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Several steps are necessary for describing a point in space, since the Earth has not a uniform shape. The ellipsoid is a simple mathematical form that roughly models the size and shape of the Earth. The datum transformation orientates the ellipsoid relative to a fixed frame. Typically, a datum will fit a specific country very well, but perhaps not the adjacent country. Datum, therefore, usually means a national datum system. The geoid describes an equipotential surface of the Earth’s gravity potential at sea-level.
For drawing a point on a map, you have to define how the point is to be projected onto a flat surface. (The flat surface may be a plane, or the surface of a cylinder or cone, which then can be unwound mathematically.) The map projection is a pure mathematical process. Since no projection can depict the Earth’s surface without distortion, you may use completely different projections and projection types, depending on your task or the location on Earth of the respective points.
A geodetic Coordinate System Zone, therefore, consists of the components:
A coordinate system is a three-dimensional reference frame that locates objects in space. It consists of a set of transformations that allow GNSS positions (given in terms of the WGS 84 ellipsoid) to be transformed to projection Northings and Eastings with Elevations above the geoid.
A coordinate system consists of a datum transformation, a geoid model allocation, and a coordinate projection definition.
The datum transformation you use must already be defined in the database. It defines the datum that the coordinate projection is based on.
You usually assign an existing geoid model to a coordinate system. Alternatively, you can define a constant geoidal separation or specify that no geoid model is used.
There are many coordinate projection types that you can allocate to a coordinate system. Different countries (and regions) use different projection types to minimize distortion when deriving projection coordinates.
Coordinate projection methods change latitude and longitude values on the chosen datum to projection coordinate values. The chosen geoid model provides the elevations for the projection coordinates.
A site consists of an existing coordinate system plus an extra set of parameters for horizontal and vertical adjustments. Together these provide the best fit of GNSS data to a specific area or site.
Because the additional corrections are only valid for a limited area, that area is called a site or local site.
A coordinate system is designed to apply over a large area and does not provide for variations that occur in local coordinates. (Local variations can result from a build-up of errors in existing control, for example.) If you apply the extra correction transformations, you can correct for the local variations and do new work that fits with the existing control. You can shift coordinates obtained using GNSS so that they better fit coordinates in the existing map grid that were obtained using traditional surveying methods.
Most Trimble applications perform the necessary calculations for you, and save the definitions to the database.
For monitoring purposes, often pure local coordinate systems are used instead of a global projection system. These systems are purely local in the sense that no geocentric reference is given. You can freely define origin, scale and false easting/northing of a local coordinate system. To do so, you select the Default projection (Transverse Mercator) coordinate system option.
The Trimble Coordinate System Manager software is an editor that gives you access to your geodetic database. The coordinate system database is stored as a file called Current.csd. Use it to edit coordinate systems and to create a site.
This software is supplied with a number of Trimble surveying and mapping systems. For most users, the projections, zones, and datum transformations that are supplied with those systems will be all they need. If you use the Coordinate System Manager software as well, however, you have the freedom to:
Use the Coordinate System Manager to:
In the coordinate system database, coordinate system zones are grouped in Coordinate System Groups. Groups are usually created for each country, but you can also have groups of a special type of coordinate system, such as Universal Transverse Mercator (UTM), Universal Polar Stereographic (UPS), or New Zealand Map Grid.
Basically, Trimble 4D Control Server applications use the Earth-Centered-Earth-Fixed Cartesian coordinates when determining coordinates. This is the three-dimensional coordinate system used for satellite positioning. The origin of the system is the center of the mass of the earth. The X direction is the Greenwich meridian (0° longitude), the Y direction is 90° east longitude, and the Z direction is the northerly rotational axis of the earth. The current GNSS version of this system is ITRF 2008. The geodetic system zone definition, therefore, is irrelevant for GNSS processing.
However, there are several Trimble 4D Control Server modules, which use geodetic coordinate system zone or site information, if available. Among these are, for example, the following:
The stand-alone Trimble Coordinate System Manager utility provides the core coordinate system database. To access the Coordinate System Manager software from the Trimble 4D Control Server UI, do one of the following:
Coordinate System Manager button. For more information on the Coordinate System Manager, refer to its help system.
For selecting a coordinate system form the database available with the Coordinate System Manager to be applied for terrestrial points (optical measurements), do the following:
- The Select Coordinate System Type dialog, if you have never before selected a coordinate system
- The Select Coordinate System dialog, if your current coordinate system is not Default:Default.
Note - Changing coordinate systems significantly, might result in considerable jumps in displacement data. This may occur, for example, if one changes to a projection or calibrated site that is not purely North-orientated.