Provided by: libgeo-coordinates-osgb-perl_2.20-1.1_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.