from the National Yard Grid to the National (metric) Grid.

Cassini (Delamere)

This MoT map extract shows the surrounding alphanumeric grid

Earlier Ordnance Survey GB maps within the primary period of SABRE Maps (such as the OS One Inch Popular Edition and the English, Welsh and London MoT maps) use the Cassini projection, with an origin near Delamere Forest in Cheshire (just north of where the A556 branches off the A54) - hence the Cassini (Delamere) term.

Cassini (Delamere) maps generally date from the start of our time period to the introduction of the National Grid, although some are from the same time period as the National Yard Grid. These maps are distinctly unhelpful for us, as they do not have printed on them any form of national co-ordinate system; but instead have a printed alphanumeric grid surrounding them. An an example, on MoT maps of England and Wales there are usually 13½ squares west to east ("1" to "14" with 14 being half width, and 9 or 12 squares north to south ("A" to "J" or "M", "I" is not used).

It is however, quite possible to georeference these maps, and the SABRE Maps toolsets all support Cassini (Delamere) maps. You will, however, need to find the corner co-ordinates as explained below via the SABRE Maps Co-ordinate Finder.

Bonne (Scotland)

The Scottish MoT maps use the Bonne (Scotland) Projection, which is centred on 57°30′N 4°W which is in the middle of nowhere in Inverness-shire, east of Inverness and southwest of Nairn. This projection is generally used for older maps of Scotland, with Cassini (Delamere) being preferred for the OS One Inch Scottish Popular Edition mapping.

They again have the alphanumeric grid surrounding them, and is supported by all SABRE Maps toolsets, including the SABRE Maps Co-ordinate Finder.

Bonne (Ireland)

Similarly to Great Britain, Ireland had a pre-Grid projection that was used heavily, that of Bonne (Ireland). This uses a similar kind of projection to the Scottish Bonne mapping, but with an origin of 53°30′N 8°W; which is within Lough Ree, near Athlone, pretty much on the border between County Roscommon and County Westmeath.

They are also often surrounded by an alphanumeric grid (with the exception of later maps).

This projection is again fully supported by all SABRE Maps tools, including the Co-ordinate Finder.

Cassini (War Office)

Also known as the Cassini Grid, Modified British System and the War Office Grid, this is the purple grid printed across military edition mapping used during World War 2 and the immediate post-war period. It was actually created in 1927, but eventually maps with the overprint ended up in civilian hands as large numbers were based upon standard mapping. There are, for example, New Popular Edition maps with the Cassini Grid printed across them.

There was also a similar grid for Ireland, known as the War Office Irish Grid.

However, the vast majority of maps showing the Cassini Grid are based around other mapping on different projections, and so the Cassini Grid is currently not supported on the SABRE Maps tile generation tools. However, the Grid Calibrator tool does offer some limited support for the Cassini Grid so that it can be used to add additional calibration points for maps where all standard projection information (such as the National Grid) have been removed. The Grid Calibrator will converts full Cassini Grid co-ordinates for you to the National (metric) Grid. Avoid using the Cassini (War Office) data where at all possible for georeferencing. There is zero support at present for the War Office Irish Grid.

Standard civilian editions are preferred for SABRE Maps use.

Other projections used by Ordnance Survey

Some other mapping that was originally produced by the Ordnance Survey during the 20th century uses different projections, most notably the Channel Islands mapping. These are generally based on Cassini projections, with different origins. For example, Guernsey maps have an origin of Fort George, hence Cassini (Fort George); whilst Jersey maps have an origin of South Hill Battery, hence Cassini (South Hill Battery).

Later maps of the Channel Islands use the Universal Mercartor Grid, using Zone 30N technically based on the ED50 International Spheroid.

Very early Ordnance Survey maps are also based around various different Cassini origins, based around counties or groups of counties.

None of these additional projections are currently supported by the SABRE Maps toolset; although the UTM grid references are visible using the The SABRE Maps Co-ordinate Finder.

Maps of all these types must be georeferenced using the OpenStreetMap Calibrator.

How to find co-ordinates for maps with archaic projections

The SABRE Maps Co-ordinate Finder

As discussed above, many older maps don't have a nice set of grid-co-ordinates ready for us to use.

For the most common mapping series that we have on SABRE Maps, the corner co-ordinates can be found on the main article on that particular map series such as OS Quarter Inch or OS Popular Edition. If unsure, these can usually be found via the appropriate Coverage Project.

For other, less common, map series, we have written something to help with this - The SABRE Maps Co-ordinate Finder. This is a small web page that displays an OpenStreetMap base map, but calculates the location of the mouse pointer and provides the co-ordinates in a number of different systems.

Thanks to the agreement of The Charles Close Society, the original creators of the data, the SABRE Maps Co-ordinate Finder also has held within it the outlines of the most common relevant OS, OSI and OSNI sheets in order to be able to quickly locate the locations of each map. This sheet information can be accessed from the menu in the top right hand corner of the Finder.

In order to use it, you need to simply place the mouse pointer at the location of the top left corner of the relevant paper map, and note down the co-ordinates in the correct system. Then do the same for the bottom right corner of the paper map. These values are then required for the georeferencing process itself. If this is challenging (for example, because the corner of the map shows sea), then as long as the correct projection is being used then the map will be square to that projection - so as long as you have the top, left, bottom and right edge locations, then that is sufficient. The top value can then be ascertained from any location along the top edge of the map, and so on.

One really important thing to remember when georeferencing maps with archaic projections is that they are best thought of as if they are "imperial" sizes - in other words don't think in decimal terms, but in fractions such as halves, quarters and eighths as the map sheetlines are generally in these locations. For example, if the Co-ordinate Finder indicates a reference of 25.6852, then the "real" reference will probably be 25.6875 (also known as 25¹¹⁄₁₆ miles...)

Note that the Cassini (Delamere) and the two Bonne projection co-ordinates are given in miles rather than feet within the SABRE Maps Co-ordinate Finder, as that is the correct value for the SABRE Maps tile creation tools.

The SABRE Maps Co-ordinate finder tool uses the Co-ordinate converter developed by Ed Fielden for Fielden Maps; whilst the Sheetlines vector files are by courtesy of and agreement with The Charles Close Society.

Maps with disguised projections

This OSI Half Inch has Irish Grid References on it, but is actually to the Irish Bonne projection, as can be seen by the values not being to a 1000m marker

As can be seen from the above, map projections are something that fall in and out of fashion, and so there are a number of maps that pretend to be of a more modern type than they actually are. These maps are usually relatively easy to spot, as the references at each corner don't match up, or gridlines printed on a map are skewed across the map.

One of the most common mapping in the period SABRE Maps is interested in that have these issues are OS One Inch Scottish Popular Editions, where the post-war maps show the National Grid on them but are actually still on the Cassini (Delamere) projection as per the pre-war revisions. However, probably the longest-lasting disguised mapping is that of OSI Half Inch mapping, which were initially created before independence and were still being updated into the mid-1980s. The Irish Grid first began to be shown on these maps in 1955, but similar to the Scottish Popular maps, the grid is slightly skew to the mapping and can be seen by the different references in the corners.

Another disguised map series is the OS Quarter Inch Fourth Series, which was created on National Yard Grid lines; but converted (with very bold WITH NATIONAL GRID wording) to the metric National Grid after World War 2. It is easily spotted simply on the fact that the grid references in the corners are not round numbers. Fortunately in this case, because the two grids are the same bar the measurements, they can be safely georeferenced in whatever system is printed on that particular map.

Occasionally however, a map is disguised because a projection or co-ordinate system drops out of favour. The OS District Map of Greater London, first published in 1936, was on National Yard Grid lines, with all the grid references expected. However, the 1945 and 1946 revisions are from after the Yard Grid fell out of favour, and so no grid of any kind is shown. Fortunately, as they are on exactly the same sheetlines as the earlier revisions with the Yard Grid printed on them, the values from those maps can be used.

For SABRE Maps purposes, maps should always be georeferenced using their "real" projection, otherwise errors and loss will occur.

How do I know which map uses which projection?

In order to assist with calibration, the following is a list of types of OS/OSI/OSNI mapping found on SABRE Maps (or will be within the next few years), along with the projection types and any relevant notes.

One Inch / 1:50,000

Name	Series	Projection Type	Notes
OS One Inch	Third Edition (England and Wales)	Cassini (Delamere)
OS One Inch	Third Edition (Scotland)	Bonne (Scotland)
OS One Inch	Fourth Edition (England and Wales)	Cassini (Delamere)
OS One Inch	Fourth Edition (Scotland)	Bonne (Scotland)
OS One Inch	Popular (England and Wales)	Cassini (Delamere)
OS One Inch	Popular (Scotland)	Cassini (Delamere)	Post-war editions have road numbers and the National Grid printed on them, but are still on the pre-war projections, so still need to be calibrated as Cassini (Delamere)
OS One Inch	Fifth Edition	National Yard Grid
OS One Inch	New Popular Edition	National Grid
OS One Inch	Seventh Series	National Grid
OS 1:50,000		National Grid	Also known as Landranger
OSI One Inch	Third Edition	Bonne (Ireland)
OSI 1:50,000		Irish Grid
OSNI One Inch	Popular Edition	Bonne (Ireland)	Some editions have the Irish Grid printed on them, but they are still on the previous projection, so will need to be calibrated as Bonne (Ireland)
OSNI One Inch	Third Series	Irish Grid
OSNI 1:50,000		Irish Grid

Half Inch

Name	Series	Projection Type	Notes
OS Ministry of Transport	England and Wales	Cassini (Delamere)
OS Ministry of Transport	Scotland	Bonne (Scotland)
OS Half Inch	District (Relief) - England and Wales	National Yard Grid
OS Half Inch	District (Relief) - Scotland	Bonne (Scotland)
OS Half Inch	Greater London	National Yard Grid	Later revisions do not have the National Yard Grid printed on them, but are on exactly the same lines as the pre-war editions
OS Half Inch	Great Britain	National Grid	Only five sheets published, plus later Snowdonia Tourist Map
OSI Half Inch	Ireland	Bonne (Ireland)	Later maps have the Irish Grid printed on them, but are on the previous lines (as can be seen by the non-matching values in the corners)
OSNI Half Inch	Second Series	Irish Grid

Quarter Inch / 1:250,000

Name	Series	Projection Type	Notes
OS Quarter Inch	Third Edition	Cassini (Delamere)	No road numbers printed
OS Quarter Inch	Third Edition (New Series)	Cassini (Delamere)	Printed on different sheetlines to the above and can be told easily by the presence of road numbers
OS Quarter Inch	Fourth Edition	National Yard Grid	Pre-war editions
OS Quarter Inch	Fourth Edition with National Grid	National Grid	Post-war editions but on the same Yard Grid sheetlines as the pre-war editions as can be seen by the strange corner values. Can however safely be calibrated as National Grid.
OS Quarter Inch	Fifth Series	National Grid
OS Routemaster		National Grid	When this series drops out of copyright, maps will be challenging to scan and georeference as early sheets were printed back-to-back with no central fold
OSI Quarter Inch		Bonne (Ireland)
OSI 1:250,000		Irish Grid
OSNI Quarter Inch		Bonne (Ireland)	Includes NI Road Map
OSNI Quarter Inch		Irish Grid	From 1957 onwards
OSNI 1:250,000		Irish Grid

Ten Mile / 1:625,000

Name	Series	Projection Type	Notes
OS Ten Mile Road Map		National Yard Grid	1932-1944, plus 1947 edition
OS Ten Mile Road Map		National Grid	1946, plus from 1948. These were designed to use the National Yard Grid (hence the odd corner co-ordinates), but only ever released using the metric National Grid.
OS Route Planning Map		National Grid	From 1964. Later editions called Routeplanner.

Two Inch / Three Inch / 1:25,000

Name	Series	Projection Type	Notes
OS Ministry of Transport		Cassini (Delamere)	London only
OS Isles of Scilly		National Yard Grid	One Inch Fifth Edition style
OS 1:25,000		National Grid	Also known as "Pathfinder" and "OS Two and a Half Inch Map"
Three Inch Map of Guernsey		Cassini (Fort George)	Prior to 1958, maps use the Cassini Projection with the origin being at Fort George. The SABRE Maps toolsets currently do not support Cassini (Fort George) and are unlikely to.
Three Inch Map of Guernsey		ED50	From 1958 maps use UTM Gridlines on "International Spheroid", which is another term for ED50. The SABRE Maps Co-ordinate Finder supports ED50 UTM, but the toolsets currently do not, so conversion to WGS84 Lat/Lon is required.
Two Inch Map of Jersey		Unknown	Maps have gridlines of lat/lon at one minute intervals, but the values do not match OSGB36 or ED50. ED50 is closest, but approximately 100m out. Suspicion is that the maps are Cassini (South Hill Battery), which is unsupported within the SABRE Maps toolsets.

Calibration file format

Calibration files are a series of comma separated values. Each co-ordinate is on one line. They look slightly different depending on whether you are using lat / lon or grid references.

Grid references

A grid reference file looks something like this:

os,400000:250000,300,200
os,500000:250000,1200,200
os,400000:150000,300,900
os,500000:150000,1200,900

The os tells the calibrator this is an OSGB grid reference co-ordinate, whilst osi would be used for the Irish Grid. The next values are the metres east and metres north of the grid reference, followed by the x and y co-ordinate the point is on the map image. (Note, the grid reference values are separated by a colon, not a comma; this is because grid references can also be specified using the two letters + number format, which is not generally used by SABRE Maps).

Archaic maps

A file for Cassini (Delamere), or Scottish or Irish Bonne projections looks something like this:

bni,30.879,30.765,345,411
bni,84.879,30.765,8417,425
bni,30.879,-5.235,335,5797
bni,84.879,-5.235,8416,5800

The bni tells the calibrator this is an Irish Bonne map, whilst bns would be used for a Scottish Bonne map, and cas for a Cassini (Delamere) map. The next values are the number of miles north and east of the origin point, followed by the x and y co-ordinate the point is on the map image. Note that the distance values are separated by a comma in these cases.

Latitude / longitude

A lat / lon file looks something like this:

p,51.8,-0.5,300,200
p,51.8,-0.1,1200,200
p,51.7,-0.5,300,900
p,51.7,-0.1,1200,900

The "p" tells the calibrator this is a lat / lon co-ordinate. The next values are the latitude and longitude of the point, followed by the x and y co-ordinate the point is on the map image.

In all of the examples, the calibration defines the four corners of a map - top left, top right, bottom left and bottom right. The four corner co-ordinates are the most important ones to include in a calibration file, otherwise the tools have no idea where the map is supposed to be displayed in the world - at all.

Mixture of projections

A mixed file contains co-ordinates in a number of different projections. For example, the Scottish Popular Edition maps have the corners and edges specified using the Cassini (Delamere) projection, but with the National Grid overlaid. For example:

cas,-81.69886,207.26894,643,847
cas,-53.69886,207.26894,8676,838
cas,-81.69886,189.26894,652,6026
cas,-53.69886,189.26894,8689,6018
gbos1936,230000,700000,1342,1620
gbos1936,240000,700000,3126,1597
gbos1936,250000,700000,4909,1576
gbos1936,260000,700000,6688,1558

Note, that the National Grid co-ordinates here use a different system to a file containing only grid references.

Tips for specific map series

Route Planning and Ten Mile Maps

Ordnance Survey Route Planning Maps, especially the northern sheet, have large inset areas upon them, for example the Orkney and Shetland Islands. These areas need calibrating appropriately for them, but they also have the effect of "covering up" the main sheet gridlines.

The best way for dealing with this issue is:

1. Use GIMP, Paint.NET or similar image manipulation tool to copy the insets into separate image files (with their own calibration files)
2. Create a copy of the original with the insets removed and replaced with blank (white) space
3. Draw lines between the visible grid squares
4. Use this copied image to work out the calibration points
5. Create another copy of the original, this time with insets removed and replaced with black (standard "nothing here" background colour)
6. Use this to actually run the warp / calibration to get the KML files

OS 1:25,000

When creating calibration or annotation files for 1:25k maps, do not use the Lat/Lon given on the maps themselves - they are to OSGB36 format, and not to WGS84; and the difference is enough to offset the maps (or annotation) by quite a large amount.

Pre-National Grid (or pre-Irish Grid) mapping

As explained above, the georeferencing tools used by SABRE Maps understands a number of old projections in use by the various Ordnance Surveys before the relevant National Grids.

One really important thing to remember when using Cassini (Delamere); or Scottish or Irish Bonne projections is that they are best treated as if they are "imperial" sizes - in other words don't think in decimal terms, but in fractions such as halves, quarters and eighths as the map sheetlines are generally in these locations.