Geoida Help

Job Configuration

This option is used to enter or edit description and specify configuration details for the current job. The Job Configuration window is displayed when this option is selected, and is also displayed when creating a new job with the New option.

Many jobs will be found to require the same or similar configuration settings, particularly if always located within the same country, region or zone and where the spheroid and datums remain constant. To pre-set default configuration values to minimise changes that might otherwise be necessary for every new job, select the Job Configuration Defaults option in the Definitions Menu menu and set the values and settings most commonly required.

Coordinate System

All points computed are stored in the job database according to the type defined for the job. If points of the opposite type are entered or imported, the coordinates are converted to that of the job before storing.

Points are coordinated and inverse measurements are extracted in different ways according to the coordinate system defined for the job as follows:

• Plane - Simple plane geometry
• Grid Projection - Application of standard non-simplified formulae for arc-to-chord and line scale factor corrections to plane bearings and distances
• Geographic - Rigorous Vincenty direct and reverse formulae

Note that the Grid Projection drop-down list may potentially be populated with a large number of projections as defined in the Definitions menu Projection option. To eliminate unwanted projections that would otherwise clutter the list with projection names that may never be used, refer to the Projection option and cross the Display in lists checkbox for only those projections that are of interest for typical projects.

Associating grid projection with Geographic job

When the Coordinate System for a job is set as Geographic, it is not essential to associate any grid projection with it, but if it will be required at any time to list or export points as grid coordinates for a particular projection, it will require an associated projection. A standard UTM projection will be defined by default (the Grid Projection field may appear blank), or a projection whose geodetic datum matches that selected for the job may be automatically set by Geoida and appear in the disabled Grid Projection field. To set a specific (matching) projection, temporarily set the Coordinate System to Grid Projection to enable the drop-down projection list, select the projection required (must be the correct datum), then reset the coordinate type to Geographic - the required projection ID will be retained even though it will now appear disabled. If the projection remains unselected and blank, a standard UTM grid will be defined so that grid coordinates etc may still be produced without error; the definition of this grid is NOT defined in the Definitions Projection library but is hard-coded in Geoida

Earth Radius and Latitude

Earth Radius is used in the correction for curvature during the reduction of distances and height differences. If the job is defined as Plane, an approximate radius value may be entered into the Earth Radius field, or may be calculated for the selected latitude in Approx. Latitude which computes a value for the WGS84 spheroid. If the job is defined as Grid Projection or Geographic, a new radius value is determined for each computed point, thus the Earth Radius field here is not needed and is disabled. Although a radius value is still computed from the spheroid for the current datum at the indicated latitude and the value is displayed in the Earth Radius field, the approximate latitude is used primarily to locate the first display window when a new job is created.

Geoid-spheroid separation (N) and Deflections from Vertical

When a job is configured for a spheroidal horizontal datum and for either spheroidal or geoidal (orthometric) vertical datum, geoid-spheroid separation (N-value) and gravitational deflections may be interpolated from any available model.

If the Interpolation Method selected is Bi-cubic or Bilinear, then interpolation is made using the selected geoid grid-file which must be associated with the same spheroid as defined for the job. If no model is available then the various interpolation options will not be available. Geoida reads the AUSGeoid text-based grid file formats and a number of other country-specific and general text and binary formats.

CAUTION - Ensure that the geoid grid-file selected corresponds to current published data and matches the geodetic datum defined for the job. Using an inconsistent or out-of-date file may result in erroneous interpolations.

If the Interpolation Method selected is Fixed-point TIN then a pre-defined geoid model will not be necessary but a model will require creation within the current job using the Form TIN Geoid Model option in the Points menu - refer to the Form TIN Geoid Model topic for more information.

Coordinate-type conversion

IMPORTANT: As noted in WARNING above, care must be exercised to avoid inadvertently or inadvisably changing job configuration properties in a job in which processing has already been done. However, it is possible to convert the coordinate type from one type to another PROVIDED THAT the geodetic and vertical datums are not changed in the process. Note that this is not a transformation but merely changing the form in which the point coordinates are represented. It must also be remembered that all points in the job will undergo a conversion from one coordinate type to the other and repeated conversion to and fro may result in loss of coordinate precision.

To change the coordinate type:

1. Select Job Configuration
2. Change the coordinate type between Grid Projection & Geographic as required (Datum & Spheroid MUST NOT be changed!)
3. In the View menu select Reset (Fix) Bounds - displays (eg) “135 points were converted to the current job type”
4. If converting from Geographic to Grid, points will still be displayed as geographic at this point; to display as grid, select Plot Geographic in the View menu and un-tick to display as grid.