Contour Tab Functions


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Contour tab controls
  1. General Comments about the Contour Tab
  2. Contour List
  3. Gridding constant
  4. Contour Interval
  5. Gridding Method
  6. Weighted Option
  7. Hemisphere Option
  8. Grid Size
  9. Contour Mode
  10. Create Contour
  11. Delete Contour
  12. Delete All Contours
  13. Visible
  14. Hide Contours
  15. Contour NameEdit Box
  16. Select Colour
  17. Lower Cut-Off
  18. Upper Cut-Off
  19. Regrid
  20. Redraw
  21. Rename Pick

General Comments about the Contour Tab

Almost all functions involving contours are found under the contour tab. The calculations involved in creating contours can be slow. For this reason a contour, once created, exists as an entity which must be, for example, explicitly deleted or updated. In would not be appropriate to have the computer try to update a contour set, for example, whenever a new point was entered.
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Contour List

The contour list box shows existing contours. As with the data list or the set list, clicking enables a contour to be selected.
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Gridding constant

During the gridding process a value is assigned to each grid point based on the data points within a certain radius. This radius is called the ‘gridding constant’ and is input in degrees.
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Contour Interval

Suppose that the gridding process assigns values to grid points that vary between 0 and 6.7. How many contour lines (at different values) should be drawn? The contour interval is the ‘vertical difference’ between contours. Thus is there is a contour with the value 3 and the contour interval is 1.5 you would expect the next contour to represent the value 4.5.
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Gridding Method

When calculating grid values only those points within the radius set as the ‘ Gridding constant’ are considered. Within this radius the contribution of a data point can either be constant (the traditional effect obtained by moving a circular opening around on a equal area plot) or (better) the contribution of each data point can be scaled according to the distance from the grid point (distance of 0 gives a scaling factor of 1, distance of the gridding constant gives a scaling factor of 0).
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Weighted Option

In general, for contouring purposes, all data points are treated equally. However, the presence of ‘weighted’ options indicates that one can contour in such a way that the values assigned to grid points takes into account the weights assigned to the data points. This might be of use if one assessed sampling biases or the continuity of sample structures.
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Hemisphere Option

Cauldron gives three options here. Contouring using the ‘both’ option is appropriate where one is dealing with joint data which has no sense of ‘way up. If on the other hand, you wished to contour poles to steeply dipping beds, some of which were overturned, then you could create one contour set for the right way up beds using the lower hem option and a second set (possibly in a different colour) using the upper hem option.
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Grid Size

The method of contouring used here involved the calculation of a ‘density function’ for each point on a grid. If the grid size is 120 then there would be 120 x 120 points on this grid (though many fall outside the circle). A large grid gives smoothly drawn, accurate contour lines. There is a price, however: time to calculated the grid, time to draw the contour lines, time to send contour line data to the printer, memory space in the computer and printer and disk space if the contours are to be saved. Increasing the grid size (and recalculating the grid) should generally be one of the last steps, and taken only when a very high standard of print is required –perhaps using the poster print function,
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Contour Mode

The contouring tab is an overcrowded one. This option helps reduce clutter by making certain controls appear or disappear.
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Create Contour

This option creates a new contour set (grids and draws) using the options selected.
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Delete Contour

Deletes the currently picked contour.
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Delete All Contours

Deletes all contours in memory.
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Visible

Controls the visibility of selected contour set. Within one file (project) you may wish to create a large number of contour sets, but not show them all at the same time. This tool helps manage the visibility of individual contour sets.
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Hide Contours

Because the process of redrawing contours can be slow, this function enables you to switch all the contouring off on the screen. It does not use effect the visibility values which are set for each contour set.
For example, suppose that you have three contour sets, one of which is set invisible -you obviously see two contour sets. Now if you 'hide' the contours you see none. If you un-hide (reveal) the contours you again see your two visible contours. To see the third set you must select it in the contour list box and change its visibility.
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Contour NameEdit Box

As with data and (data) sets contour sets can have 'names' or 'remarks' attached to them in their internal data structure.
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Select Colour

This launches the familiar colour picker dialog to select the colour of the contour set.
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Lower Cut-Off

The first contour of a contour set (that with the lowest 'value' or 'population density') is that determined by the lower cut-off. Increasing this value will mean that contours will not be drawn for low values i.e. where there are few data points. For a change in value to be seen the contour only needs to be redrawn, not regridded.
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Upper Cut-Off

The contour drawing algorithm starts at the lowest value (see lower cut-off) and then draws a series of contours which depends on the contour interval. The process stops when either there are no more contours that can be drawn or the upper cut-off is reached. Note that a negative value for this (the default value) implies there is no cut-off.
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Regrid

Gridding is the first and potentially most time consuming part of the contouring process. The time taken is proportional to the number of data points and the square of the grid size. Regridding causes the grid values to be recalculated according to the current values on the contour tab to be used and the current data. It is functionally equivalent to deleting the contour and creating a new one afresh. Redrawing will automatically occur.
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Redraw

Drawing is the second and generally quickest part of the contouring process, the first being gridding. If you wish to change the drawing parameters (lower and upper cut-offs, colour, contour interval) then this option will avoid the slower regridding. It will not take into account changes to the data set, or changes to gridding parameters (gridding method, gridding constant ..).
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Rename Pick

This updates the name/remark attached to the contour set.
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Copyright: The copyright (2001) of this page is owned by Dr Nigel Stuart of Resource Dynamics, Alberta, Canada. You are welcome to use, reproduce it in part or in whole. Should you reproduce it in part please ensure that credit/blame for authorship is given. A link to the Resource Dynamics home page would be appreciated.