noble (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 and data
published by the Ordnance Survey of Great Britain.
VERSION
2.20
SYNOPSIS
use Geo::Coordinates::OSGB qw(ll_to_grid grid_to_ll);
($easting,$northing) = ll_to_grid($lat,$lon);
($lat,$lon) = grid_to_ll($easting,$northing);
DESCRIPTION
These modules convert accurately between OSGB national grid references and coordinates given in latitude
and longitude.
The default "ellipsoid model" used for the conversions is the de facto international standard WGS84.
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 accurate
British National grid references for use with one of the Ordnance Survey's paper maps. And vice versa,
of course.
The module is implemented purely in Perl, and should run on any platform with Perl version 5.8 or better.
In this description, the abbreviations `OS' and `OSGB' mean `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 map sheets defined in the accompanying maps module.
This code is written for the British national grid system. It is of no use outside Britain. In fact
it's only really useful in the areas covered by the OS's main series of maps, which exclude the Channel
Islands and Northern Ireland.
SUBROUTINES/METHODS
The following functions can be exported from the "Geo::Coordinates::OSGB" module:
grid_to_ll
ll_to_grid
Neither of these is exported by default.
Main subroutines
"ll_to_grid(lat, lon)"
"ll_to_grid" translates a latitude and longitude pair into a grid easting and northing pair.
When called in a list context, "ll_to_grid" returns the easting and northing as a list of two. When
called in a scalar context, it returns a single string with the numbers separated by a space.
The arguments should be supplied as real numbers representing decimal degrees, like this:
my ($e,$n) = ll_to_grid(51.5, -2.1); # (393154.801, 177900.605)
Following the normal mathematical convention, positive arguments 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);
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. However, since reasonable
latitudes for the OSGB are in the range 49 to 61, and reasonable longitudes in the range -9 to +2,
"ll_to_grid" accepts the arguments in either order; if your longitude is larger than your latitude, then
the values of the arguments will be silently swapped.
You can also supply the arguments as named keywords (but be sure to use the curly braces so that you pass
them as a reference):
my ($e,$n) = ll_to_grid( { lat => 51.5, lon => -2.1 } );
The easting and northing will be returned as the orthogonal distances in metres from the `false point of
origin' of the British Grid (which is a point some way to the south-west of the Scilly Isles). The
returned pair refers to a point on the usual OSGB grid, which extends from the Scilly Isles in the south
west to the Shetlands in the north.
my ($e,$n) = ll_to_grid(51.5, -2.1); # (393154.807, 177900.595)
my $s = ll_to_grid(51.5, -2.1); # "393154.807 177900.595"
If the coordinates you supply are in the area covered by the OSTN transformation data, then the results
will be rounded to 3 decimal places, which corresponds to the nearest millimetre. If they are outside
the coverage then the conversion is automagically done using a Helmert transformation instead of the OSTN
data. The results will be rounded to the nearest metre in this case, although you probably should not
rely on the results being more accurate than about 5m.
With the older OSTN02 dataset, coverage extended only to about 3km offshore, but the current OSTN15
dataset extends coverage to the whole grid area from (0,0) to (700000, 1250000), so you have be really
far away to get whole metres. Even points well away from land, like this one:
# A point in the sea, to the north-west of Coll
my $s = ll_to_grid(56.75,-7);
will get an accurate conversion. With OSTN02 that returned "94471 773206" but with OSTN15 you get
"94469.597 773209.464". For your sake, I hope you are never in a situation at sea off Coll where the 3
metres difference is important.
The numbers returned may be negative if your latitude and longitude are far enough south and west, but
beware that the transformation is less and less accurate or useful the further you get from the British
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 from Grid.pm. Like this.
my $s = ll_to_grid(51.5, -2.1); # "393154.807 177900.595"
$s = format_grid(ll_to_grid(51.5,-2.1)); # "ST 931 779"
$s = format_grid_GPS(ll_to_grid(51.5,-2.1)); # "ST 93154 77900"
$s = format_grid_map(ll_to_grid(51.5,-2.1)); # "ST 931 779 on A:173, B:156, C:157"
"ll_to_grid()" also takes an optional argument that sets the ellipsoid model to use. This defaults to
`WGS84', the name of the normal model for working with normal GPS coordinates, but if you want to work
with the traditional latitude and longitude values printed around the edges of OS maps before 2015 then
you should add an optional shape parameter like this:
my ($e, $n) = ll_to_grid(49,-2, {shape => 'OSGB36'});
Incidentally, if you make this call above you will get back "(400000, -100000)" which are the coordinates
of the `true point of origin' of the British grid. You should get back an easting of 400000 for any
point with longitude 2W since this is the central meridian used for the OSGB projection. However you
will get a slightly different value unless you specify "{shape => 'OSGB36'}" because the WGS84 meridians
are not quite the same as OSGB36.
"grid_to_ll(e,n)"
The routine "grid_to_ll()" takes an easting and northing pair representing the distance in metres from
the `false point of origin' of the OSGB grid and returns a pair of real numbers representing the
equivalent longitude and latitude coordinates in the WGS84 model.
Following convention, positive results are North of the equator and East of the prime meridian, negative
numbers are South and West. The fractional parts of the results represent decimal fractions of degrees.
No special processing is done in scalar context because there is no obvious assumption about how to round
the results. You will just get the length of the list returned, which is 2.
The arguments must be an (easting, northing) pair representing the absolute grid reference in metres from
the point of origin. You can get these from a traditional grid reference string by calling
"parse_grid()" first.
my ($lat, $lon) = grid_to_ll(parse_grid('SM 349 231'))
An optional last argument defines the ellipsoid model to use just as it does for "ll_to_grid()". This is
only necessary is you are working with an ellipsoid model other than WGS84. Pass the argument as a hash
ref with a `shape' key.
my ($lat, $lon) = grid_to_ll(400000, 300000, {shape => 'OSGB36'});
If you like named arguments then you can use a single hash ref for all of them (this is strictly
optional):
my ($lat, $lon) = grid_to_ll({ e => 400000, n => 300000, shape => 'OSGB36'});
The results returned will be floating point numbers with the default Perl precision. Unless you are
running with long double precision floats you will get 13 decimal places for latitude and 14 places for
longitude; but this does not mean that the calculations are accurate to that many places. The OS online
conversion tools return decimal degrees to only 6 places. A difference of 1 in the sixth decimal place
represents a distance on the ground of about 10 cm. This is probably a good rule of thumb for the
reliability of these calculations, but all the available decimal places are returned so that you can
choose the rounding that is appropriate for your application. Here's one way to do that:
my ($lat, $lon) = map { sprintf "%.6f", $_ } grid_to_ll(431234, 312653);
Additional subroutines
"set_default_shape(shape)"
The default ellipsoid shape used for conversion to and from latitude and longitude is `WGS84' as used in
the international GPS system. This default is set every time that you load the module. If you want to
process or produce a large number latitude and longitude coordinates in the British Ordnance Survey
system (as printed round the edges of OS Landranger and Explorer maps before 2015) you can use
"set_default_shape('OSGB36');" to set the default shape to OSGB36. This saves you having to add "{ shape
=> 'OSGB36' }" to every call of "ll_to_grid" or "grid_to_ll".
You can use "set_default_shape('WGS84');" to set the default shape back to WGS84 again when finished with
OSGB36 coordinates.
"ll_to_grid_helmert(lat, lon)"
You can use this function to do a conversion from WGS84 lat/lon to the OS grid without using the whole
OSTN data set. The algorithm used is known as a Helmert transformation. This is the usual coordinate
conversion algorithm implemented in most consumer-level GPS devices, which generally do not have enough
memory space for the whole of OSTN. It is based on parameters supplied by the OS; they suggest that in
most of the UK this conversion is accurate to within about 5m.
my ($e, $n) = ll_to_grid_helmert(51.477811, -0.001475); # RO Greenwich
The input must be decimal degrees in the WGS84 model, with latitude first and longitude second. The
results are rounded to the nearest whole metre. They can be used with "format_grid" in the same way as
the results from "ll_to_grid".
This function is called automatically by "ll_to_grid" if your coordinates are WGS84 and lie outside the
OSTN polygon.
"grid_to_ll_helmert(e,n)"
You can use this function to do a slightly quicker conversion from OS grid references to WGS84 latitude
and longitude coordinates without using the whole OSTN data set. The algorithm used is known as a
Helmert transformation. This is the usual coordinate conversion algorithm implemented in most consumer-
level GPS devices. It is based on parameters supplied by the OS; they suggest that in most of the UK
this conversion is accurate to within about 5m.
my ($lat, $lon) = grid_to_ll_helmert(538885, 177322);
The input must be in metres from false point of origin (as produced by "parse_grid") and the results are
in decimal degrees using the WGS84 model.
The results are returned with the full Perl precision in the same way as "grid_to_ll" so that you can
choose an appropriate rounding for your needs. Four or five decimal places is probably appropriate in
most cases. This represents somewhere between 1 and 10 m on the ground.
This function is called automatically by "grid_to_ll" if the grid reference you supply lies outside the
OSTN polygon. (All such spots are far out to sea). The results are only useful close to mainland
Britain.
Importing all the functions
You can import all the functions defined in "OSGB.pm" with an ":all" tag.
use Geo::Coordinates::OSGB ':all';
EXAMPLES
use Geo::Coordinates::OSGB qw/ll_to_grid grid_to_ll/;
# Latitude and longitude according to the WGS84 model
($lat, $lon) = grid_to_ll($e, $n);
# and to go the other way
($e, $n) = ll_to_grid($lat,$lon);
See the test files for more examples of usage.
BUGS AND LIMITATIONS
The formulae supplied by the OS and used for the conversion routines are specifically designed to be
close floating-point approximations rather than exact mathematical equivalences. So after round-trips
like these:
($lat1,$lon1) = grid_to_ll(ll_to_grid($lat0,$lon0));
($e1,$n1) = ll_to_grid(grid_to_ll($e0,$n0));
neither "$lat1 == $lat0" nor "$lon1 == $lon0" nor "$e1 == $e0" nor "$n1 == $n0" exactly. However the
differences should be very small.
The OS formulae were designed to give an accuracy of about 1 mm of error. This means that you can rely
on the third decimal place for grid references and about the seventh or eighth for latitude and longitude
(although the OS themselves only provide six decimal places in their results).
For all of England, Wales, Scotland, and the Isle of Man the error will be tiny. All other areas, like
Northern Ireland, the Channel Islands or Rockall, and any areas of sea more than a few miles off shore,
are outside the coverage of OSTN, so the simpler, less accurate transformation is used. The OS state
that this is accurate to about 5m but that the parameters used are only valid in the reasonably close
vicinity of the British Isles.
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
The only error message you will get from this module is about the ellipsoid shape used for the
transformation. If you try to set "{shape => 'blah'}" the module will croak with a message saying
"Unknown shape: blah". The shape should be one of the shapes defined: WGS84 or OSGB36.
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
Perl 5.10 or better.
INCOMPATIBILITIES
None known.
LICENSE AND COPYRIGHT
Copyright (C) 2002-2017 Toby Thurston
OSTN transformation data included in this module is freely available from the Ordnance Survey but remains
Crown Copyright (C) 2002
This program is free software; you can redistribute it and/or modify it under the terms of the GNU
General Public License as published by the Free Software Foundation; either version 2 of the License, or
(at your option) any later version.
This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even
the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public
License for more details.
You should have received a copy of the GNU General Public License along with this program; if not, write
to the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
AUTHOR
Toby Thurston -- 29 Oct 2017
toby@cpan.org
SEE ALSO
See Geo::Coordinates::OSGB::Grid for routines to format grid references.
The UK Ordnance Survey's explanations on their web pages.
See Geo::Coordinates::Convert for a general approach (not based on the OSGB).