geographic coordinate system


May 19, 2022

The geographic coordinate system (Geographic coordinate system) is a coordinate system for representing points on the earth and celestial bodies. Geographic coordinates are usually expressed as a combination of latitude and longitude, which represents a horizontal position on the surface of the earth, as a spheroid (earth ellipsoid), and altitude, which represents a vertical position.

Latitude and longitude

Latitude is the angle between the perpendicular (vertical line) of a given surface point of the earth and the equatorial plane. Lines connecting points at the same latitude are called parallels and form concentric circles parallel to the equator. The North Pole is 90 ° north latitude and the South Pole is 90 ° south latitude. The parallel of 0 ° is the equator and is the basic plane of the spherical coordinate system. The equator divides the earth into the northern and southern hemispheres. Longitude is the angle between the meridian drawn from the North Pole to the South Pole through a given surface point of the Earth and the prime meridian. All meridians draw a semicircle, are not parallel, and gather at one point in the North Pole and the South Pole. The meridian (Greenwich meridian) that passes directly under the Greenwich Observatory near London has been selected as the first meridian. The points to the east of this are the eastern hemisphere, and the points to the west are the western hemisphere. The longitude of Greenwich's antipodes is 180 ° west longitude and 180 ° east longitude. By combining these two coordinates, you can specify any place on the earth, but the height and depth are not considered.

Universal Transverse Mercator and Universal Polar Projection

The Universal Transverse Mercator (UTM) and Universal Polar Polar Projection (UPS) coordinate systems both use a Cartesian grid based on distance metric, projected from the Earth's surface with the properties of a conformal map. UTM is not injective, but compound fire, each corresponding to 60 areas. UPS is used to represent polar regions that are not covered by UTM. In Japan, apart from UTM, the country is divided into 19 regions and a "planar Cartesian coordinate system" is set, which is projected onto a plane by the Gauss-Kruger projection.

Stereographic projection coordinate system

In the Middle Ages, the stereographic coordinate system was used for voyages, but the stereographic coordinate system was replaced by the latitude-longitude system. Although no longer used in voyages, stereographic coordinate systems are still used in the field of materials science to represent the geometry of crystals.

Geodesy altitude

It is necessary to determine the vertical height from the surface of the earth in order to complete the identification of the geomorphological position on the earth, inside and above. Since the terms "surface" and "vertical" are ambiguous, more accurate geodetic datums such as above sea level are generally used. Each country has its own geodetic system. For example, Japan is based on the Japan's Geodetic Origin.

Cartesian coordinate system

All points represented in the spherical coordinate system can be represented in the three-dimensional Cartesian coordinate system. This is not a good way to record points on a map, but it is useful for distance calculations and other mathematical operations. The origin is usually centered on the earth ellipsoid. In Japan, it is set as a "ground-center Cartesian coordinate system".

Figure of the Earth

The shape of the earth is not a true sphere, but roughly the shape of an earth ellipsoid. It is close to the shape of a sphere, but the equator bulges and the diameter around the equator is 0.3% larger than the diameter of the poles. The short axis almost coincides with the rotation axis. The cartographer can select the ellipsoid that is most suitable for describing the area in which the map is made. They then select the most suitable coordinate system to represent the ellipsoid on a plane. Early voyagers flatten the sea