trusty (3) Geo::Coordinates::OSGB.3pm.gz
NAME
Geo::Coordinates::OSGB - Convert coordinates between Lat/Lon and the British National Grid
An implementation of co-ordinate conversion for England, Wales, and Scotland based on formulae published
by the Ordnance Survey of Great Britain.
These modules will convert accurately between an OSGB national grid reference and lat/lon coordinates
based on the OSGB geoid model. (For an explanation of what a geoid model is and why you should care,
read the Theory section below.) The OSGB geoid model fits mainland Britain very well, but is rather
different from the international WGS84 model that has rapidly become the de facto universal standard
model thanks to the popularity of GPS devices and maps on the Internet. So, if you are trying to
translate from an OSGB grid reference to lat/lon coordinates that can be used in Google Earth, Wikipedia,
or some other Internet based tool, you will need to do two transformations: first translate your grid ref
into OSGB lat/lon; then nudge the result into WGS84. Routines are provided to do both of these
operations, but they are only approximate. The inaccuracy of the approximation varies according to where
you are in the country but may be as much as several metres in some areas.
To get more accurate results you need to combine this module with its companion Geo::Coordinates::OSTN02
which implements the transformation that now defines the relationship between GPS survey data based on
WGS84 and the British National Grid. Using this module you should be able to get results that are
accurate to within a few centimetres, but it is slightly slower and requires more memory to run.
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.
VERSION
Examine $Geo::Coordinates::OSGB::VERSION for details.
SYNOPSIS
use Geo::Coordinates::OSGB qw(ll_to_grid grid_to_ll);
# Basic conversion routines
($easting,$northing) = ll_to_grid($lat,$lon);
($lat,$lon) = grid_to_ll($easting,$northing);
DESCRIPTION
These modules provide a collection of routines to convert between coordinates expressed as latitude &
longtitude and map grid references, using the formulae given in the British Ordnance Survey's excellent
information leaflet, referenced below in the Theory section. There are some key concepts explained in
that section that you need to know in order to use these modules successfully, so you are recommended to
at least skim through it now.
The module is implemented purely in Perl, and should run on any Perl platform.
In this description `OS' means `the Ordnance Survey of Great Britain': the British government agency that
produces the standard maps of England, Wales, and Scotland. Any mention of `sheets' or `maps' refers to
one or more of the 204 sheets in the 1:50,000 scale `Landranger' series of OS maps.
This code is fine tuned to the British national grid system. You could use it elsewhere but you would
need to adapt it. Some starting points for doing this are explained in the Theory section below.
SUBROUTINES/METHODS
The following functions can be exported from the "Geo::Coordinates::OSGB" module:
grid_to_ll ll_to_grid
shift_ll_into_WGS84 shift_ll_from_WGS84
parse_grid
parse_trad_grid format_grid_trad
parse_GPS_grid format_grid_GPS
parse_landranger_grid format_grid_landranger
parse_ISO_ll format_ll_trad
format_ll_ISO
None of these is exported by default, so pick the ones you want or use an ":all" tag to import them all
at once.
use Geo::Coordinates::OSGB ':all';
ll_to_grid(lat,lon)
When called in a void context, or with no arguments "ll_to_grid" does nothing.
When called in a list context, "ll_to_grid" returns two numbers that represent the easting and the
northing corresponding to the latitude and longitude supplied.
The parameters can be supplied as real numbers representing decimal degrees, like this
my ($e,$n) = ll_to_grid(51.5, 2.1);
Following the normal convention, positive numbers mean North or East, negative South or West. If you
have data with degrees, minutes and seconds, you can convert them to decimals like this:
my ($e,$n) = ll_to_grid(51+25/60, 0-5/60-2/3600);
Or you can use a single string in ISO 6709 form, like this:
my ($e,$n) = ll_to_grid('+5130-00005/');
To learn exactly what is matched by this last option, read the source of the module and look for the
definition of $ISO_LL_PATTERN. Note that the neither the "+" or "-" signs at the beginning and in
the middle, nor the trailing "/" may be omitted.
If you have trouble remembering the order of the arguments, or the returned values, note that
latitude comes before longitude in the alphabet too, as easting comes before northing.
The easting and northing will be returned as a whole number of metres from the point of origin of the
British Grid (which is a point a little way to the south-west of the Scilly Isles).
If you want the result presented in a more traditional grid reference format you should pass the
results to one of the grid formatting routines, which are described below. Like this.
$gridref = format_grid_trad(ll_to_grid(51.5,-0.0833));
$gridref = format_grid_GPS(ll_to_grid(51.5,-0.0833));
$gridref = format_grid_landranger(ll_to_grid(51.5,-0.0833));
However if you call "ll_to_grid" in a scalar context, it will automatically call "format_grid_trad"
for you.
It is not needed for any normal work, but "ll_to_grid()" also takes an optional argument that sets
the ellipsoid model to use. This normally defaults to `OSGB36', the name of the normal model for
working with British maps. If you are working with the highly accurate OSTN02 conversions supplied
in the companion module in this distribution, then you will need to produce pseudo-grid references as
input to those routines. For these purposes you should call "ll_to_grid()" like this:
my $pseudo_gridref = ll_to_grid(51.2, -0.4, 'WGS84');
and then transform this to a real grid reference using "ETRS89_to_OSGB36()" from the companion
module. This is explained in more detail below.
format_grid_trad(e,n)
Formats an (easting, northing) pair into traditional `full national grid reference' with two letters
and two sets of three numbers, like this `TQ 102 606'. If you want to remove the spaces, just apply
"s/\s//g" to it.
$gridref = format_grid_trad(533000, 180000); # TQ 330 800
$gridref =~ s/\s//g; # TQ330800
If you want the individual components call it in a list context.
($sq, $e, $n) = format_grid_trad(533000, 180000); # (TQ,330,800)
Note the easting and northing are truncated to hectometers (as the OS system demands), so the grid
reference refers to the lower left corner of the relevant 100m square.
format_grid_GPS(e,n)
Users who have bought a GPS receiver may initially have been puzzled by the unfamiliar format used to
present coordinates in the British national grid format. On my Garmin Legend C it shows this sort of
thing in the display.
TQ 23918
bng 00972
and in the track logs the references look like this "TQ 23918 00972".
These are just the same as the references described on the OS sheets, except that the units are
metres rather than hectometres, so you get five digits in each of the easting and northings instead
of three. So in a scalar context "format_grid_GPS()" returns a string like this:
$gridref = format_grid_GPS(533000, 180000); # TQ 33000 80000
If you call it in a list context, you will get a list of square, easting, and northing, with the
easting and northing as metres within the grid square.
($sq, $e, $n) = format_grid_GPS(533000, 180000); # (TQ,33000,80000)
Note that, at least until WAAS is working in Europe, the results from your GPS are unlikely to be
more accurate than plus or minus 5m even with perfect reception. Most GPS devices can display the
accuracy of the current fix you are getting, but you should be aware that all normal consumer-level
GPS devices can only ever produce an approximation of an OS grid reference, no matter what level of
accuracy they may display. The reasons for this are discussed below in the section on Theory.
format_grid_landranger(e,n)
This routine does the same as "format_grid_trad", but it appends the number of the relevant OS
Landranger 1:50,000 scale map to the traditional grid reference. Note that there may be several or
no sheets returned. This is because many (most) of the Landranger sheets overlap, and many other
valid grid references are not on any of the sheets (because they are in the sea or a remote island.
This module does not yet cope with the detached insets on some sheets.
In a list context you will get back a list like this: (square, easting, northing, sheet) or (square,
easting, northing, sheet1, sheet2) etc. There are a few places where three sheets overlap, and one
corner of Herefordshire which appears on four maps (sheets 137, 138, 148, and 149). If the GR is not
on any sheet, then the list of sheets will be empty.
In a scalar context you will get back the same information in a helpful string form like this "NN 241
738 on OS Sheet 44". Note that the easting and northing will have been truncated to the normal
hectometre three digit form. The idea is that you'll use this form for people who might actually
want to look up the grid reference on the given map sheet, and the traditional GR form is quite
enough accuracy for that purpose.
parse_trad_grid(grid_ref)
Turns a traditional grid reference into a full easting and northing pair in metres from the point of
origin. The grid_ref can be a string like 'TQ203604' or 'SW 452 004', or a list like this "('TV',
'435904')" or a list like this "('NN', '345', '208')".
parse_GPS_grid(grid_ref)
Does the same as "parse_trad_grid" but is looking for five digit numbers like 'SW 45202 00421', or a
list like this "('NN', '34592', '20804')".
parse_landranger_grid(sheet, e, n)
This converts an OS Landranger sheet number and a local grid reference into a full easting and
northing pair in metres from the point of origin.
The OS Landranger sheet number should be between 1 and 204 inclusive (but I may extend this when I
support insets). You can supply "(e,n)" as 3-digit hectometre numbers or 5-digit metre numbers. In
either case if you supply any leading zeros you should 'quote' the numbers to stop Perl thinking that
they are octal constants.
This module will croak at you if you give it an undefined sheet number, or if the grid reference that
you supply does not exist on the sheet.
In order to get just the coordinates of the SW corner of the sheet, just call it with the sheet
number. It is easy to work out the coordinates of the other corners, because all OS Landranger maps
cover a 40km square (if you don't count insets or the occasional sheet that includes extra details
outside the formal margin).
parse_grid(grid_ref)
Attempts to match a grid reference some form or other in the input string and will then call the
appropriate grid parsing routine from those defined above. In particular it will parse strings in
the form '176-345210' meaning grid ref 345 210 on sheet 176, as well as 'TQ345210' and 'TQ 34500
21000' etc. You can in fact always use "parse_grid" instead of the more specific routines unless you
need to be picky about the input.
grid_to_ll(e,n) or grid_to_ll(grid_ref)
When called in list context "grid_to_ll()" returns a pair of numbers representing longitude and
latitude coordinates, as real numbers. Following convention, positive numbers are North and East,
negative numbers are South and West. The fractional parts of the results represent fractions of
degrees.
When called in scalar context it returns a string in ISO longitude and latitude form, such as
'+5025-00403/' with the result rounded to the nearest minute (the formulae are not much more accurate
than this). In a void context it does nothing.
The arguments must be an (easting, northing) pair representing the absolute grid reference in metres
from the point of origin. You can get these from a grid reference string by calling "parse_grid()"
first.
An optional last argument defines the geoid model to use just as it does for "ll_to_grid()". This is
only necessary is you are working with the pseudo-grid references produced by the OSTN02 routines.
See Theory for more discussion.
format_ll_trad(lat, lon)
Takes latitude and longitude in decimal degrees as arguments and returns a string like this
N52:12:34 W002:30:27
In a list context it returns all 8 elements (hemisphere, degrees, minutes, seconds for each of lat
and lon) in a list. In a void context it does nothing.
format_ll_ISO(lat, lon)
Takes latitude and longitude in decimal degrees as arguments and returns a string like this
+5212-00230/
In a list context it returns all 6 elements (sign, degrees, minutes for each of lat and lon) in a
list. In a void context it does nothing.
parse_ISO_ll(ISO_string)
Reads an ISO 6709 formatted location identifier string such as '+5212-00230/'. To learn exactly what
is matched by this last option, read the source of the module and look for the definition of
$ISO_LL_PATTERN. Note that the neither the "+" or "-" signs at the beginning and in the middle, nor
the trailing "/" may be omitted. These strings can also include the altitude of a point, in metres,
like this: '+5212-00230+140/'. If you omit the altitude, 0 is assumed.
In a list context it returns ($lat, $lon, $altitude). So if you don't want or don't need the
altitude, you should just drop it, for example like this:
my ($lat, $lon) = parse_ISO_ll('+5212-00230/')
In normal use you won't notice this. In particular you don't need to worry about it when passing the
results on to "ll_to_grid", as that routine looks for an optional altitude after the lat/lon.
shift_ll_from_WGS84(lat, lon, altitude)
Takes latitude and longitude in decimal degrees (plus an optional altitude in metres) from a WGS84
source (such as your GPS handset or Google Earth) and returns an approximate equivalent latitude and
longitude according to the OSGM02 model. To determine the OSGB grid reference for given WGS84
lat/lon coordinates, you should call this before you call "ll_to_grid". Like so:
($lat, $lon, $alt) = shift_ll_from_WGS84($lat, $lon, $alt);
($e, $n) = ll_to_grid($lat,$lon);
You don't need to call this to determine a grid reference from lat/lon coordinates printed on OSGB
maps (the so called "graticule intersections" marked in pale blue on the Landranger series).
This routine provide a fast approximation; for a slower, more accurate approximation use the
companion Geo::Coordinates::OSTN02 modules.
shift_ll_into_WGS84(lat, lon, altitude)
Takes latitude and longitude in decimal degrees (plus an optional altitude in metres) from an OSGB
source (such as coordinates you read from a Landranger map, or more likely coordinates returned from
"grid_to_ll()") and adjusts them to fit the WGS84 model.
To determine WGS84 lat/lon coordinates (for use in Wikipedia, or Google Earth etc) for a given OSGB
grid reference, you should call this after you call "grid_to_ll()". Like so:
($lat, $lon) = grid_to_ll($e, $n);
($lat, $lon, $alt) = shift_ll_into_WGS84($lat, $lon, $alt);
This routine provide a fast approximation; for a slower, more accurate approximation use the
companion Geo::Coordinates::OSTN02 modules.
THEORY
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 Dec 2010) and available at
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.
The routines are intended for use in Britain with the Ordnance Survey's National Grid, however they are
written in an entirely generic way, so that you could adapt them to any other ellipsoid model that is
suitable for your local area of the earth. There are other modules that already do this that may be
more suitable (which are referenced in the "See Also" section), but the key parameters are all defined at
the top of the module.
$ellipsoid_shapes{OSGB36} = [ 6377563.396, 6356256.910 ];
use constant ORIGIN_LONGITUDE => RAD * -2; # lon of grid origin
use constant ORIGIN_LATITUDE => RAD * 49; # lat of grid origin
use constant ORIGIN_EASTING => 400000; # Easting for origin
use constant ORIGIN_NORTHING => -100000; # Northing for origin
use constant CONVERGENCE_FACTOR => 0.9996012717; # Convergence factor
The ellipsoid model is defined by two numbers that represent the major and minor radius measured in
metres. The Mercator grid projection is then defined by the other five parameters. The general idea is
that you pick a suitable point to start the grid that minimizes the inevitable distortion that is
involved in a Mercator projection from spherical to Euclidean coordinates. Such a point should be on a
meridian that bisects the area of interest and is nearer to the equator than the whole area. So for
Britain the point of origin is 2W and 49N (in the OSGB geoid model) which is near the Channel Islands.
This point should be set as the "ORIGIN_LONGITUDE" and "ORIGIN_LATITUDE" parameters (as above) measured
in radians. Having this True Point of Origin in the middle and below (or above if you are antipodean)
minimizes distortion but means that some of the grid values would be negative unless you then also adjust
the grid to make sure you do not get any negative values in normal use. This is done by defining the
grid coordinates of the True Point of Origin to be such that all the coordinates in the area of interest
will be positive. These are the parameters "ORIGIN_EASTING" and "ORIGIN_NORTHING". For Britain the
coordinates are set as 400000 and -100000, so the that point (0,0) in the grid is just to the south west
of the Scilly Isles. This (0,0) point is called the False Point of Origin. The fifth parameter affects
the convergence of the Mercator projection as you get nearer the pole; this is another feature designed
to minimize distortion, and if in doubt set it to 1 (which means it has no effect). For Britain, being
so northerly it is set to slightly less than 1.
The British National Grid
One consequence of the True Point of Origin of the British Grid being set to "+4900-00200/" 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 "plotmaps.pdf" picture supplied with this package. The effect
of moving the False Point of Origin to the far south west is that all grid references always positive.
Strictly grid references are given as whole numbers of metres from this point, 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 identified in 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
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, and HU the Shetlands. Note that it 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.
Within each of these large squares, we only need five digit coordinates --- from (0,0) to (99999,99999)
--- to refer to a given square metre. For daily use however we don't generally need such precision, so
the normal recommended usage 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 for local use with a particular map, this does not usually matter. 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
of in the corner of every Landranger 1:50,000 scale map.
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 (although beware that the GPS fix is unlikely to be accurate down to the last metre).
Geoid 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. There is therefore a fundamental problem in
making any maps of the earth that its curved surface being mapped must be distorted at least slightly in
order to get it to fit onto the 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). This projection
is in itself is a fairly lengthy bit of maths, but what makes it extra complicated is 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 than a sphere. This makes the maths even longer, but the
real problem is that the earth is not a regular oblate spheroid either, but an irregular lump that
closely resembles an oblate spheroid and which is constantly (if slowly) being rearranged by plate
tectonics. So the best we can do is to pick an imaginary regular oblate spheroid that provides a good
fit for the region of the earth that we are interested in mapping. The British Ordnance Survey did this
back in 1830 and have used it ever since as the base on which the National Grid for Great Britain is
constructed. You can also call an oblate spheroid an ellipsoid if you like. The general term for an
ellipsoid model of the earth is a "geoid".
The first standard OSGB geoid is known as "Airy 1830" after the year of its first development. It was
revised in 1936, and that version, generally known as OSGB36, is the basis of all current OSGB mapping.
In 2002 the model was redefined (but not functionally changed) as a transformation from the international
geoid model WGS84. This redefinition is called OSGM02. For the purposes of these modules (and most
other purposes) OSGB36 and OSGM02 may be treated as synonyms.
The general idea is that you can establish your latitude and longitude by careful observation of the sun,
the moon, the planets, or your GPS handset, and that you then do some clever maths to work out the
corresponding grid reference using a suitable geoid. These modules let you do the clever maths, and the
geoid they use is the OSGM02 one. This model provides a good match to the local shape of the Earth in
the British Isles, but is not designed for use in the rest of the world; there are many other models in
use in other countries.
In the mid-1980s a new standard geoid model was defined to use with the fledgling global positioning
system (GPS). This model is known as WGS84, and is designed to be a compromise model that works equally
well for all parts of the globe (or equally poorly depending on your point of view --- for one thing
WGS84 defines the Greenwich observatory in London to be not quite on the zero meridian). Nevertheless
WGS84 has grown in importance as GPS systems have become consumer items and useful global mapping tools
(such as Google Earth) have become freely available through the Internet. Most latitude and longitude
coordinates quoted on the Internet (for example in Wikipedia) are WGS84 coordinates.
One thing that should be clear from the theory is that there is no such thing as a single definitive set
of coordinates for every unique spot on earth. There are only approximations based on one or other of
the accepted geoid models, however for most practical purposes good approximations are all you need. In
Europe the official definition of WGS84 is sometime referred to as ETRS89. For all practical purposes in
Western Europe the OS advise that one can regard ETRS89 as identical to WGS84 (unless you need to worry
about tectonic plate movements).
Practical implications
If you are working exclusively with British OS maps and you merely want to convert from the grid to the
latitude and longitude coordinates printed (as faint blue crosses) on those maps, then all you need from
these modules are the plain "grid_to_ll()" and "ll_to_grid()" routines. On the other hand if you want to
produce latitude and longitude coordinates suitable for Google Earth or Wikipedia from a British grid
reference, then you need an extra step. Convert your grid reference using "grid_to_ll()" and then shift
it from the OSGB model to the WGS84 model using "shift_ll_into_WGS84()". To go the other way round,
shift your WGS84 lat/lon coordinated into OSGB, using "shift_ll_from_WGS84()", before you convert them
using "ll_to_grid()".
If you have a requirement for really accurate work (say to within a millimetre or two) then you need to
use the OS's transformation matrix called OSTN02. This monumental work published in 2002 re-defined the
British grid in terms of offsets from WGS84 to allow really accurate grid references to be determined
from really accurate GPS readings (the sort you get from professional fixed base stations, not from your
car's sat nav or your hand-held device). The problem with it is that it defines the grid in terms of a
deviation in three dimensions from a pseudo-grid based on WGS84 and it does this separately for every
square km of the country, so the data set is huge and takes a second or two to load even on a fast
machine. Nevertheless a Perl version of OSTN02 is included as a separate module in this distribution
just in case you really need it (but you don't need it for any "normal" work). Because of the way OSTN02
is defined, the sequence of conversion and shifting works differently from the approximate routines
described above.
Starting with a really accurate lat/lon reading in WGS84 terms, you need to transform it into a pseudo-
grid reference using "ll_to_grid()" using an optional argument to tell it to use the WGS84 geoid
parameters instead of the default OSGB parameters. The Geo::Coordinates::OSTN02 package provides a
routine called "ETRS89_to_OSGB36()" which will shift this pseudo-grid reference into an accurate OSGB
grid reference. To go back the other way, you use "OSGB36_to_ETRS89()" to make a pseudo-grid reference,
and then call "grid_to_ll()" with the WGS84 parameter to get WGS84 lat/long coordinates.
($lat, $lon, $height) = (51.5, -1, 10);
($x, $y) = ll_to_grid($lat, $lon, 'WGS84');
($e, $n, $elevation) = ETRS89_to_OSGB36($x, $y, $height);
($x, $y, $z) = OSGB36_to_ETRS89($e, $n, $elevation);
($lat, $lon) = grid_to_ll($x, $y, 'WGS84');
EXAMPLES
# to import everything try...
use Geo::Coordinates::OSGB ':all';
# Get full coordinates in metres from GR
($e,$n) = parse_trad_grid('TQ 234 098');
# Latitude and longitude according to the OSGB geoid (as
# printed on OS maps), if you want them to work in Google
# Earth or some other tool that uses WGS84 then adjust results
($lat, $lon) = grid_to_ll($e, $n);
($lat, $lon, $alt) = shift_ll_into_WGS84($lat, $lon, $alt);
# and to go the other way
($lat, $lon, $alt) = shift_ll_from_WGS84($lat, $lon, $alt);
($e, $n) = ll_to_grid($lat,$lon);
# In both cases the elevation is in metres (default=0m)
# Reading and writing grid references
# Format full easting and northing into traditional formats
$gr1 = format_grid_trad($e, $n); # "TQ 234 098"
$gr1 =~ s/\s//g; # "TQ234098"
$gr2 = format_grid_GPS($e, $n); # "TQ 23451 09893"
$gr3 = format_grid_landranger($e, $n);# "TQ 234 098 on Sheet 176"
# or call in list context to get the individual parts
($sq, $e, $n) = format_grid_trad($e, $n); # ('TQ', 234, 98)
# parse routines to convert from these formats to full e,n
($e,$n) = parse_grid('TQ 234 098');
($e,$n) = parse_grid('TQ234098'); # spaces optional
($e,$n) = parse_grid('TQ',234,98); # or even as a list
($e,$n) = parse_grid('TQ 23451 09893'); # as above..
# You can also get grid refs from individual maps.
# Sheet between 1..204; gre & grn must be 3 or 5 digits long
($e,$n) = parse_grid(176,123,994);
# With just the sheet number you get GR for SW corner
($e,$n) = parse_grid(184);
# Reading and writing lat/lon coordinates
($lat, $lon) = parse_ISO_ll("+52-002/");
$iso = format_ll_ISO($lat,$lon); # "+520000-0020000/"
$str = format_ll_trad($lat,$lon); # "N52:00:00 W002:00:00"
BUGS AND LIMITATIONS
The conversions are only approximate. So after
($a1,$b1) = grid_to_ll(ll_to_grid($a,$b));
neither "$a==$a1" nor "$b==$b1". However "abs($a-$a1)" and "abs($b-$b1)" should be less than 0.00001
which will give you accuracy to within a metre. In the middle of the grid 0.00001 degrees is
approximately 1 metre. Note that the error increases the further away you are from the central meridian
of the grid system.
The "format_grid_landranger()" does not take account of inset areas on the sheets. So if you feed it a
reference for the Scilly Isles, it will tell you that the reference is not on any Landranger sheet,
whereas in fact the Scilly Isles are on an inset in the SW corner of Sheet 203. There is nothing in the
design that prevents me adding the insets, they just need to be added as extra sheets with names like
"Sheet 2003 Inset 1" with their own reference points and special sheet sizes. Collecting the data is
another matter.
Not enough testing has been done. I am always grateful for the feedback I get from users, but especially
for problem reports that help me to make this a better module.
DIAGNOSTICS
Should this software not do what you expect, then please first read this documentation, secondly verify
that you have installed it correctly and that it passes all the installation tests on your set up,
thirdly study the source code to see what it's supposed to be doing, fourthly get in touch to ask me
about it.
CONFIGURATION AND ENVIRONMENT
There is no configuration required either of these modules or your environment. It should work on any
recent version of perl, on any platform.
DEPENDENCIES
None.
INCOMPATIBILITIES
None known.
LICENSE AND COPYRIGHT
Copyright (C) 2002-2013 Toby Thurston
OSTN02 transformation data is freely available but remains Crown Copyright (C) 2002
This library is free software; you can redistribute it and/or modify it under the same terms as Perl
itself.
AUTHOR
Toby Thurston -- 04 Oct 2013
toby@cpan.org
SEE ALSO
The UK Ordnance Survey's theory paper referenced above.
See Geo::Coordinates::Convert for a general approach (not based on the above paper).
See Geo::Coordinates::Lambert for a French approach.