2. Understanding CRS, GCS and PCS¶
This chapter gives a summary of the concepts of Coordinate Reference System (CRS) (also referred to Spatial Reference System (SRS)), Geographic Coordinate System (GCS) and Projected Coordinate System (PCS). For a complete explanation of these concepts please refer to the QGIS guide: Coordinate Reference Systems.
Coordinate Reference System (CRS): 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 two types of CRS; GCS and PCS:
Geographic Coordinate System (GCS): 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). 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.
Look at task Setting CRS, GRS or PCS to illustrate the practical implications of using different coordinate systems during design.