Provided by: libgeo-coordinates-osgb-perl_2.20-2_all 
NAME
Geo::Coordinates::OSGB::Background - Background and extended description
VERSION
2.20
DESCRIPTION
These notes are part of Geo::Coordinates::OSGB, a Perl implementation of latitude and longitude co-
ordinate conversion for England, Wales, and Scotland based on formulae and data published by the Ordnance
Survey of Great Britain.
These modules will convert accurately between an OSGB national grid reference, and coordinates given in
latitude and longitude using the WGS84 model. This means that you can take latitude and longitude
readings from your GPS receiver, (or read them from Wikipedia, or Google Earth, or your car's sat-nav),
and use this module to convert them to an accurate British National grid reference for use with one of
the Ordnance Survey's paper maps. And vice versa, of course.
These notes explain some of the background and implementation details that might help you get the most
out of them.
The algorithms and theory for these conversion routines are all from A Guide to Coordinate Systems in
Great Britain published by the OSGB, April 1999 (Revised December 2010) and available from the Ordnance
Survey website <http://www.ordnancesurvey.co.uk/>. You may also like to read some of the other
introductory material there. Should you be hoping to adapt this code to your own custom Mercator
projection, you will find the paper called Surveying with the National GPS Network, especially useful.
Upgrading from V2.09 or earlier
These modules suffered a major overhaul in V2.10 which changed the semantics and interface. The
motivation for the change was to simplify the interface, to make WGS84 the default model for latitude and
longitude, and to speed up conversions. This section explains what you might have to change to get your
old code to work with V2.10 and above.
• The module structure changed from V2.09 to V2.10 so it would probably be a good idea to clean up any
old installation you have and re-install a new version so that none of the old files hangs about.
The simplest way to do this is to use the "cpanm" application.
• The parsing and formatting routines have been split into a separate module, so before V2.10 you could
do this:
use Geo::Coordinates::OSGB ':all'; # Don't do this any more
now you need to do this:
use Geo::Coordinates::OSGB qw(grid_to_ll ll_to_grid);
use Geo::Coordinates::OSGB::Grid qw(parse_grid format_grid);
• The old "OSTN02.pm" module has been removed. The data and the functions it provided are now
integrated into "OSGB.pm". The old "OSGB36_to_ETRS89" and "ETRS89_to_OSGB36" functions are now built
into "ll_to_grid" and "grid_to_ll"; the OSTN data is now used automatically when needed.
• The main conversion routines now assume the WGS84 model for lat/lon coordinates. If you still want
to work with the OSGB36 model for lat/lon, then you need to add the "{ shape => 'OSGB36' }" option
when converting. In other words if you did this before:
my ($e, $n) = ll_to_grid(52.5, -2);
then to get exactly the same results, you now need to do this
my ($e, $n) = ll_to_grid(52.5, -2, { shape => 'OSGB36' } );
On the other hand, if you are working with the WGS84 model, then before V2.10 you had to do this:
# Old and exact
my ($e, $n) = ETRS89_to_OSGB36(ll_to_grid(52.5, -2));
or this:
# Old and approximate
my ($e, $n) = ll_to_grid(shift_ll_from_WGS84(52.5, -2));
now you can just do:
my ($e, $n) = ll_to_grid(52.5, -2);
This means that the most likely use case is the default. In other words you can take lat/lon from
your GPS and get correct grid references by default
• The functions "shift_ll_into_WGS84" and "shift_ll_from_WGS84" are no longer provided. They were not
very accurate, and they were confusing. "ll_to_grid" and "grid_to_ll" now do the necessary
adjustments for you automatically.
• The functions to parse and format grid references are moved to "OSGB/Grid.pm". They have also been
radically simplified, so that there are now only two functions: "parse_grid" and "format_grid". The
old names like "parse_GPS_grid" and "format_grid_trad" are retained with their former meanings, but
they are just synonyms for the two core functions.
• The functions to parse and format lat/lon have been removed. They were not really a core function of
this module. There are other modules on CPAN to deal with latitude and longitude. I also give an
example of how to format decimal degrees as degrees, minutes, and seconds in "examples/bngl.pl".
Coordinates and ellipsoid models
This section explains the fundamental problems of mapping a spherical earth onto a flat piece of paper
(or computer screen). A basic understanding of this material will help you use these routines more
effectively. It will also provide you with a good store of ammunition if you ever get into an argument
with someone from the Flat Earth Society.
It is a direct consequence of Newton's law of universal gravitation (and in particular the bit that
states that the gravitational attraction between two objects varies inversely as the square of the
distance between them) that all planets are roughly spherical; if they were any other shape gravity
would tend to pull them into a sphere. On the other hand, most useful surfaces for displaying large
scale maps (such as pieces of paper or screens) are flat. Therefore the fundamental problem in making a
map of the earth is that the curved surface being mapped must be distorted at least slightly in order to
get it to fit onto a flat map.
This module sets out to solve the corresponding problem of converting latitude and longitude coordinates
(designed for a spherical surface) to and from a rectangular grid (for a flat surface). A spherical
projection is a fairly simple but tedious bit of trigonometry, but the problem is complicated by the fact
that the earth is not quite a sphere. Because our planet spins about a vertical axis, it tends to bulge
out slightly in the middle, so it is more of an oblate spheroid (or ellipsoid) than a sphere. This makes
the arithmetic even more tedious, but the real problem is that the earth is not a regular ellipsoid
either; it is an irregular lump that closely resembles an ellipsoid and which is constantly (if slowly)
being rearranged by plate tectonics. So the best we can do is to pick an imaginary regular ellipsoid
that provides a good fit for the region of the earth that we are interested in mapping.
An ellipsoid model is defined by a series of numbers: the major and minor semi-axes of the solid, and a
ratio between them called the flattening. There are four ellipsoid models that are relevant to the UK:
OSGB36
The OSGB36 ellipsoid is a revision of work begun by George Airy the Astronomer Royal in 1830, when
the OS first undertook to make a series of maps that covered the entire country. It provides a good
fit for most of the British Isles.
EDM50
The European standard ellipsoid is a compromise to get a good fit for most of Western Europe. This
is not used by these modules.
WGS84
As part of the development of the GPS network by the American military in the 1980s a new world-wide
ellipsoid called WGS84 was defined. This fits most populated regions of the world reasonably well.
(Technically the ellipsoid is called GRS80, and WGS84 refers to the whole World Geodetic System that
is based on it, plus some very nerdy modifications, but for the purposes of this module it's just a
label).
ETRS89
The European Terrestrial Reference System is also based on GRS80, and for our purposes is identical
to WGS84. The technical difference is that in the ETRS89 system assumes that the Eurasian tectonic
plate is the reference, whereas WGS84 assumes that the American plate is the reference. But this
makes no practical difference whatsoever for the use of these modules.
The latitude and longitude marked on OS maps printed before 2015 are given in the OSGB36 model. The
latitude and longitude you read from your GPS device, or from Wikipedia, or Google Earth are in the WGS84
model. So the point with latitude 51.4778 and longitude 0 in the OSGB36 model is on the prime meridian
line in the courtyard of the Royal Observatory in Greenwich, but the point with the same coordinates in
the WGS84 model is about 120 metres away to the south-east, in the park.
In these modules the shape used for the projection of latitude and longitude onto the grid is WGS84
unless you specifically set it to use OSGB36.
The British National Grid and OSTN02 / OSTN15
A Mercator grid projection like the British National Grid is defined by the five parameters defined as
constants at the top of the module.
• True point of origin Latitude and Longitude = 49N, 2W
• False origin easting and northing = 400000, -100000
• Convergence factor = 0.9996012717
One consequence of the True Point of Origin of the British Grid being set to 49N, 2W is that all the
vertical grid lines are parallel to the 2W meridian; you can see this on the appropriate OS maps (for
example Landranger sheet 184), or on the PDF picture supplied with this package in the "examples" folder.
The effect of moving the False Point of Origin to the far south west is to make all grid references
positive.
Strictly speaking, grid references are given as the distance in metres from the False Point of Origin,
with the easting always given before the northing. For everyday use however, the OSGB suggest that grid
references need only to be given within the local 100km square as this makes the numbers smaller. For
this purpose they divide Britain into a series of 100km squares, each identified by a pair of letters:
TQ, SU, ND, etc. The grid of the big squares actually used is something like this:
HP
HU
HY
NA NB NC ND
NF NG NH NJ NK
NL NM NN NO NP
NR NS NT NU
NW NX NY NZ OV
SC SD SE TA
SH SJ SK TF TG
SM SN SO SP TL TM
SR SS ST SU TQ TR
SV SW SX SY SZ TV
SW covers most of Cornwall, TQ London, HU the Shetlands, and there is one tiny corner of a beach in
Yorkshire that is in OV. The system has the neat feature that N and S are directly above each other, so
that most Sx squares are in the south and most Nx squares are in the north. The system logically extends
far out in all directions; so square XA lies south of SV and ME to the west of NA and so on. But it
becomes less useful the further you go from the central meridian of 2W.
Within each of the large squares, we only need five-digit coordinates --- from (0,0) to (99999,99999) ---
to refer to a given square metre. But general use rarely demands such precision, so the OSGB
recommendation is to use units of 100m (hectometres) so that we only need three digits for each easting
and northing --- (000,000) to (999,999). If we combine the easting and northing we get the familiar
traditional six figure grid reference. Each of these grid references is repeated in each of the large
100km squares but this does not usually matter for local use with a particular map. Where it does
matter, the OS suggest that the six figure reference is prefixed with the identifier of the large grid
square to give a `full national grid reference', such as TQ330800. This system is described in the notes
in the corner of every Landranger 1:50,000 scale map.
This system was originally devised for use on top of the OSGB36 model of latitude and longitude, so the
prime meridian used and the coordinates of the true point of origin are all defined in that system.
However as part of standardizing on an international GPS system, the OS have redefined the grid as a
rubber sheet transformation from WGS84. There is no intrinsic merit to using one model or another, but
there's an obvious need to be consistent about which one you choose, and with the growing ubiquity of GPS
systems, it makes sense to standardize on WGS84.
The grid remains the primary reference system for use with maps, but the OS has always also printed a
latitude and longitude `graticule' around the edges of the large scale sheets. Traditionally these
coordinates have been given in the OSGB36 model, but since 2015 the OS has been printing revised editions
of Explorer and Landranger sheets with WGS84 coordinates instead. The legend of my recently purchased
copy of Explorer 311 has this paragraph under the heading `The National Grid Reference System':
• Base map constructed on Transverse Mercator Projection, Airy Ellipsoid, OSGB (1936) Datum. Vertical
datum mean sea level. The latitude, longitude graticule overlay is on the ETRS89 datum and is
compatible with the WGS84 datum used by satellite navigation devices.
If your map does not have the last sentence you can assume that it shows OSGB36 latitude and longitude.
Of course, this change makes no difference to the grid itself.
The differences between the OSGB36 and WGS84 models are only important if you are working at a fairly
small scale. The average differences on the ground vary from about -67 metres to + 124 meters depending
on where you are in the country.
Square Easting difference Northing difference
-------------------- ------------------------- ------------------
HP 109 66
HT HU 100 106 59 62
HW HX HY 73 83 93 51 48 47
NA NB NC ND 61 65 81 89 40 39 38 40
NF NG NH NJ NK 57 68 79 92 99 30 29 28 26 26
NL NM NN NO 56 66 79 91 18 17 15 15
NR NS NT NU 66 77 92 100 3 2 1 0
NW NX NY NZ 70 77 92 103 -9 -8 -10 -13
SC SD SE TA 77 93 104 112 -19 -22 -23 -24
SH SJ SK TF TG 79 91 103 114 124 -35 -34 -35 -38 -40
SM SN SO SP TL TM 72 80 90 101 113 122 -49 -47 -46 -46 -46 -47
SS ST SU TQ TR 80 90 101 113 121 -57 -56 -57 -57 -59
SW SX SY SZ TV 71 79 90 100 113 -67 -64 -62 -62 -62
The chart above shows the mean difference in each grid square. A positive easting difference means the
WGS84 Lat/Lon is to the east of OSGB36; a positive northing difference means it is to the north of
OSGB36. At a scale of 1:50,000, 124 meters is 2.48 mm, and at 1:25,000 it is 4.96 mm, so the difference
is readily visible if you compare new and old editions of the same map sheet.
The transformation from WGS84 to OSGB36 published in 2002 was called OSTN02 and consisted of a large data
set that defined a three dimensional shift for each square kilometre of the country. This dataset was
revised (apparently to give a better fit) in 2015 and the revised dataset is called OSTN15.
To get from WGS84 latitude and longitude to the grid, you project from the WGS84 ellipsoid to a pseudo-
grid and then look up the relevant shifts from OSTN15 and adjust the easting and northing accordingly to
get coordinates in the OSGB grid. Going the other way is slightly more complicated as you have to use an
iterative approach to find the latitude and longitude that would give you your grid coordinates.
It is also possible to use a three-dimensional shift and rotation called a Helmert transformation to get
an approximate conversion. This approach is used automatically by these modules for locations that are
undefined in OSTN15, and, if you want to, you can explicitly use it anywhere in the UK by using the
"grid_to_ll_helmert" and "ll_to_grid_helmert" routines.
Modern GPS receivers can all display coordinates in the OS grid system. You just need to set the display
units to be `British National Grid' or whatever similar name is used on your unit. Most units display
the coordinates as two groups of five digits and a grid square identifier. The units are metres within
the grid square. However you should note that your consumer GPS unit will not have a copy of the whole
of OSTN15 in it. To show you an OSGB grid reference, your GPS will be using either a Helmert
transformation, or an even more approximate Molodenksy transformation to translate from the WGS84
coordinates it is getting from the satellites.
Note that the OSGB (and therefore this module) does not cover the whole of the British Isles, nor even
the whole of the UK, in particular it covers neither the Channel Islands nor Northern Ireland. The
coverage that is included is essentially the same as the coverage provided by the OSGB "Landranger"
1:50000 series maps. The coverage of the OSTN02 data set was slightly smaller, as the OS did not
originally define the model for any points more than about 2km off shore. The main difference in OSTN15
is that coverage is extended to the whole rectangle from grid point (0,0) to (700000,1250000), although
the accuracy far off shore should not be relied on more than about +/- 5m.
Implementation of OSTN shift data
The OSTN15 is the definitive transformation from WGS84 coordinates to the British National Grid. It is
published as a large text file giving a set of corrections for each square kilometre of the country. The
OS also publish an algorithm to use it which is described on their website. Essentially you take WGS84
latitude and longitude coordinates and project them into an (easting, northing) pair of coordinates for
the flat surface of your grid. You then look up the corrections for the four corners of the relevant
kilometre square and interpolate the exact corrections needed for your spot in the square. Adding these
exact corrections gives you an (easting, northing) pair in the British grid.
The distributed data also includes a vertical height correction as part of the OSGM15 geoid module, but
this is not used in this module, so it is omitted from the module in order to save space.
The table of data supplied by the Ordnance Survey contains 876951 rows with entries for each km
intersection between (0,0) and (700000, 1250000). In OSTN02, 567472 of these entries referred to square
kms that are more than 10 km away from the British mainland (either in the sea or in Eire) and these were
set to zero, indicating that OSTN02 is not defined at these places. In order to save more space, these
were omitted from the beginning and end of each row in the data as stored in my original implementation.
The last 21 rows (north of Shetland) were all zeroes, so these were omitted as well.
My implementation of the OSTN02 data was included in the "OSGB.pm" module after the "__DATA__" line, and
was read using the magic "<DATA>" file handle. In tests this proved to be the fastest way to load all
that data, by a long way.
There were 1229 rows of data, and each row contained up to 701 pairs of shift data encoded as pairs of
integers representing the shift in mm. Leading and trailing zeros were omitted, and the number of
leading zeros omitted was stored in the first three characters of each row. The integer pairs were all
coded in a home-grown version of base32 using the character set "0123456789:;<=>?@ABDEFGHIJKLMNO". This
allowed integers up to 32767 to be stored in three bytes of printable ASCII. that could be stored inside
"OSGB.pm". Decoding them was very slightly slower than decoding hex strings, but using 3 bytes integer
instead of 4 reduced the memory and loading time by 25%.
When the OSGB revised OSTN02 to OSTN15, they filled in all the zeros, so this complicated approach was no
longer justified. In version 2.20 and above of this module, the shift data sets are included in the
module's `share` directory and loaded at run time. This turns out to be faster and simpler.
Accuracy, uncertainty, and speed
This section explores the limits of accuracy and precision you can expect from this software.
Accuracy of readings from GPS devices
If you are converting readings taken from your own handheld GPS device, the readings themselves will not
be very accurate. To convince yourself of this, try taking your GPS on the same walk on different days
and comparing the track: you will see that the tracks do not coincide. If you have two units take them
both and compare the tracks: you will see that they do not coincide.
The accuracy of the readings you get will be affected by cloud cover, tree cover, the exact positions of
the satellites relative to you (which are constantly changing as the earth rotates), how close you are to
sources of interference, like buildings or electricity installations, not to mention the ambient
temperature and the state of your rechargeable batteries.
To get really accurate readings you have to invest in some serious professional or military grade
surveying equipment.
How big is 0.000001 of a degree?
In the British Isles the distance along a meridian between two points that are one degree of latitude
apart is about 110 km or just under 70 miles. This is the distance as the crow flies from, say, Swindon
to Walsall. So a tenth of a degree is about 11 km or 7 miles, a hundredth is just over 1km, 0.001 is
about 110m, 0.0001 about 11m and 0.00001 just over 1 m. If you think in minutes, and seconds, then one
minute is about 1840 m (and it's no coincidence that this happens to be approximately the same as 1
nautical mile). One second is a bit over 30m, 0.1 seconds is about 3 m, and 0.0001 second is about 3mm.
Degrees Minutes Seconds * LATITUDE *
1 = 110 km 1 = 1.8 km 1 = 30 m
0.1 = 11 km 0.1 = 180 m 0.1 = 3 m
0.01 = 1.1 km 0.01 = 18 m 0.01 = 30 cm
0.001 = 110 m 0.001 = 2 m 0.001 = 3 cm
0.0001 = 11 m 0.0001 = 20 cm 0.0001 = 3 mm
0.00001 = 1.1 m 0.00001 = 2 cm
0.000001 = 11 cm 0.000001 = 2 mm
0.0000001 = 1 cm
Degrees of latitude get very slightly longer as you go further north but not by much. In contrast
degrees of longitude, which represent the same length on the ground as latitude at the equator, get
significantly smaller in northern latitudes. In southern England one degree of longitude represents
about 70 km or 44 miles, in northern Scotland it's less than 60 km or about 35 miles. Scaling everything
down means that the fifth decimal place of a degree of longitude represents about 60-70cm on the ground.
Degrees Minutes Seconds * LONGITUDE *
1 = 60-70 km 1 = 1.0-1.2 km 1 = 17-20 m
0.1 = 6-7 km 0.1 = 100-120 m 0.1 = 2 m
0.01 = 600-700 m 0.01 = 10-12 m 0.01 = 20 cm
0.001 = 60-70 m 0.001 = 1 m 0.001 = 2 cm
0.0001 = 6-7 m 0.0001 = 10 cm 0.0001 = 2 mm
0.00001 = 60-70 cm 0.00001 = 1 cm
0.000001 = 6-7 cm
How accurate are the conversions?
The OS supply test data with OSTN15 that comes from various fixed stations around the country and that
form part of the definition of the transformation. If you look in the test files "06-osgb-*.t" you can
see how it is used for testing these modules.
In all cases translating from the WGS84 coordinates to the national grid and vice versa is accurate to
the millimetre, so these modules are at least as accurate as the OSGB software that produced the test
data.
The main difference between the OSTN02 transformation and OSTN15 is that the model fits the whole grid
area better. With OSTN02 the conversions were (very slightly) less accurate for places west of 7W.
Translating from the given grid coordinates to WGS84 latitude and longitude coordinates was accurate to
1mm for all of England, Wales, Scotland and the Isle of Man, but `round trip' testing (by generating
random grid references, converting them to WGS84 latitude and longitude and then converting them back to
grid easting and northing), showed that beyond of 6W (that is in the Scilly Isles and the Hebrides), the
error creeps up to about 4mm if you go as far as St Kilda (at about 8.57W). The new OSTN15 numbers are
all very slightly different, so that converting any given latitude and longitude in WGS84 gives a grid
reference that may be a few mm different. But OSTN15 no longer shows greater round trip errors in the
far west. The accuracy of round trip conversions is less than 1mm for all of the OSGB test points in both
directions.
Outside the rectangle covered by OSTN15, this module uses the small Helmert transformation recommended by
the OS. The OS state that, with the parameters they provide, this transformation will be accurate up to
about +/-5 metres, in the vicinity of the British Isles.
You can also use this transformation within the OSTN15 rectangle by calling the "grid_to_ll_helmert" and
"ll_to_grid_helmert" routines. If you compare the output from these routines to the output from the more
accurate routines that use OSTN15 you will find that the differences are between about -3.6 metres and
+5.1 metres depending on where you are in the country. In the South East both easting and northing are
underestimated, in northern Scotland they tend to be overestimated.
How fast are the conversions?
In general the answer to this question is "probably faster than you need", but if you have read this far
you might be interested in the results of my benchmarking. The slowest part of these routines used to be
the loading of the OSTN15 data set but I have put considerable effort into making this faster since about
version 2.08 of this module, so that now the accurate routines the use the OSTN15 data are pretty much
the same as the approximate routines ("ll_to_grid" is slightly faster, "grid_to_ll" is slightly slower).
The tests used to be a bit generous to my code because of the caching effect. In order to speed up the
OSTN data lookups, the routines used to keep a cache of the data fetched, so that if you convert a
sequence of coordinates in the same square km, the second and subsequent lookups will get the data from a
local hash instead of having to read the table. The benchmark test (using the standard Benchmark.pm
approach) consists of getting Perl to run as many conversions as possible for about 5 CPU seconds, and as
a result the cache hit ratio was probably rather exaggerated. Nevertheless this might still be
reasonably representative if you are converting, say, the steps in a GPX track, where successive steps
are highly likely to be the same square km as the one before. Moreover, since converting to OSTN15, I
have removed the cache look ups so I'm no longer getting any such boost.
Last year (2016) with version 2.16 of this module, a typical bench mark run on my development machine (a
Mac Mini server from 2011) using the Apple-supplied Perl 5.16 gave:
Subroutine calls per sec ms per call
----------------------------------------------
ll_to_grid 41677 0.024
ll_to_grid_helmert 42145 0.024
grid_to_ll 18131 0.055
grid_to_ll_helmert 35793 0.028
On my newer work machine (a MBP from 2015) using the newer Perl 5.18 supplied with macOS Sierra, I got
slightly better numbers with V2.16
Subroutine calls per sec ms per call
----------------------------------------------
ll_to_grid 50980 0.020
ll_to_grid_helmert 68267 0.015
grid_to_ll 25831 0.039
grid_to_ll_helmert 54371 0.018
But after a bit more work to simplify the way that the cache is implemented I get this on the same MBP
with the version 2.19:
Subroutine calls per sec ms per call
----------------------------------------------
ll_to_grid 69099 0.0145
ll_to_grid_helmert 66158 0.0151
grid_to_ll 29114 0.0343
grid_to_ll_helmert 53969 0.0185
Now, on the same MBP, but with the new OSTN15 data set and the simpler data look up I get:
Subroutine calls per sec ms per call
----------------------------------------------
ll_to_grid 70730 0.0141
ll_to_grid_helmert 63308 0.0158
grid_to_ll 32238 0.0310
grid_to_ll_helmert 55286 0.0181
In my opinion, this justifies my decision to make the accurate OSTN conversion the default. The
approximate Helmert-based routine is no quicker for ll to grid conversions.
It is always going to be slightly slower converting from grid to ll, due to the iterative nature of the
algorithm that is built into OSTN15. So the approximate routine is likely to remain slightly faster.
Having said that none of the routines is really slow, since even "grid_to_ll" averages under 60
microseconds per call on the older machine. `Your mileage may vary', of course.
The routines have been tested with various versions of Perl, including recent versions with the
"uselongdouble" option enabled. Using a locally compiled version of Perl 5.22 with ordinary doubles, I
saw a small improvement on the Helmert routines, but the default routines are about the same. On the
same system, long doubles slowed everything down by about 10%, and made no difference to the round trip
precision of the routines. Since the formulae were specifically designed for ordinary double precision
arithmetic, Perl's default arithmetic is more than adequate.
Maps
Since Version 2.09 these modules have included a set of map sheet definitions so that you can find which
paper maps your coordinates are on.
See Geo::Coordinates::OSGB::Maps for details of the series included. The first three series are OS maps:
A - OS Landranger maps at 1:50000 scale;
B - OS Explorer maps at 1:25000;
C - the old OS One-Inch maps at 1:63360.
Landranger sheet 47 appears in the list of keys as "A:47", Explorer sheet 161 as "B:161", and so on. As
of 2015, the Explorer series of incorporates the Outdoor Leisure maps, so (for example) the two sheets
that make up the map `Outdoor Leisure 1' appear as "B:OL1E" and "B:OL1W".
Thanks to the marketing department at the OS and their ongoing re-branding exercise several Explorer
sheets have been promoted to Outdoor Leisure status. So (for example) Explorer sheet 364 has recently
become `Explorer sheet Outdoor Leisure 39'. Maps like this are listed with a combined name, thus:
"B:395/OL54".
Many of the Explorer sheets are printed on both sides. In these cases each side is treated as a separate
sheet and distinguished with suffixes. The pair of suffixes used for a map will either be N and S, or E
and W. So for example there is no Explorer sheet "B:271", but you will find sheets "B:271N" and
"B:271S". The suffixes are determined automatically from the layout of the sides, so in a very few cases
it might not match what is printed on the sheet but it should still be obvious which side is which.
Where the map has a combined name the suffix only appears at the end. For example: "B:386/OL49E" and
"B:386/OL49W".
Several sheets also have insets, for islands, like Lundy or The Scilly Isles, or for promontories like
Selsey Bill or Spurn Head. Like the sides, these insets are also treated as additional sheets (albeit
rather smaller). They are named with an alphabetic suffix so Spurn Head is on an inset on Explorer sheet
292 and this is labelled "B:292.a". Where there is more than one inset on a sheet, they are sorted in
descending order of size and labelled ".a", ".b" etc. On some sheets the insets overlap the area of the
main sheet, but they are still treated as separate map sheets.
Some maps have marginal extensions to include local features - these are simply included in the
definition of the main sheets. There are, therefore, many sheets that are not regular rectangles.
Nevertheless, the module is able to work out when a point is covered by one of these extensions.
In the examples folder there is an extended example showing how to work with the map data as supplied
through the "Maps.pm" interface.
The source files for the map data are in the "maps" directory. The script that builds "Maps.pm" from the
source files is "build/make_maps_code".
It's probably fair to say that currently (in early 2016) everything about the maps is still experimental
and may change in the future.
For each series there are two files:
• "catalogue" which defines: an index number for each map (unique to this project); the sheet number
for the map - not necessarily an integer; the sheet title as a UTF8 string; the current ISBN number
for maps that have them.
• "polygons" which has: index numbers that match the corresponding "catalogue"; sheet numbers that
match the corresponding "catalogue"; a flag - an integer to show the status of the entry (no formal
meaning); followed by a MULTIPOLYGON in WKT format.
There is one line in each of the files for each map in the series. The data format for the sheet
polygons is Well Known Text (as defined in Wikipedia). The set of polygons for each map is defined as a
MULTIPOLYGON; a list of POLYGONS. There are no holes in any of the polygons. A missing polygon list is
recorded as "EMPTY" (this is part of WKT).
The units are metres from the false point of origin (which is some miles west of the Scilly Isles). So
the south west corner of Landranger sheet 204, which has traditional grid reference SW 720 140 is defined
in this data as "172000 14000". This is essentially what "parse_grid" returns. No leading zeros needed.
The polygon starts at the south west corner of the sheet and is recorded anticlockwise. WKT insists that
the first pair is repeated at the end to close the polygon. So a simple 40km square Landranger sheet with
no insets or extensions, such as Sheet 152, whose SW corner is at SP 530 300 is recorded as
(((453000 230000, 493000 230000, 493000 270000, 453000 270000, 453000 230000)))
If the sheet boundary is more complicated than a square, the polygon consists of a coordinate pair for
each corner. Extensions - where the coloured printing spills over the neat edge - are included as part
of the main polygon in the appropriate place, always moving anticlockwise. Extensions don't have to be
rectilinear but they are made up of straight lines. Extensions consisting of administrative boundaries
and labels are ignored. If in doubt, use common sense.
If an inset is drawn on the map sheet with its own grid margin then it is recorded as a separate polygon
following the WKT format, even if it overlaps the main sheet.
On OS Landranger maps, the first (and last) pair should always be the SW corner, if an extension affects
the SW corner, start and end with the regular corner pair even if they are technically redundant. This
allows me to find the SW corners currently defined for the Landranger maps easily. In other series,
start somewhere near the SW and go anticlockwise.
perl v5.36.0 2022-12-06 Geo::Coordinate...SGB::Background(3pm)