As previously discussed, geospatial databases are spatially defined on Three-Dimensional representations of the Earth’s surface and visualised on Two-Dimensional forms for civil engineering projects, such as maps and topographic plans. Furthermore, it is desirable to perform data modelling in a simple Two-Dimensional coordinate system rather than on spherical surfaces whenever possible. For these reasons, whenever geodetic data are used for data modelling, specific mathematical functions must be applied to transform the Earth’s spherical surface into a flat surface. This is carried out using Map Projections, which are defined as a mathematical function (or mapping equations) that transforms geodetic coordinates (ϕg, λg) on the reference ellipsoid into the Two-Dimensional coordinate system (X, Y) on a plane surface as given by Eq. (16.1). The Two-Dimensional coordinates based on a map projection plane are usually called grid coordinates.