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Coordinates and coordinate systems

General introduction and definitions

Describing a betoken on the existent-globe Earth equally a point on the World model

Several steps are necessary for describing a point in space, since the World 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 stock-still frame. Typically, a datum volition fit a specific country very well, just possibly not the adjacent country. Datum, therefore, commonly means a national datum organization. The geoid describes an equipotential surface of the World'south gravity potential at sea-level.

Displaying a signal in space on a two-dimensional map

For drawing a betoken on a map, you lot have to define how the indicate is to be projected onto a flat surface. (The apartment surface may be a plane, or the surface of a cylinder or cone, which and so tin can be unwound mathematically.) The map project is a pure mathematical process. Since no project can depict the Earth's surface without distortion, yous may use completely dissimilar 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:

• datum
• ellipsoid
• projection
• geoid (optional)

Coordinate system

A coordinate system is a three-dimensional reference frame that locates objects in space. It consists of a set up 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 yous use must already exist defined in the database. It defines the datum that the coordinate project is based on.

You usually assign an existing geoid model to a coordinate organization. Alternatively, you tin can define a constant geoidal separation or specify that no geoid model is used.

There are many coordinate project types that you tin allocate to a coordinate arrangement. Unlike countries (and regions) use dissimilar projection types to minimize distortion when deriving projection coordinates.

Coordinate projection methods change latitude and longitude values on the chosen datum to project coordinate values. The chosen geoid model provides the elevations for the projection coordinates.

Site definition

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 but valid for a limited area, that area is chosen a site or local site.

A coordinate system is designed to apply over a big surface area and does not provide for variations that occur in local coordinates. (Local variations can result from a build-up of errors in existing command, for example.) If you employ the extra correction transformations, you can right for the local variations and practise new work that fits with the existing control. Y'all tin 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 salve the definitions to the database.

Local coordinate organization

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 lot tin freely define origin, scale and false easting/northing of a local coordinate system. To practice so, yous select the Default projection (Transverse Mercator) coordinate organisation pick.

Trimble Coordinate System Managing director

The Trimble Coordinate System Manager software is an editor that gives you admission to your geodetic database. The coordinate system database is stored equally 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 exist all they need. If you utilise the Coordinate System Director software as well, however, you have the freedom to:

• browse – know what parameters you are using as yous work
• change – although y'all cannot edit the definitions that are supplied, yous tin duplicate records and use the duplicated record to make changes
• create – plant a site if you are working in an area that is not adequately covered by the definitions provided

Use the Coordinate System Manager to:

• create or edit a site, zone, datum, ellipsoid, or geoid
• select the projection used by a zone
• select the ellipsoid of any datum
• specify vertical and/or horizontal adjustments to a zone and save them as a named site
• select a geoid model
• assign the files to be used past a geoid model or datum transformation

In the coordinate system database, coordinate system zones are grouped in Coordinate System Groups. Groups are usually created for each country, simply you can also accept groups of a special blazon of coordinate system, such as Universal Transverse Mercator (UTM), Universal Polar Stereographic (UPS), or New Zealand Map Grid.

The usage of coordinate systems within this software

Basically, Trimble 4D Command Server applications utilise the Earth-Centered-Globe-Fixed Cartesian coordinates when determining coordinates. This is the three-dimensional coordinate organization 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° e longitude, and the Z direction is the northerly rotational axis of the earth. The current GNSS version of this organization is ITRF 2008. The geodetic organization zone definition, therefore, is irrelevant for GNSS processing.

Nonetheless, there are several Trimble 4D Control Server modules, which use geodetic coordinate system zone or site information, if available. Amid these are, for example, the following:

• The Terrestrial Engine RT: If the known coordinates (control coordinates) of points are given in a coordinate system zone or as calibrated site coordinates, this coordinate system will exist practical for calculating the approximate bespeak coordinates for monitored points.
• The Integrity Monitor: Displacement values (which consequence from the internal adjustment process) are in the selected coordinate organization zone or calibrated site.

Managing coordinate systems

The stand-alone Trimble Coordinate Organisation Managing director utility provides the core coordinate system database. To access the Coordinate Arrangement Manager software from the Trimble 4D Control Server UI, do one of the following:

• From the main menu, select the File > Coordinate System Manager command.
• At the toolbar, click the Coordinate Organization Director push button.

For more than information on the Coordinate System Manager, refer to its help organisation.

Selecting the coordinate arrangement

For selecting a coordinate system grade the database available with the Coordinate Organisation Manager to exist applied for terrestrial points (optical measurements), do the following:

1. From the main menu, select the File > Arrangement Properties command.
   The Organization Properties dialog appears.
2. Edit the Select coordinate system property: Set the focus into its value field and click the browse button. This opens the Trimble coordinate system choice sorcerer with one of the post-obit pages:

   - The Select Coordinate System Type dialog, if you lot take never before selected a coordinate system

   - The Select Coordinate Arrangement dialog, if your current coordinate system is not Default:Default.

3. Use the options of the Trimble coordinate organisation choice wizard to specify the coordinate system and zone, a calibrated site, or a default / modified (Transverse Mercator) projection and the geoid model to be applied.

Note - Irresolute coordinate systems significantly, might event in considerable jumps in displacement data. This may occur, for instance, if one changes to a projection or calibrated site that is not purely Northward-orientated.

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Source: http://help.t4d.trimble.com/documentation/manual/version4.6/server/CoordSys_GEN_Introduction.htm

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