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2D Grid

The 2D Grid function offers a quick way of gridding and visualizing one or more theme as images.

To use 2D Grid first select a model selection or a theme in the Workspace Manager and then select 2D Grid from the Visualization ribbon.

The Grid Settings window for 2D Grid consists of five tabsheets. One with the theme selection, three with the grid settings, and finally the usual one with image settings.

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

The options for each tab are listed below:

Theme(s)

The themes to be included in the batch job are selected here. If 2D Grid was started from a theme, it will be preselected.

Use the Theme Type filter to select among different theme types and select themes in the list box below. Use shift+click to select a range of themes or ctrl+click to select multiple individual themes. It is also possible to Filter on theme name or Select All themes

The No. of processors drop-down box controls how many gridding jobs will be run at a time, e.g. this can be increased on high end machines for higher speed.

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 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.

Note that batch gridding all grids are created using the same grid routine (though the variogram is fitted automatically for each theme if Kriging is selected). The Active Theme selection allows the user to test calculate variograms for individual themes, but it is otherwise not used.

Image

The image tab holds the path and a preview to the currently selected color scale. The user can:

  • Press the Open button to browse for a color scale. This opens the Color Scale Selector (ColorScaleSelector).
     
  • Press the Edit button to edit a color scale. This opens the Color Scale Wizard (ColorScaleWizard) where the user can customize existing or create entirely new color scales.
     
  • Press the Save button to save the selected color scale to a file. By default this will be the Aarhus Workbench color scale format (*.awc), but it is also possible to save the color information to a few other standard formats like

    Geosoft TBL (*.tbl)
    ER Mapper LUT (*.lut)
    ArcGIS CLR (*.clr)


Below the standard color scale options there are a number of image creation related options:

  • When Interpolate pixels is checked, the user can interpolate the pixels of of the grid to create a smoother image than the grid spacing specified during gridding. When checked the Grid subdivision factor dropdown becomes active. Based on the used subdivision factor, the resolution of each pixel and the width and height of the image is calculated below. Odd subdivision factors will match the existing pixel border, even subdivision factors will have half a pixel on either side of the existing pixel borders.
     
  • When White as transparent color is checked (it is enabled by default) white it set as transparent in the grid.
     
  • When Shadows is checked, the user can click the Shadows button to create shade theme images (ShadeTheme) rather than the normal images. This feature has been disabled. It can still be used when creating individual images (NewImage).
     
  • The Translucency drop down makes it possible to created image with a degree of translucency. Higher values equals higher translucency.
     
  • When Add layers to GIS after creation is checked, the layers will be added as part of the batch operation, otherwise the layers can be added later from the workspace explorer when they are needed.
     

When the OK button is hit, the Aarhus Workbench prompts for a suffix to the names of the new image. When the names are specified a gridding job is started on the selected number of CPUs. When all grids have been completed the user can press Close and the grid and image nodes will then be added in the Workspace Manager below the relevant theme nodes. The images are also automatically created as layers and appears in the GIS Layer Control for the relevant map.