Geodetic References: Datum

In order to determine the locations on the surface of the earth, we need to choose a mathematical form that expresses the shape and size of the earth itself, which we call the name Reference Surface. One of these reference shapes might be the sphere, which has long been used to find locations that do not require great precision and to generate maps with a scale of no more than 1:1 million. In addition, for relatively small regions (less than 50 square kilometers), the (Plane) form can be used as a reference, particularly in (Surveying Plane) applications. The ellipsoid is the reference shape used for high-accuracy positioning or generating accurate maps.

Geodetic Datum

Geodesic experts have made numerous attempts over the last two centuries to discover the most appropriate ellipsoid that best depicts the shape of the earth. And, as fresh geodetic measurements are collected by scientists or international agencies, new values for the elements of the definition of ellipsoids (whether b, a, or f, a) are calculated, resulting in the existence of numerous ellipsoid models.

Each country, when first creating its geodetic or cadastral structure in order to begin producing maps, frequently chose the most recent ellipsoid available at the time to serve as the reference surface for its map system. If another ellipsoid developed after a few years, this country would be unable to alter its reference surface and replicate and publish all of its maps for technological and material reasons. But what is the reference? Any ellipsoid is known to be as close to portraying the surface of the planet as possible at the global level; that is, the differences between it and the geoid vary from place to place on the earth's surface, but they are the smallest possible at the global level. When a country chooses a particular ellipsoid, it wants the difference between it and the geoid to be as small as possible within its boundaries, regardless of how large the deviations are in other parts of the world. To attain this purpose, each country resorted to significantly changing the ellipsoid's reference point, Re-Position. In this situation, after making this simple alteration, the ellipsoid is no longer what it was, but it has evolved into something new, which we name a reference (a geodetic datum, a local datum, or any datum, simply). That is, any country's national reference is nothing more than a global ellipsoid whose status has been altered in some way to fit this country and be the closest representation of the shape of the geoid (the true form of the Earth) in this country. It should also be emphasized that the fewer variations there are between a country's local reference and the geoid, the more accurate the maps generated using this reference. The fact that there are numerous local references for various nations, all of which rely on the same global ellipsoid but alter its status in various ways, should also be recognized in this context.

Types of Datum

The references that we have talked about so far are what we can call the "horizontal datum," which are for determining locations in the horizontal plane. When dealing with coordinates in the vertical plane (ie, heights), we need another type of reference, which is the vertical datum. The geoid is the vertical reference used by many countries around the world, therefore finding it requires finding the location where the average marine surface is zero.

Geodetic Coordinate System

the geographic latitude and longitude of a place on the surface of the earth, calculated by geodetic measurement of the distance (mostly using the triangulation method) and the bearing (azimuth) from a number of other sites whose geographic coordinates are known. On the surface of a reference ellipsoid, which represents the form and size of the earth, geodetic coordinates are computed. Due to errors in the measurements of the accepted ellipsoid and deviations from the perpendicular, they deviate slightly from latitude and longitude as determined by astronomical methods. A point's altitude is taken into account in addition to its geodetic coordinates. It is determined from the surface of the chosen reference ellipsoid, and the amount of its geoidal divergence from this ellipsoid determines how high above sea level it is.

A line that is perpendicular to the geoid and goes through any chosen point above the surface of a geoid is created. The following set of geodetic parameters are defined using this line, known as the geodetic local vertical:

Geodetic altitude is the distance from the selected point to the reference geoid, measured along the geodetic local vertical, and is positive for points outside the geoid.

Geodetic latitude is measured in the plane of the local meridian from the true Equator of the Earth to the geodetic local vertical, which is measured positive north from the Equator.

Longitude: measured positive eastward in the plane of the Earth's real Equator from the Greenwich meridian to the local meridian.

The geodetic coordinate system additionally defines the following parameters for reference:

Geocentric latitude is measured in the plane of the local meridian from the Earth's actual Equator to the line connecting the coordinate system's geometric center to the point where the geodetic local vertical intersects the surface of the geoid.

Declination is the angle measured in the plane of the local meridian from the true Equator of the Earth to the center of the coordinate system.