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';