An example of a geocentric reference frame relative to the geoid – not to scale. While the geocentric reference frame is a good approximation of the entire geoid, the regional reference frame is usually a better approximation in a regional area (Anzlic Committee on Surveying and Mapping (ICSM), 2016). Figure 3 and 4 show the differences between a geocentric reference frame and a regional reference frame. This enables positions in the regional area to be described with greater accuracy. For example, rather than aligning the center of the reference frame with the center of mass of the Earth, the reference frame can be shifted such that its surface coincides with the surface of the geoid in a regional area. A more accurate representation – based on the approximate mean sea level – is called the geoid, but requires gravity measurements and the determination of the gravity potential, the theory for which will not be covered in this article.Īlthough the ellipsoidal and spheroidal models are only approximations of the Earth’s true geometry, they can still be used to determine local coordinates with relative accuracy.

The surface and shape of the Earth is highly irregular and difficult to represent mathematically, which means ellipsoidal and spheroidal models are only approximations of the Earth’s true geometry. A mathematical model to determine Cartesian coordinates relative to an ellipsoidal frame (Sanz Subirana, Juan Zornoza, & Hernández-Pajares, 2013). Now consider an ellipsoid as a reference frame, which also has a known mathematical model that can describe the position of any point at height h above its surface, as shown in Figure 2. A mathematical model to determine Cartesian coordinates relative to a spherical frame (Weisstein, 2016). The sphere has a known mathematical model that can describe the position of any point at height h above its surface, as shown in Figure 1. Consider a perfect sphere as a reference frame with the sphere’s center as the origin of that frame. DatumsĪ datum is a reference frame that enables a system of coordinates to describe positions in three-dimensional space. The following sections describe datums and projections in more detail. A coordinate system and its datum are therefore required to determine accurate locations, and unless a perfect scale model of the Earth is used, it will be necessary to use a projection to visually represent those locations on a different surface. There are many datums and coordinate systems, each representing a different level of accuracy in different regions around the globe. A system of coordinates is then used to describe those measurements relative to the datum, and a projection is the visual representation of those measurements on a different surface. A datum is simply a foundation and reference for spatial measurements. The terms datum, projection and coordinate system are often misinterpreted and misunderstood. This article describes the theory behind datums, projections and coordinate systems, and highlights the importance of using the correct one for a given purpose. Understanding the spatial data acquisition methods and the idiosyncrasies of individual applications will allow users to take the correct action to make decisions that ensure positional integrity and accuracy in exploration geophysics.

The location of a coordinate in one datum will be different for the same coordinate in another datum. Assumptions are dangerous, and not all spatial data is acquired and delivered in the same way.

There are also numerous applications for analyzing and interpreting spatial data, all of which have different methods for representing a three-dimensional position in two-dimensional space. There are numerous datums and projections to which the coordinates are referenced, and using the correct one can make the difference between successful exploration and expensive legal fees. Unfortunately the latitude and longitude of a location (and the height) are not enough to describe where on Earth it is. The interpretation of fault zones, the surface and bottom locations of a well, and the position and orientation of seismic ground control points and receivers are all dependent on being able to accurately describe where they are. Accurately describing locations on Earth is essential to exploration geophysics.

Define datum