Explanation of the Coordinate Systems Used in Precise Drone Mapping
WGS84: The Global GPS Reference
The GPS WGS84 coordinate system, often referred to simply as WGS84, is a widely used global geodetic reference system and coordinate system for specifying locations on the Earth’s surface. WGS84 stands for “World Geodetic System 1984,” and it was developed and is maintained by the United States Department of Defense. It serves as the foundation for global positioning systems (GPS) and various mapping and navigation applications worldwide.
Earth Model
WGS84 assumes that the Earth is an ellipsoid, a slightly flattened sphere, to represent the Earth’s shape accurately. This ellipsoid is known as the WGS84 ellipsoid, and it approximates the Earth’s shape very closely.
Coordinate Representation
WGS84 uses a system of latitude and longitude to specify locations on the Earth’s surface. Latitude measures north-south position, while longitude measures east-west position. Latitude values range from -90° (South Pole) to +90° (North Pole), and longitude values range from -180° to +180°.
Datum
WGS84 is based on a specific reference point, known as the geodetic datum. This datum is a mathematical model that defines the reference point, the shape of the Earth’s ellipsoid, and the orientation of the coordinate axes. The WGS84 datum was updated in 1984 to improve accuracy and alignment with other global geodetic systems.
Accuracy & Compatibility
WGS84 is designed to provide a highly accurate and consistent reference system for global navigation and positioning. It is widely adopted and accepted as the de facto standard for geospatial data interchange. Many GIS platforms, mapping software, and GPS devices use WGS84 by default.
Coordinate Transformations
When dealing with maps or data in different coordinate systems, it may be necessary to perform coordinate transformations to convert between WGS84 and other local or regional coordinate systems.
Elevation
In addition to latitude and longitude, WGS84 also includes a system for specifying elevation (height) above or below the reference ellipsoid, typically represented in meters.
Note: While WGS84 provides a globally consistent reference, the Earth’s surface is not perfectly smooth. In precision applications, localized datums and projections are often preferred to account for geoid undulation and gravity variations.
Ellipsoid Height vs. Orthometric (Sea Level) Height
WGS84 Ellipsoid (Geodetic) Height: elevation above the WGS84 reference ellipsoid, measured along the ellipsoid normal. This is commonly what raw GPS/GNSS solutions output and is widely used in geodesy and precise surveying workflows.
Orthometric (Geopotential) Height: elevation above the geoid (an equipotential surface that approximates mean sea level). This is the elevation most people think of as “height above sea level” and what appears on topographic maps. Obtaining orthometric height requires applying a geoid model to convert from ellipsoid height.
In practice: h_ellipsoid = H_orthometric + N_geoid, where N_geoid is the geoid undulation at the location. Modern GNSS post-processing applies the appropriate geoid model (e.g., GEOID in the U.S.) to derive orthometric heights from ellipsoid heights.
Takeaways for Drone Mapping
- Know what vertical datum your client expects (ellipsoid vs. orthometric) and which geoid model to apply.
- Record the coordinate reference system (CRS), geoid model, and processing steps in your delivery notes.
- When integrating with local engineering workflows, transform WGS84 positions to the local project CRS (e.g., Illinois State Plane East/West in US feet).