What Is a GIS Datum? Understanding Geographic and Geodetic Datums
Geographic Information Systems (GIS), surveying, cartography, and remote sensing depend on the accuracy of the spatial reference system to depict locations on the surface of the Earth. A key element of these systems is a GIS datum, which provides a framework for geographic coordinates and locates spatial features.
A GIS datum allows defining the shape, size, and orientation of the Earth. A GIS datum enables GIS software to understand the coordinates of latitude, longitude, and elevation. Without a GIS datum, spatial data obtained using GPS, satellite imagery, aerial photographs, and topographical maps would not be aligned.
The knowledge of geographic and geodetic datums is critical for those who use coordinate reference systems (CRS).
What Is a GIS Datum?
A GIS datum is a mathematically defined reference system that enables one to describe the position and orientation of coordinates on Earth. This definition sets the reference surface and origin of the coordinate system to be able to determine the geographical location precisely.
A datum usually describes a number of parameters associated with the geometry and characteristics of Earth, including its shape, size, orientation, and position. Depending on the specific type of datum, it can use an ellipsoid shape model of Earth or a geocentric reference system based on the Earth's center of mass.
In the process of applying a datum for GIS purposes, it is necessary to understand that a datum provides coordinate values relative to the real world. For example, the geographic position that could be defined by coordinates will depend on the chosen datum.
It should be noted that a datum is not a coordinate system or map projection.
What Is a Geodetic Datum?
A geodetic datum refers to the system that describes the basis of measurement of geographical positions close to the Earth’s surface. The geometrical values needed to determine the coordinates are provided by the datum system.
Modern geodetic datums use a reference ellipsoid as the mathematical model to approximate the shape of the Earth since the Earth is not perfectly spherical.
The geodetic datum can be described as either a local or regional geodetic datum or a global geocentric datum.
Local and Regional Geodetic Datums
A local and regional geodetic datum is a geodetic datum that aims to provide accuracy in positioning locations in a specific region of interest. Ellipsoidal models and orientations are chosen to fit the Earth's surface in the region.
In the past, most countries have used the local geodetic datum in their national mapping due to the fact that the local datum would allow minimum errors in positioning within the country's territory.
Global Geocentric Datums
Global geocentric datums are datums whose reference frames are located close to the center of mass of the Earth. Such datums are useful for mapping, positioning, navigation, and Earth observation on a global scale.
The most common global geodetic datum is the WGS 84 (World Geodetic System 1984). This datum is associated with GPS positioning and allows the depiction of geographic coordinates worldwide.
Another important datum in this class is the NAD83 (North American Datum of 1983). The NAD83 datum has several realizations, so you should specify the particular realization and epoch of coordinates if needed.
What Is a Geographic Datum?
A geographic datum provides the reference through which the geographic coordinates, especially the latitude and longitude, are interpreted. The geographic datum gives the link between the value of the coordinates and the reference model of the Earth.
For example, the same coordinate value given in latitude and longitude according to WGS 84 can have slight differences from the same coordinate value given in another datum, such as a local datum.
Geographic datums are especially important where different types of data are combined, including those from GPS surveys, satellite imagery, cadastral maps, and national geographic databases.
Although the two terms are used interchangeably, the geodetic datum is actually the wider term. The geographic coordinate reference system utilizes the geodetic datum to provide the reference for latitude and longitude.
Geographic Datum vs. Geodetic Datum
The two terms are closely related, but understanding their roles helps prevent confusion when working with GIS datasets.
Feature | Geographic Datum | Geodetic Datum |
Primary role | Provides the reference for interpreting geographic coordinates | Defines the geodetic reference framework for positioning |
Coordinates | Commonly associated with latitude and longitude | Supports geographic and other coordinate representations |
Reference model | Uses an Earth reference model defined by the datum | May use an ellipsoid and a defined geodetic reference frame |
Applications | Geographic mapping and spatial data integration | Surveying, GNSS, geodesy, and precise positioning |
Examples | WGS 84 geographic coordinate reference system | WGS 84 and NAD83 geodetic reference systems |
In practical GIS workflows, the distinction depends on the terminology used by a particular software platform or standard. The most important consideration is identifying the complete coordinate reference system, including its datum and coordinate definitions.
Types of GIS Datums
There are several kinds of datums and corresponding datums that exist in geospatial data processing.
Horizontal Datum
The horizontal datum is the one that is used as a reference to find locations on the surface of the Earth using latitude and longitude or horizontal coordinates.
The horizontal datums play a crucial role in road map making, boundary maps, infrastructure maps, etc.
Some examples are WGS 84 and NAD83. Sometimes the horizontal datasets are in different datums, and it requires transformation of the coordinates.
Vertical Datum
The vertical datum provides the basis upon which elevation or depth is measured.
Some applications where the vertical datum is relevant include the creation of DEMs, flood modeling, construction, hydrology, and engineering surveys.
Examples of vertical datums include mean sea level datums and gravity-height datums. For example, NAVD 88 is one of the vertical datums used in parts of the United States.
There is a need to differentiate ellipsoidal height from orthometric height. Most GNSS receivers give ellipsoidal height, while some engineering and topographical data have the orthometric height, which approximates elevation above the geoid.
The formula to express the difference between the two is;
h = H + N
Where:
h = Ellipsoidal Height
H = Orthometric Height
N = Geoid Height above the Reference Ellipsoid
A proper geoid model is needed to make the conversion between these heights.
Geocentric Reference Frames
Modern global positioning systems use geocentric reference frames that determine coordinates based on the Earth's center of mass. Geocentric reference frames enable global navigation, orbit determination of satellites, and geospatial measurement with high precision.
While WGS 84 is related to GPS, ITRF is an example of a scientific reference frame that is applied for precise geodesy.
As the Earth is changing over time because of tectonic movement and other geophysical processes, precise coordinates might need a reference epoch in addition to a datum or reference-frame realization.
What Is the Difference Between a Datum, Coordinate System, and Map Projection?
These terms are connected to one another, although they serve distinct purposes in GIS.
Datum: It provides a reference framework for positioning on the surface of the Earth.
Geographic coordinate system (GCS): Specifies geographic coordinates, such as latitudes and longitudes, in degrees and using a geodetic datum.
Projected coordinate system (PCS): Positioning of locations is performed using planar coordinates such as eastings and northings via a map projection.
Map projection: Coordinates from a curved surface are converted to coordinates on a plane.
For example, WGS 84 is applicable for use in the geographic coordinate system in conjunction with latitudes and longitudes. Also, the same underlying geodetic datum can be used in any appropriate projected coordinate system, e.g., WGS 84 / UTM Zone 33N.
There is bound to be distortion when the curved shape of the Earth is flattened to a plane due to a map projection.
A GIS datum is a critical element of spatial data management in that it is responsible for establishing the reference frame through which geographic location is determined. Geographic or geodetic datums provide coordinate references, while horizontal and vertical datums cater to different location needs.
Knowledge of datums including WGS 84, NAD83, NAD27, and ETRS89 enables GIS experts to integrate spatial data, analyze satellite imagery, conduct surveys without mistakes, and prevent errors in coordinate alignment.
In order to have effective GIS analysis, one should be aware of the datum source, choose the right coordinate reference system, transform where necessary, and confirm the results using reference data.
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