Geoid-Spheroid Separation

The requirement to extract geoid-spheroid separation (N) values depends on the configuration of a job and the type of computation to be made.

If the job is configured with both ellipsoidal horizontal and ellipsoidal vertical datums, it may be required to simply determine N-values for individual or multiple points only for reporting or listing purposes. In this case, the Apply to obs check box in Job Configuration would be uncrossed to indicate that no correction for geoid-spheroid separation is to be applied.

Figure 1: 'Apply Geoid' setting in Job Configuration

Where it is required to compute projection or geographic coordinates for observed points, the field observations must be reduced to the spheroid, and if the height components are related to a geoidal (or orthometric spirit levelled, gravimetric reference surface such as mean sea level) vertical datum, then it will normally be necessary to correct for the geoid-spheroid separation to maintain correctness and accuracy.

So, if the vertical datum is a geoidal datum, then N-values should be involved in the observation reduction processes when coordinating points, particularly if there is a significant separation between the geoidal and ellipsoidal surfaces. To ensure that the geoid-spheroid separation is accounted for, the Apply to obs check box in Job Configuration must be crossed in order to incorporate the necessary correction into the reductions.



A geoid model is required before N-values can be extracted, and the type of model is determined by the Interpolation Method in Job Configuration.

Figure 2: Selecting Geoid Interpolation Method in Job Configuration

There are two types of model.


When entering geoid-spheroid separation data it is of vital importance that it applies to the correct spheroid. Permanent mark summary sheets may show horizontal and vertical details for more than one datum and spheroid. It is vital that the values for the correct datum and spheroid be used or errors will result.

The display of the current geoid model is controlled by the Geoid model checkbox in Display Settings - refer to this topic for more information.

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Choice of interpolation method

Bicubic grid interpolation is the best method to use by reason of it being the most accurate method; it should be the method of choice wherever possible and is suitable for large or small areas. The bilinear grid method is less accurate but is used in the bicubic grid method at grid corners and edges. The triangle method is suitable for smaller areas when a grid model is not available but the geoid values for control points are already known. With a triangle model care should be taken if geoid values are extrapolated for points outside the model area.

Grid-file interpolation method

Interpolation of geoid and deflection values from a grid-file model may use either of the bicubic or bilinear methods.

CAUTION - Ensure that the geoid grid-file selected in Job Configuration 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.

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Data file optimisation - Bilinear & bicubic interpolation

If Interpolation Method is set to Bilinear or Bicubic interpolation, one of the following optimisations may be applied, depending on the format of the gridded geoid data:

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Interpolation and data reduction

Direct interpolation of geoid-spheroid separation for individual or multiple points may be done at any time in a number of different ways once a model has been formed or is otherwise available. For individual points (i.e., one at a time) use the Interrogate Point option, the Modify function in List Points / Export option, or when creating new points (or provisional points for least-square adjustment) use the Digitise Provisional Points option. To interpolate for multiple points at once, use Interpolate Geoid values in Window.

When processing survey field observations, interpolation of N-value is made differently depending on the processing option to be used but is generally made in the following stages:

  1. an initial value for reducing a distance observation to spheroid, and
  2. deriving the N-value for the resulting computed point, according to the processing option:
    • Radiations and Traverse (Bowditch or None adjustment) - derive N-value at first approximation of the new point's coords
    • Least-Squares adjustment - derive N-value for first provisional coords before reducing distance observation
  3. evaluate a final value for all non-fixed points upon exit from the option.

Note - Vertical gravitational deflections are available only from the Australian AUSGeoid .DAT/.TXT formats and are interpolated together with the undulations and applied to individual points, however the deflections are otherwise applied only to certain observation types in Least Squares Network Adjustment - refer to Corrections Applied.

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Interpolation tips

The following tips apply only to the Australian AUSGeoid .DAT/.TXT or South African .DAT formats. All other formats are either binary formats or are ASCII text formats that Geoida will automatically convert to binary; binary files are handled differently by Geoida and do not require creation of windowed files.

  1. Creating new points with the Digitise Provisional Points option - If the current geoid file is a windowed portion of a larger original file and a new point falls outside the windowed area, Geoida will automatically reassess the geoid extents from the original file (named in the inserted GEOIDA_LOCAL_WINDOW keyword line - see above) and create a new windowed file to cover the larger job area. This could potentially occur with every new point created. To prevent this happening, create the first new points at the limits of the area and work inwards. Geoida will not re-window the geoid area for new points falling inside an existing windowed area. Note - new points may also be created with numerous other options, but even though these points may fall outside an existing windowed geoid area, the geoid extents are not reassessed automatically - see next item.

  2. To manually reassess the windowed area after new points have been inserted, imported or computed, use the Reset Geoid Window option, or double-click an existing point outside the existing area (Interrogate Point option) and click the Interpolate button. This will cause Geoida to re-assess the windowed area by re-reading from the original geoid file.

  3. If points are deleted that effectively reduces the area covered by the remaining points in the job, Geoida does not automatically reduce the windowed geoid area. To re-size the windowed area downwards, use the Reset Geoid Window option; this will usually create a new, smaller file. If there is no change in the file, it may be necessary to use the Reset (Fix) Bounds option first (Refresh/Review Options).

  4. If the size of the windowed geoid area cannot be reduced, it may be because there are other points that still exist outside of the area currently displayed on-screen. Locate these points by Zoom Bounds (Zoom Options) or the List Points / Export option. It may also be necessary to check the Zoom Point Range option to ensure that all available points are displayed.

  5. Sometimes Geoida may be found to be frequently attempting to re-size the geoid extents but the windowed area remains unchanged at the same as the maximum size of the original area. This may be frustrating to the user because of the time taken for each failed re-assessment, but may occur because there are valid points that exist in the job but fall outside of the geoid extents as defined by the original file. Or the user may wish to retain the nominated original file and geoid window as-is and prevent the window being down-sized to a smaller area. Geoida can be prevented from making any further attempts to re-size the geoid extents by the use of the 'NO_RESIZE' keyword in either of two ways:

    1. Windowed file: Edit the file and add the [NO_RESIZE] tag at the end of the GEOIDA_LOCAL_WINDOW header line, eg
    2. GEOIDA_LOCAL_WINDOW,"D:\Geoida Reference\Geoid Files\SH50-14.dat",[NO_RESIZE]
    3. Original (non-windowed) file: Insert the tag [NO_RESIZE] into the file name, eg
    4. SH50-14[NO_RESIZE].dat
    The required geoid file with either of these changes applied should then be re-selected in Job Configuration. Note that the NO_RESIZE keyword is not case-sensitive but must be enclosed in the square brackets.

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