# 2. Understanding Coordinate Reference System/ Spatial Reference System used in GIS#

This chapter gives a summary of the concepts of Coordinate Reference System (CRS) (also referred to Spatial Reference System (SRS)). For a better explanation of Geographic Coordinate System (GCS) and Projected Coordinate System (PCS) please refer to the QGIS guide: Coordinate Reference Systems. For more information you can also refer to this short article.

**Coordinate Reference System (CRS)/ Spatial Reference System (SRS)**: A reference system to pin-point any point on earth by specifying a set of two to three numbers (horizontal position (east or west), vertical position (north or south) & elevation ). There are generally four types of CRS; GCS, PCS, Earth-centered Earth-fixed (ECEF) and Engineering Coordinate System (ECS):**Geographic Coordinate System (GCS)/ Geodetic**: Uses a 3D spherical surface; an ellipsoid, to pin-point location on the earth. A GCS is defined by a Datum (ellipsoid model), prime meridian and the unit of the coordinates. Different GCS uses different ellipsoid to define the shape of the earth, different longitude as its prime meridian, and different units for coordinates. Usually, degrees of Latitude and Longitude are used as coordinates to specify any point on the map. The World Geodetic System 1984 (WGS84) is the most commonly used GCS, map services like Google Map uses WGS84.**Projected Coordinate System (PCS)**: The projected GCS on a 2D plane (map). PCS is applied on a large area, usually on the scale of a country or region. For a smaller projection, such as for a small scale engineering project, an ECS is used. In a PCS, the units for the coordinates are in metres or feet. There are many different projections. Different projections can lead to many surveying errors, as it is not possible to accurately project a spherical surface onto a 2D plane. Different projections will produce different distortions. A projection is usually chosen based on the scale, extent and resolution of the region to be projected. One interesting example that shows the consequences of using differing projection is the use of the Mercator projection drastically shrinking the size of Africa as compared to other countries in the world. Typically, every country will decide on the optimal projection to used. As urban designers and architects, we just have to follow the the standard PCS. Singapore uses the SVY21 coordinate system.**Earth-centered Earth-fixed (ECEF)/Geocentric Coordinate System**: Wikipedia explanation:*‘A three-dimensional cartesian coordinate system that models the Earth as a three-dimensional object, measuring locations from a center point, usually the center of mass of the Earth, along x, y, and z axes aligned with the equator and the prime meridian. This system is commonly used to track the orbits of satellites, because they are based on the center of mass. Thus, this is the internal coordinate system used by Satellite navigation systems such as GPS to compute locations using multilateration.’***Engineering Coordinate System (ECS)**: Wikipedia explanation:*‘A cartesian coordinate system (2-D or 3-D) that is created bespoke for a small area, often a single engineering project, over which the curvature of the Earth can be safely approximated as flat without significant distortion. Locations are typically measured directly from an arbitrary origin point using surveying techniques. These may or may not be aligned with a standard projected coordinate system. Local tangent plane coordinates are a type of local coordinate system used in aviation and marine vehicles.’*(EastNorthUp (ENU)) is an ECS system. A ENU is used in the visualization of 3D models in the CesiumJS library.

Look at task Setting CRS, GRS or PCS to illustrate the practical implications of using different coordinate systems during design.