Geophysics Series - How to Coordinate Conversion

Geophysics Series - How to Coordinate Conversion

Coordinate systems are integral to virtually every aspect of spatial data management and analysis. Essentially, a coordinate system provides a standardised method for defining a point in space. In two dimensions, this is typically achieved through a pair of numerical values which correlate to a point on a flat plane. In three dimensions, an additional value is added to account for depth or height.

These coordinate systems can be categorised into three main types: Cartesian, cylindrical, and spherical.

Cartesian Coordinate System: This is likely the most intuitive and widely used system. It utilises a series of perpendicular lines (or axes) to define a point in space. In 3D, these axes are commonly referred to as the X, Y, and Z (depth) axes. In geospatial systems, there is a geocentric coordinate system (EPSG:4978) which uses a cartesian system with its origin in the centre of the earth. This has applications in building virtual earth systems. The UTM system is commonly used in geospatial data and uses cartesian coordinates, but UTM is in fact a cylindrical system.
Cylindrical Coordinate System: This system is based on a circular cylinder, with a point defined by its distance from a central axis (radius), the angle around that axis (azimuth), and the height above or below a reference plane. Examples of these are the Mercator projections used for most modern maps of the world, with the cylinder axis through the poles. The UTM system is also a cylindrical system, but with a transverse cylinder (axis through the equator) rotated for each zone. Most online map systems use a Web Mercator projection.
Spherical Coordinate System: Based on a sphere, a point in this system is defined by the distance from a central point (radius), the angle from a reference direction (azimuth), and the angle from a reference plane (inclination or polar angle). WGS84 (EPSG:4326) is the latitude-longitude system we are all familiar where the coordinates are defined in degrees, minutes and seconds rather than in Metres.
The choice of a coordinate system is significant as it can greatly affect the accuracy of the model, particularly when interpreting complex geological structures. There are several ways of defining coordinate systems, but commonly EPSG codes and WKT are used.

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