WGS84 is an extensively used term in mapping. There are different situations when WGS84 is used, and people often get confused. Does WGS84 means the same thing in "WGS84/UTM zone 32N" and "GPS uses WGS84"? In this article, we will help you to better understand what WGS84 stands for.
Actually, WGS84 stands for World Geodetic System 1984 and comprises of four different things:
- An ellipsoid
- A horizontal datum
- A vertical datum
- A coordinate system
The shape of our earth is not perfectly spherical. So when we try to approximate the shape of our earth, we need to have a better model. Such a model is the ellipsoid and an example is the WGS84 ellipsoid which has a certain defined radius at the equator and a flattening at the top.
A horizontal datum
When you position the ellipsoid on a certain location with respect to the earth, on the so-called anchor point, you set a horizontal datum.
For example, the WGS84 ellipsoid with its anchor point is the WGS84 horizontal datum. The anchor point for the WGS84 horizontal datum is known to about 2 cm. Because the ellipsoid is now set with respect to the earth you'll be able to determine your geographic location.
A vertical datum
When you determine the elevation of your position you can measure that elevation with respect to the WGS84 ellipsoid. In this case WGS84 refers to a vertical datum or a vertical reference level.
Important to know here is that the elevation that comes with the pictures acquired by your drone uses WGS84 as the vertical datum. Often the end user will require the elevation model with respect to a different vertical datum. More information can be found in the article regarding "The difference between ellipsoidal and orthometric height".
A coordinate system
WGS84 can also be one type of geographic coordinate system.
The WGS84 Coordinate Systems adds Greenwich as the starting point (prime meridian) for the longitude (0°) and sets the units in degrees (°). This coordinate systems also has a unique reference code, the so-called EPSG code which is 4326.
The illustration below lists the WGS84 system with all its properties: EPSG code, Datum, Spheroid (Ellipsoid), Prime Meridian and the units.
- Leuven, Belgium is located on 50°52'47" North and 4°42'01" East in the WGS84 coordinated system
- San Antonio, United States is located on 29°25'26" North and 98°29'37" West in the WGS 84 coordinate system
Note that if you would have measured with another horizontal datum, e.g. GRS80 you would have gotten slightly different numbers.
The most known use case is GPS, which uses WGS84 as its coordinate system. Your drone also contains a GPS which is used to tag the pictures acquired with your drone with coordinates and elevations that are determined in the WGS84 coordinate system.
When you plot the geographic coordinates on a two-dimensional map you apply a map projection. So when you plot the geographic locations in WGS84 on a map where you the horizontal axis represents longitude between -180° and +180° then the vertical axis represents latitude between -90° and +90° then you get a world map in the Plate Carree projection.
Coming back to the introduction: What's the difference then between "WGS84/UTM zone 32N" and "GPS uses WGS84". This actually means that the position on the earth is in both cases establised in WGS84 coordinate system (50°52'47" North and 4°42'01"). But in the first case that number is further projected to the UTM projection system. In the 2nd case the coordinates stay as they are. More information on the difference between a projected and geographic coordinate system can be found here.