Geographic coordinates

Article

January 21, 2022

In geodesy, geographic coordinates are useful values ​​for identifying the position of a point on the earth's surface. They are latitude, longitude and altitude.

Latitude, longitude and altitude

Latitude is the angular distance of a point from the equator and longitude is the angular distance of a point from an arbitrary reference meridian along the same parallel of the place (measured in degrees). Since 1884 the fundamental meridian of reference has been conventionally fixed at Greenwich. Its longitude is therefore 0 °. Altitude is the distance, measured along the vertical of the point considered on the earth's surface, from sea level.

Measurement

Latitudes and longitudes are angular quantities and as such are measured in degrees Historically, the order in which coordinates were indicated was always the same, first the latitude and then the longitude, using different formats to write the degrees. Degrees Minutes Seconds (DMS) Everything is expressed in a sexagesimal basis. Example: the coordinates of the Colosseum are N 41 ° 53 '24 ″ E 12 ° 29' 32 ″. Sometimes, to provide more precise information, even if using the DMS notation, the seconds are expressed in decimal format. For example: N 41 ° 53'24.8280 E 12 ° 29'32.0136. Degrees Decimal Minutes (DM) Example: the above coordinates become 41 ° 53.41380 ', 12 ° 29.53356' or 41d 53.41380m, 12d 29.53356m. Decimal degrees (DD) Usually 4 to 6 decimal places. Example: the above coordinates become 41.8902300 °, 12.4922260 °. Note that the indication of the N (north) / S (south) and E (east) / W (west) hemispheres can be replaced by the sign. In particular, we will have negative values ​​for latitudes in the southern hemisphere and longitudes west of the fundamental meridian. Lately, the order longitude - latitude is increasingly used, to conform to the UTM and MGRS systems.

Reference

Since the earth is an irregular body (geoid), giving a mathematical description of its surface is difficult since we do not have the necessary data. It is usually assumed that it is similar to an ellipsoid, so that the latter closely approximates its surface (especially as regards the dimensions). The extreme variability of the earth's surface has led several scholars to propose different forms of ellipsoid, among which the most used (because it is supposed to approximate the earth's surface better) is the Hayford ellipsoid. Usually the ellipsoids are locally oriented for a terrestrial surface attributable to that of a region, a nation, a continent. For this reason, when we talk about geographical coordinates, we must also mention its datum, that is its reference ellipsoid and its orientation. Today the most used (also from Wikipedia, in the coordinates field of the localities) is the WGS84 system, that is an ellipsoid having the center coinciding with the center of mass of the earth and having the following parameters: semi-major axis: a 6378137 m; semi-minor axis: c 6356752.3142 m; crushing: f 1 / 298.257223563; geocentric gravitational constant: u 3986005 × 108 - m³ / s².

Monte Mario

In Italy it was custom (not completely disappeared) to use as a reference meridian that called Meridian of Monte Mario as it passed through the observatory of Monte Mario in Rome, located at 12 ° 27 '08.40 "E of Greenwich. reference ellipsoid was used the Hayford ellipsoid oriented precisely to Monte Mario. This coordinate system was used by the Military Geographical Institute (IGM) to update the data to the World Geodetic System 84 (WGS84) standard, on which the Global Positioning System (GPS) The WGS84 standard is based on a standard geodetic model of the Earth The EGM96 is a geoid defined by a spherical harmonic system.

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