sgo_bng_lonlat {sgo}

British National Grid (BNG) Easting/Northing to Geodetic Coordinate System (GCS)

Description

Converts Ordnance Survey grid reference easting/northing coordinates to GCS longitude/latitude (SW corner of grid square).

Usage

sgo_bng_lonlat(x, to = 4258, OSTN = TRUE, OD = FALSE)

Arguments

x

A sgo_points object with coordinates defined in the projected coordinate system BNG (EPSGs 27700 or 7405).

to

Numeric. Sets the epsg code of the destination Geodetic Coordinate System. 4258 (ETRS89) by default.

OSTN

Logical variable indicating whether use OSTN15 transformation when TRUE or a less accurate but slightly faster single Helmert transformation when FALSE.

OD

Logical variable. When TRUE, and the output contains a column with heights, then a new column is added to the result indicating the ordnance datum (OD) used on each point. It is ignored when OSTN=FALSE.

Details

The UK Ordnance Survey defined 'OSGB36' as the datum for the UK, based on the 'Airy 1830' ellipsoid. However, in 2014, they deprecated OSGB36 in favour of ETRS89 for longitude/latitude coordinates. Thus, when converting to longitude/latitude the OSGB36 datum should be always converted to ETRS89 (or WGS84).

According to the Transformations and OSGM15 User Guide, p. 8: "...ETRS89 is a precise version of the better known WGS84 reference system optimised for use in Europe; however, for most purposes it can be considered equivalent to WGS84." and "For all navigation, mapping, GIS, and engineering applications within the tectonically stable parts of Europe (including UK and Ireland), the term ETRS89 should be taken as synonymous with WGS84." This means that ETRS89 and WGS84 datums will be considered equivalent by this routine.

If, for historical reasons, longitude/latitude coordinates must have the old OSGB36 datum, then the parameter to must be set to 4277.

Note: Grid references rounded to whole metres will give latitude/longitude that are accurate to about 5 decimal places: in Great Britain, 1/100000 of a degree of latitude is about 70cm and 1/100000 of a degree of longitude is about 1m. All those coordinates outside the rectangle covered by OSTN15 will be automatically computed using the small Helmert transformation. Such coordinates will be accurate up to about +/-5 metres. Converting from BNG to lon/lat coordinates is slower than the other way around, due to the iterative nature of the algorithm that is built into OSTN15.

Value

An object of class sgo_points whose coordinates are defined as Longitude/Latitude.If OD=TRUE a column named height.datum is added to the resulting object.

References

Ordnance Survey Limited, 2018. Transformations and OSGM15 user guide

See Also

sgo_points, sgo_lonlat_bng, sgo_coordinates, sgo_transform.

Examples

p <- sgo_points(list(651409.903, 313177.270), epsg=27700)
p.89 <- sgo_bng_lonlat(p) #ETRS89 lon/lat
p.36 <- sgo_bng_lonlat(p, to=4277) #OSGB36 lon/lat