Human beings have always felt a need to represent the Earth’s surface and the elements on it, as a result of which maps emerged (IGN & UPM-LatinGEO, 2013). Coordinate systems are fundamental when working with GIS, although they are often overlooked. But, while all locations are identified using a coordinate system, the same location may have different coordinates depending on the coordinate system used by the dataset (ESRI, 2011b).
To better understand coordinate systems it is necessary to comprehend the following terms: projection, ellipsoid, geoid and datum.
The projection is intended to geometrically represent the Earth’s surface, which is curved, on a flat surface. But when this process is carried out, deformations inevitably occur and each projection generates a different type of distortion. Before drawing up a map, one of the most important decisions is to know which projection will be used, because each projection works best in a specific place. Projections can distort the shape, area, distance and direction on a map, attributes by which they can be categorized into conforms: these projections conserve the angles between meridians and parallels, but maintain the shapes. If equivalent, they maintain constant surface relationships, and if equidistant, they maintain the relations of distance (Del Bosque González et al. 2012).
An ellipsoid is the geometric figure that best adapts to the real shape of the Earth, and so it is the one that best allows a good fitting. Once a theoretical expression for the shape of the Earth is available, the next step is to determine the parameters that define it. If a sphere is used, its radius must be calculated. If an ellipsoid is taken as a reference form, the minor and major semi-axes must be measured (Olaya, 2014). In addition, an ellipsoid (also called a spheroid) may be defined as a three-dimensional shape created from a two-dimensional ellipse. The ellipse is an oval, with a major axis (the longest axis) and a minor axis (the shortest axis). If the ellipse is spun, the shape of the spun figure is the spheroid (ESRI, 2015). The geoid is defined as the surface of the Earth’s gravity field, which is approximately equal to the mean sea level. It is perpendicular to the direction of gravitational attraction since the Earth’s mass is not uniform at all points and the direction of gravity changes. The shape of the geoid is irregular (ESRI, 2015).
The datum specifies the spheroid that will be used to give the shape to the coordinate system. It also directs the spheroid to a series of land control points, ensuring the accuracy of a reference point for its intended region or spatial extent (ESRI, 2011b). The datum is generated above the selected spheroid and can incorporate local variations in elevation. With the spheroid, the rotation of the ellipse creates a totally smooth surface of the whole world. Since reality is not properly represented in this way, a local datum may incorporate local variations in elevation (ESRI, 2015).
Put simply, projection is the method used to represent the circular shape of the Earth in a plane, and the datum is the set of parameters that is used to do it.
Geographic information can be displayed according to a geographic coordinate system or a projected coordinate system. The geographic coordinate system indicates a location using the longitude and latitude based on a sphere (spheroid). The XY projected coordinate system is based on a plane (Hillier, 2011). The most commonly used geodetic reference systems are: WGS84, NAD 27, NAD 83, ED50, ETRS89, PSAD56, and SIRGAS. In addition, some coordinate systems have certain advantages, such as the ability to quickly measure distances and surfaces.
You want to learn an ArcGIS 10 Course following this manual + online + certificate = 15.99 USD