Locations on earth are often expressed in geographic degrees (latitude and longitude). But when you are surveying you need to talk in meters and feet. This is because - depending on the application - you use a geographic or projected coordinate system. This article explains some differences between both types of coordinate systems and presents what is supported in Virtual Surveyor.
Geographic Coordinate System
A Geographic Coordinate System (GCS) is a coordinate system which uses a three-dimensional spherical surface (ellipsoid) to define locations on the earth. A common choice of coordinates is latitude and longitude. For example, Leuven, Belgium is located on 50°52'47" North and 4°42'01" East in the WGS84 coordinate system.
A geographic coordinate system is constituted by a datum (DATUM), a prime meridian (PRIMEM), and unit (UNIT). The datum is constituted by a ellipsoid model (SPHEROID) and a anchor point.
An example is:
- WGS84 coordinate system with unique EPSG code 4326
Projected Coordinate System
In a Projected Coordinate System (PCS) you project the geographic coordinate that you have measured to, for example, a cylinder that you can easily roll out on a two-dimensional surface (the map). There exist many different projections and we'll not go in further detail about that here.
Typically, every country, state, or region has its optimal projected coordinate system, which minimizes distortions for particular applications like mapping.
- South central Texas in the United States uses "NAD83( NSRS2007) / Texas South Central (ftUS)" with unique EPSG code 3674
- Belgium uses "ETRS89 / Lambert 2008" with unique EPSG code 3812
A projected coordinate system is constituted by a geographic coordinate system from which it is projected (GEOGCS) and other projection parameters like the measurement unit (like meter or US Survey Foot), the projection technique, and its projection parameters.
How Does that Work in Virtual Surveyor?
- Virtual Surveyor always shows a project in a PCS and never in a GCS.
- Data sources that are in a GCS will be read and reprojected on-the-fly (OTF) to the chosen PCS.
- The PCS is shown in the status bar.
- You can set or update the Project Coordinate System to any PCS that is applicable for your project area. Explore details on how to do that here.