Geodesy and Surveying: Reference Systems and Joint Evaluation

Basic Networks: Joint Evaluation of GPS and Terrestrial Measurements

• GPS measurements are used to determine points in land surveying, and can be combined with terrestrial measurements to create and consolidate 3D engineering networks.
• Control of network points close to objects (e.g. dams, landslide slopes) is achieved by connecting them to GPS points, which provides a frame around the terrestrial network or its extension in the rearward direction.
• Disturbing influences of the subsoil and object (e.g. dead weight, water pressure) can be eliminated as far as possible by combining GPS observations with terrestrial measurements.

Reference Systems

• GPS coordinates are referenced to the geocentric WGS84, while country coordinates are referenced to a reference ellipsoid.
• GPS coordinates are three-dimensional, and three-dimensional measurements and analyses are increasingly being used for engineering surveying tasks.
• The coordinate reference is the ellipsoid, and the height reference surface is the geoid.
• A joint model is recommended to equalize terrestrial and satellite-based observations.

Evaluating GPS Data

• GPS data is evaluated in a terrestrially specified datum by transforming the GPS data into the available coordinate system (national network or local construction site or monitoring network).
• The joint equalization of GPS and terrestrial measurements is performed, eliminating the date-determining part by additional parameters (translations, rotations, scale).

Geodetic Reference System

• A geodetic reference system is a basic determination system for defining the spatial position, location, height, or gravity of points.
• A reference system consists of a coordinate system and a fixed point field.
• There are three types of geodetic reference systems: position reference system, height reference system, and gravity reference system.

Reference Surfaces

• Geodetic instruments are levelled with spirit levels and compensators to align the vertical axes in the direction of gravity.
• The level surface (equipotential surface) is an infinite number of surfaces perpendicular to gravity.
• The geodetic measurements refer to a specific level surface, known as the geoid.
• The geoid is a level surface that best approximates the actual undisturbed sea surface, imagined to continue under the continents.
• The geoid is represented by the ocean surface at rest, unaffected by tides and continuing beneath the continents.