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Grid Settings (from Grid Theme)

The geophysical themes only have values at coordinates where the underlying database query is defined. Before displaying such a theme as an image on the map, the theme must be transformed to a regular interpolated grid. This transform is controlled by the Grid Settings window.

The grid theme node appears as a subordinate to the theme node in the Workspace Manager.

To create a grid the user must first select a theme in the Workspace Manager and then from the Visualization ribbon select the Grid function. When the gridding is done, the user will automatically be asked to also create a new image (NewImage).

The mathematical operation of gridding the values are performed by the program GSTAT, for further reference please consult www.gstat.org

The Batch Gridding (BatchGridding) function offers a quick way of gridding multiple themes and creating images for each of the gridded themes.

The Grid Settings window consists of three tabs. The options for each tab are listed below.

The Search tab contains parameters concerning the search criteria of the gridding. This process is integrated in the gridding procedure and should not be confused with any search parameters in the Aarhus Workbench. The parameters are based on the similar gstat parameters:

  • Min. number of data states the minimum number of data to be used in an interpolation. If this option is checked, the Search radius must include this minimum number of data points to give a value to the grid point. If Force minimum is also checked, the Search radius given below is disregarded and the Search radius for this grid point is increased until the minimum number of data points has been reached.
     
  • Max. number of data states the maximum number of data to be used in an interpolation. If this option is checked, only the the closest data up to the maximum number of data within the Search radius will be used.
     
  • Search radius states the Search radius used.
     
  • Max. number per sector states the maximum number of data to be used in an interpolation. If this option is checked, no more than this many number of data will be used from within each sector of the Search radius.

In most cases it is sufficient to simply set a Search radius, but for Kriging the calculation time increase significantly as the number of data within the Search radius increases. It is therefore recommended to use a Max. number per sector to reduce the calculation time. This can make the difference between hours and minutes per grid, and if the used value is large enough the resulting grid will be indistinguishable from a grid made without a Max. number per sector. A value between 8 and 16 should be large enough.

Grid

The Grid tab holds the control of the elementary setup of the grid:

  • Min. X and Max. X is the minimum and maximum East-West coordinates in the grid. The user may choose only to grid a part of the theme. The default values are the minimum and maximum coordinates represented in the relevant theme.
     
  • Min. Y and Max. Y is the minimum and maximum north-south coordinates in the grid. The user may choose only to grid a part of the theme. The default values are the minimum and maximum coordinates represented in the relevant theme.
     
  • Min. Data and Max data is the min and max data value. This option is no longer available.
     
  • Reset: By pressing this the min and max coordinates for the grid is returned to default values.
     
  • By selecting Show on Map the area specified in the above mentioned fields are highlighted on the GIS Map.
     
  • The valuespecified in Node Spacing is the cell size value. In the interpolation an equally sized squared grid is used. Information about the resulting grid size is shown in the section Number of nodes X and Y which holds the number of grid cells in the East-West and North-South direction respectively.

Note that while points (DBQ positions, theme positions etc.) are drawn at their actual coordinate positions, grid cells are drawn in the center of the grid cell. Therefore a small discrepancy may be seen between the two.

Note that the actual UTM values in the grid file will be distributed between the min and max UTM values using the entered Node Spacing.

Interpolation

Two methods of interpolation are available, either Inverse Distance or Kriging. Inverse distance is actually inverse distance to a power where the user must specify the Power to be used. When performing Kriging a variogram for the Kriging has to be specified. This can either be calculated automatically, or the user can manually construct the variogram to be used.

If the Logarithmic data transform option is enabled the gridding will be done using the logarithm to data values. This is for example useful when gridding resistivity values.

If Kriging is selected a plot showing the Variogram is shown. The variogram function is created using the parameters in the Variogram Settings fields. these are:

  • Function: Which function to use to fit the variogram. Available functions are Exponential, Linear and Spherical. Available functions also include Gaussian in debug mode.
     
  • Nugget: Offers the option of adding a nugget value to the variogram. The box must be checked to use it.
     
  • Sill: Effects the vertical appearance of the variogram.
     
  • Range: Effects the horizontal of the variogram.

Pressing Autofit Variogram will make an autofit of the variogram using the selected Function type.

The variogram function can also be fitted manually by entering values.

For further information about the functionality please consult the GSTAT homepage at www.gstat.org.

The variogram setup in terms of the Number of Bins and Sampling Distance can be changed by pressing the Advanced Settings button.