Surveying and mapping the Earth involves translating a complex, three-dimensional, curved surface into usable measurements for construction, navigation, and property delineation. Because the planet is an irregular shape, simply measuring a location’s latitude, longitude, or its elevation above the ground is not enough to ensure accuracy. These measurements only become meaningful when they are tied to a shared, fixed starting framework. Without a defined common reference, every measurement would be isolated, making it impossible to align data collected by different people, at different times, or with different equipment. A specialized, agreed-upon model is necessary to provide the standardized grid that anchors all spatial information.
Defining the Surveying Datum
A geodetic datum is a set of parameters that define a coordinate system for referencing locations on Earth. It functions as the foundational mathematical model that makes coordinates usable and repeatable across large areas. This framework establishes the size, shape, and orientation of the Earth model, providing a fixed origin and orientation for all subsequent measurements.
A datum is realized through a network of physical benchmarks, such as monumented disks, whose exact coordinates are known and cataloged. Surveyors use the closest benchmark as their starting reference point for measuring new locations. Datums are classified by scope, such as a local datum, which is optimized for a small geographic area, or a global datum, like the World Geodetic System 1984 (WGS 84), designed to provide a consistent reference system for the entire planet.
Horizontal and Vertical Datums
Datums are divided into horizontal and vertical categories to define a location in three-dimensional space. A horizontal datum defines a location on the Earth’s surface, determining the latitude and longitude of a point. This type requires a mathematical model of the Earth’s shape to calculate positions, and it is primarily used for applications such as establishing boundaries, charting property lines, and navigation. Horizontal coordinates are defined relative to the center of the Earth model, which allows for the global grid system necessary for GPS coordinates. The North American Datum of 1983 (NAD 83) is a widely used example in North America.
A vertical datum defines height or elevation above or below a standard reference surface, often related to an approximation of Mean Sea Level (MSL). Vertical datums are necessary for accurate elevation assessments required for engineering projects, levee design, and determining flood zones. The North American Vertical Datum of 1988 (NAVD 88) is the standard vertical reference used throughout the United States. Combining both datums allows for the precise, three-dimensional localization of any point on the Earth.
The Ellipsoid and Geoid Reference Models
Horizontal and vertical measurements require two distinct theoretical models of the Earth: the ellipsoid and the geoid. The ellipsoid is a smooth, mathematical surface that approximates the Earth’s shape. Since the Earth is slightly flattened at the poles and bulges at the equator, the ellipsoid is an oblate spheroid, which is necessary for calculating latitude and longitude coordinates.
This model is purely geometric and is defined by parameters such as the semi-major axis (equatorial radius) and the semi-minor axis (polar radius). The ellipsoid provides a computational framework for Global Navigation Satellite System (GNSS) observations, as GPS receivers calculate the height of a point relative to this smooth surface.
In contrast, the geoid is a physical model that accounts for the Earth’s irregular mass distribution and gravitational field. It is defined as the equipotential surface of the Earth’s gravity field that best fits the global Mean Sea Level. Because the Earth’s density varies, the gravitational pull is not uniform, causing the geoid surface to be irregular.
The geoid is the required reference for accurate elevation measurements, known as orthometric heights, or heights “above sea level.” The separation between the smooth ellipsoid and the irregular geoid is called the geoid height or undulation height. Surveyors use geoid models to convert the geometric height measured by GPS (ellipsoidal height) into the gravity-based orthometric height needed for engineering and mapping.
Why Datum Consistency is Critical
Consistency across projects is necessary because datums standardize measurements. When different datums are inadvertently used for the same project, the resulting coordinates for a single location can be significantly misaligned. For example, the difference between the older NAD 27 datum and the modern WGS 84 datum can result in a horizontal shift of approximately 265 feet (80 meters).
Even between modern systems like NAD 83 and WGS 84, coordinate differences can reach up to 1.5 meters horizontally and 1 meter vertically in some regions. This discrepancy causes substantial errors in construction, property boundary delineation, and navigation, potentially leading to infrastructure failures or legal disputes. Global systems like WGS 84, which is the standard for all GPS devices, are continually refined to maintain accuracy.
National datums like NAD 83 and NAVD 88 are also periodically updated to account for the continuous movement of tectonic plates and improvements in measurement technology. The modernization of these systems is necessary because older datums can be off-center from the Earth’s true center. Using a single, consistent, agreed-upon datum ensures that all spatial data can be accurately overlaid and integrated, providing the foundation for reliable mapping and engineering.