Geographic Coordinate Systems

lastmod 2025-01-20created 2014-07-1011 edits

A geographic coordinate system is a coordinate system that enables every location on the Earth to be specified by a set of numbers.

A geographic coordinate system (地理座標系) allows every location on the Earth to be specified by three coordinate values of a spherical coordinate system aligned with the Earth's axis of rotation. Coordinates on a sphere are given using latitude and longitude. One great circle passing through the North Pole is taken as the reference, and the Royal Observatory in Greenwich, England is taken as 0 degrees longitude. How far it has rotated about the Earth's axis from that great circle is called the longitude. Angular units are used.

Spatial references, EPSG codes, and PROJ.4 parameters

Coordinate system resources

Guide to coordinate systems in Great Britain (PDF) ¹

This guide can be considered essential reading for anyone trying to work with coordinates in Great Britain. It is a gentle introduction and an excellent source of information on the often confusing subjects of GPS and Ordnance Survey coordinate systems.

Measuring the distance between two coordinates

The distance we talk about comes in several kinds: slope distance, distance on the geoid surface, spherical distance on the ellipsoid surface, and so on.
If the heights of the two points are unknown, the distance can first be calculated as a spherical distance.

• Various geodetic calculation spreadsheets in Excel from AUSLIG (Australian Surveying and Land Information Group)
• ICSM GDATM: see the Vincenty formula in the technical manual.

Calculating the distance between two WGS84 points: PHP Example

• Calculating the distance between two WGS84 points

function _deg2rad($deg)
{
    $radians = 0.0;
    $radians = $deg * M_PI/180.0;
    return($radians);
}

function geoDistance($lat1, $lon1, $lat2, $lon2, $unit="k")
{
    $theta = $lon1 - $lon2;
    $dist = sin(_deg2rad($lat1)) * sin(_deg2rad($lat2)) + cos(_deg2rad($lat1)) * cos(_deg2rad($lat2)) * cos(_deg2rad($theta));
    $dist = acos($dist);
    $dist = rad2deg($dist);
    $miles = $dist * 60 * 1.1515;
    $unit = strtolower($unit);

    if ($unit == "k") {
        return ($miles * 1.609344);
    } else {
        return $miles;
    }
}

See also

• Longitude And Latitude: how to express longitude and latitude.
• Vincentys Formulae: about the Vincenty formula.
• Geodetic datum (Datum)
• GIS
• Azimuth
• Gizmo

Favorite site

• Wikipedia (en) About geographic coordinate systems
• Coordinate conversion service
• PROJ.4 (coordinate system conversion library)
• PROJ.4 Javascript
• Spatial reference (coordinate system reference)

References