Datum, coordinate representation, and map projection
These terms are often confused, but they describe different parts of the positioning workflow.
- Datum or reference frame: defines the Earth model, origin, orientation, and scale used for coordinates. Examples include WGS84 and ITRF.
- Coordinate representation: describes how the position is expressed, such as latitude and longitude, ECEF, or projected grid coordinates such as UTM.
- Coordinate format: describes how coordinates are written, such as decimal degrees or degrees, minutes, seconds.
- Map projection: describes how curved Earth coordinates are converted onto a flat map. Examples include Web Mercator, UTM zones, and local grid systems.
A map offset is often caused by a coordinate reference system mismatch, not by poor GNSS accuracy. For example, a receiver may calculate an accurate position, but the position may appear shifted if the map, correction service, survey control point, or application software uses a different reference frame, projection, or transformation.
Heights: ellipsoidal height and MSL height
GNSS height can be confusing because different systems may use different vertical references.
- Ellipsoidal height, usually written as h, is the height above the mathematical ellipsoid.
- Orthometric height, often called MSL height, is height above mean sea level. It is usually derived from ellipsoidal height using a geoid model.
- Geoid separation, usually written as N, is the difference between the ellipsoid and the geoid at a specific location.
The simplified relationship is:
H = h − N
Where:
- H is orthometric height, or MSL height.
- h is the ellipsoidal height.
- N is the geoid separation.
GNSS position calculations are based on an ellipsoid. Depending on the receiver message or software, the output may provide ellipsoidal height, MSL height, or both.
If a vertical offset is fairly constant, check whether one system is using ellipsoidal height while another is using MSL height. Many maps, GIS tools, and customer applications display MSL or geoid-based height, while GNSS or receiver-specific messages may provide ellipsoidal height.
How to get rid of consistent offset in position
A consistent offset in the same direction and magnitude usually points to a reference frame, configuration, or installation mismatch rather than random GNSS noise.
Common causes include:
- Reference frame mismatch between the receiver, correction service, map, or survey control point.
- Incorrect datum transformation in the application or GIS software.
- Wrong map projection or grid zone.
- Antenna phase center or mounting offset not accounted for.
- Comparing the ellipsoidal height with the MSL height.
Recommended checks:
- Confirm the reference frame used by the correction service.
- Confirm the reference frame used by the map, GIS software, or customer application.
- Confirm that the same coordinate representation and projection are used throughout the workflow.
- Check whether any datum transformation is being applied automatically.
- Check the antenna model, antenna phase center, and mounting offsets.
- For vertical offsets, confirm whether the system is using ellipsoidal height or MSL height.
Summary
Datum, coordinate representation, coordinate format, and map projection are separate concepts. The datum defines the reference frame. The coordinate representation defines how the position is expressed. The coordinate format defines how the coordinates are written. The projection defines how the position is shown on a flat map.
GNSS position calculations are based on an reference ellipsoid, but applications may transform that position into another reference frame, projection, or height reference. When an offset is stable and repeatable, check coordinate references before assuming the GNSS output is inaccurate.