North American Overlay Mapping

Layer Notes - Latitude/Longitude Grid

The 'LatLon' foreground overlay allows us to find the location of any point on the currently selected map. The colors chosen were designed to be visible against a range of backgrounds, without obscuring more important detail. Numbers are included to identify the lines. Those identifying lines of Longitude (North-South) are of three digits, whilst those identifying lines of Latitude (East-West) are of two digits.

To further sub-divide the 1° grid, use one of the two Graticules. For information about their use and capabilities, see the Graticule help page.

For more exact positional information, the latitude/longitude of your current cursor-position is shown in the Status Bar, at the bottom of your browser. For more information, visit the Status Bar help-page. Depending on the map selected, this ranges in accuracy from ±0.005° (small scale) to ±0.04° (Overview).

A bit of history: Although the first map reference system was a simple rectangular grid, developed by the Chinese, later map-makers realised that they had to take account of the earth as a sphere. The Greeks derived from the Babylonians (present-day Iraq) the idea of dividing a circle into 360°, and the Greek geographer Eratosthenes (c.276-194 B.C.) was the first to calculate the circumference of the earth, and was reported to have made a world map based on the concept of the earth's sphericity. From this, the Greeks went on to develop the system of spherical coordinates still in use today.

The poles at each end of the earth's axis provide the reference points in its rotation in relation to the celestial sphere. Parallel circles around the earth are degrees of Latitude, and express the idea of distance North or South of the Equator. Lines of Longitude running North or South through the poles express East-West distances. One Meridian is chosen as the meridian of origin, known as the Prime Meridian. In 1884, at an international conference in Washington DC, most countries agreed to adopt the Prime Meridian through the Royal Greenwich Observatory in England, and to calculate Longitude to 180° East and West of Greenwich.