Project geodetics

One of the most important aspects of a data collection project is that of the coordinate systems used - of the geodatabase feature classes, of the map, and of the correction sources (both real-time and post-processed). The combination of these, the information necessary to interoperate between Trimble and Esri, and the availability of datum transformations necessary to work between different coordinate systems, will almost always determine what coordinate system will be used in the field application.

As part of Creating and managing projects, users are presented with a Project Geodetics step in the Project Wizard. Although this step does include contextual help within the add-in itself, the information is also included here.The following information is provided in the Project Geodetics step in the Project Wizard when and is repeated here.

The Esri ArcGIS definition of the horizontal coordinate system used by the geodatabase feature classes in the project, expressed in its individual components. Full definition includes: Coordinate System (projected or geographic), Geographic Coordinate System, Datum, Vertical Coordinate System, Matching Trimble Datum (if any).,

The Esri ArcGIS definition of the horizontal coordinate system used in the data frame of the map document for display. Full definition includes: Coordinate System (projected or geographic), Geographic Coordinate System..

Geodetic workflow defines the way GNSS coordinates are stored in Terraflex and Terra Office and how they are converted when required. Depending on the geodatabase coordinate system above, the following geodetic workflows are available:

Store all position data in TerraFlex and Terra Office in the global WGS 1984 datum and apply transformations wherever required. Best used when the datum of the correction source does not match that of the project. Transformations in the field-side of the workflow will be done with Trimble components while transformations in the office-side of the workflow will be done with Esri ArcGIS components (see next section).

Real-time Accuracy Source

Corrected positions from local sources (VRS, RTK) will be transformed between their datum (as specified) and that of the project using standard methods. Corrected positions from global sources (RTX and SBAS) will not be transformed.

Post-processed Accuracy Source

Post-processing will use base station reference positions in the global WGS 1984 datum and results will be transformed to the project datum. This is affected by the ‘Use reference position from’ setting in the ‘Processing Options’ area of a ‘Processing Profile’. The profile can be set to use reference positions from base station Rinex files only, from the Trimble CBS database only, or from either source using the best match for project geodetics.

Store all position data in TerraFlex and Terra Office in the supported local datum and thus minimize the number of datum transformations required in the whole workflow. Best used when all correction sources share the same datum as the project OR when using Trimble RTX.

Real-time Accuracy Source

Corrected positions from global sources (RTX and SBAS) will be transformed using the most accurate, time-dependent methods. Corrected positions from local sources (VRS, RTK) will not be transformed and as a result, only those that specify the same datum can be included with the project.

Post-processed Accuracy Source

Post-processing will use base station reference positions that match the local datum and results will not be transformed. Only base stations that have reference positions in the local datum will be usable for post-processing. This is affected by the ‘Use reference position from’ setting in the ‘Processing Options’ area of a ‘Processing Profile’. The profile can be set to use reference positions from base station Rinex files only, from the Trimble CBS database only, or from either source using the best match for project geodetics.

Store all position data in TerraFlex and Terra Office in the supported local datum but allow office-side transformations between the project’s local coordinate system and that of the geodatabase using Esri ArcGIS components (see next section). This option is only available for select regions where the use of related local coordinate systems is common. In some cases, the TerraFlex project in the field will also be configured with the same coordinate system as the geodatabase with only the internal storage of position data using the related local datum (for working with local correction sources).

Real-time Accuracy Source

Corrected positions from global sources (RTX and SBAS) will be transformed using the most accurate, time-dependent methods. Corrected positions from local sources (VRS, RTK) will not be transformed and as a result, only those that specify the same datum can be included with the project.

Post-processed Accuracy Source

Post-processing will use base station reference positions that match the storage datum and results will be transformed to the alternate local datum used in the geodatabase. Only base stations that have reference positions in the storage datum will be usable for post-processing. This is affected by the ‘Use reference position from’ setting in the ‘Processing Options’ area of a ‘Processing Profile’. The profile can be set to use reference positions from base station Rinex files only, from the Trimble CBS database only, or from either source using the best match for project geodetics.

Based on the Geodetic Workflow setting and the way datums and datum transformations are modeled in Esri ArcGIS components, it may be necessary to select the following datum transformations:

  • Transformation for geodatabase (storage)

    The datum transformation that will be used to transform between Terra Office and geodatabase feature geometries at the time features are stored or retrieved to/from geodatabase.

  • Transformation for map (display)

    The datum transformation that will be used to display Terra Office feature geometries and positions (in a session) as temporary graphic elements on the map document data frame.

The coordinate system that will be used for the project in both the TerraFlex mobile application and the Connect Map Viewer web application (where it is known as a workspace). This affects storage of data as well as coordinate display, measurement, and navigation.

The setting for how feature heights should be handled both field-side in TerraFlex and office-side in Terra Office. This affects both feature height metadata and the Z component of geometries in 3D, or Z-enabled feature classes.

  • Use height above ellipsoid (HAE)

    Store all feature heights and Z values as-is, without conversion from the GNSS (as height-above-ellipsoid).

  • Use height above mean sea level (MSL)

    Convert feature heights and Z values from height-above-ellipsoid to an orthometric height (typically called MSL) by using the Trimble geoid model selected. Geoids available are filtered based on the current extents of the map in view.

  • Use fixed separation (meters, office only)

    Store all feature heights and Z values with a fixed separation value (in meters) applied to the height-above-ellipsoid value. This is only available in the office-side workflow in Terra Office (values in TerraFlex will remain height-above-ellipsoid).

For more information on geodetics related to high-accuracy GIS data collection workflows, refer to https://geospatialresources.trimble.com/blog/coordinate-systems-101-the-importance-of-geodetics-for-high-accuracy-gis-data-collection.