Sample a Grid

Use the Sample a Grid option (geogxnet.dll(Geosoft.GX.GridUtils.SampleAGrid;Run)*) to sample a grid along the designated X, Y locations of the current database and create a new channel containing the sampled grid data.

The option is available from the following locations:

  • Grid and Image > Utilities menu
  • Tie Line Levelling > Levelling Utilities menu with the Geophysics Levelling extension

Sample a Grid dialog options

Input grid

Grid file to sample

Script Parameter: GRIDSAMP.GRID

Grid sampled channel

Output sampled grid channel. By default the channel will have the same name as the grid

Script Parameter: GRIDSAMP.Z

Application Notes

*The GX tool will search in the "...\Geosoft\Desktop Applications \gx" folder. The GX.Net tools, however, are embedded in the geogxnet.dll located in the "...\Geosoft\Desktop Applications \bin" folder. If running this GX interactively, bypassing the menu, first change the folder to point to the "bin" folder, then supply the GX.Net tool in the specified format.

It is often useful to be able to sample a data value from a grid based on the (x,y) location in a database.

For example, you can extract the digital terrain along a survey path from SRTM grids. A more complex application of this tool is in micro-levelling of survey data.

Sample a VOXI Model from a Grid

Be aware of the sampling content of the grid to avoid creating a moire effect that depends on the ratio of the data grid cell size and the voxel cell size.

When creating the observation values from an import grid, use grid sampling (sample at a specific (x,y)) to sample at the location directly above and centered on the model cell.

Micro-levelling

Micro-levelling is intended to remove more subtle levelling errors that remain in data after normal levelling corrections have been applied. This error appears as a long wavelength along the survey lines and a short wavelength related to the survey line separation across the survey lines. Such error can be removed from a gridded data set using the de-corrugation techniques available in the Geosoft Mapping System.

The following procedure describes how to use grid de-corrugation to apply micro-levelling correction to the original line data.

  1. Apply base station levelling and tie line levelling to produce a best-effort levelling of the data. There may still be some minor levelling errors that have not been corrected.

  2. Grid the survey line data (excluding tie lines) to produce a raw levelled grid.

  3. Apply decorrugation filters to the grid using MAGMAP to produce a 'micro-levelled' grid. This normally involves the application of a Butterworth high-pass filter (4 times the line separation) and a directional cosine filter to produce a level error grid, which is in turn subtracted from the original data grid to produce a de-corrugated result. You may have to adjust the parameters of the Butterworth high-pass filter, or apply BIGRID non-linear filters to the original grid to remove very high-power features from the data. The result is an acceptably levelled gridded product.

  4. Run GRIDSAMP to extract the 'micro-levelled' values from the levelled grid and place them in a new channel of the database.

  5. Subtract the 'micro-levelled' data from the raw levelled data to produce a raw level error channel.

  6. The raw level error channel will contain some short-wavelength features that have a geological source and which result from line gridding error. These must be removed by applying an appropriate low-pass filter using the LOWPASS GX. The result is the final micro-level correction.

  7. Subtract the micro-level correction from the raw levelled data. The result is final levelled and micro-levelled data. The micro-levelled data can now be gridded and processed as a final product.