Contour grid
Use this function to extract contours from the Grid Model.
To run this function: Choose Surfaces > Contouring > Contour grid, or...
The CREATE GRID CONTOURS function has the following features:
(a) You can define the set of contours to extract either by an interval or by a range;
(b) You can choose to extract raw contours or you can smooth the contours using cubic splines;
(c) You may define spatial constraints, eg inside string or above a certain x value, so that you only extract contours from a particular area of the grid in the current pass;
(d) The output contours are ordered so that separate contours with the same value are stored as different segments in the same string.
(e) You have great flexibility in defining the drawing styles for the contours. You can define drawing styles for contours and index contours. Index contours allow you to highlight specific intervals in your contours, that is you can choose to create contours every 2 metres but have the contour at each 10 metre interval drawn in a distinct style (by defining an index interval of 10). You can choose colour, thickness and pattern for contours and index contours. You can also label the contours and index contours with the contour value, this is done by defining the frequency (in terms of distance along the contour) and size of the value labels. The drawing styles must be saved to a new styles file. This is so that when you save the contours to a string file and later recall the string file, the drawing styles are consistent.
(f) The created contours are automatically displayed on the screen in the graphics layer chosen by the user. You may create contours in several passes. The contour graphics layers can be saved to string files using the SAVE FILE button on the GRID menu. You can also toggle their display using the LAYERS Status button on the GRID menu.
Note:If you extract raw unsmoothed contours from the Grid Model, the contours are guaranteed not to cross. If you choose the option of smoothing the contours using cubic splines, the extra curvature introduced by the smoothing may, in certain extreme cases, cause the contours to cross. If you intend to use the contours for further detailed calculations, you are therefore advised to use the unsmoothed raw contours.
Contour display layer
Enter the graphics layer in which to store the created contours.
When you select APPLY on the DEFINE LAYER TO DISPLAY CONTOURS form a check is made to see if the contour display layer that you have chosen already exists. If it does then the VERIFY CREATION OF CONTOUR LAYER form is displayed. This form reminds you that the display layer that you have chosen already exists and gives you the option of selecting APPLY to continue and overwrite the layer, or selecting CANCEL and returning to the DEFINE LAYER TO DISPLAY CONTOURS form.
When you have chosen a satisfactory display layer and applied the appropriate form, a progress feedback bar is displayed with the label:
Finding grid max and min values
This is displayed while the maximum and minimum values over the grid are found.
At the completion of the progress feedback the EXTRACT CONTOURS FROM A GRID form is displayed.
GRID INFORMATION
Minimum contour value
Display only. The minimum value found in the grid.
Maximum contour value
Display only. The maximum value found in the grid.
CONTOUR DEFINITION
Define contours by (I)nterval or (R)ange?
Enter I or R to define the contours to be extracted by an interval or a range respectively.
Enter I in the Define contours by (I)nterval or (R)ange? field to display:
Minimum contour
Enter the minimum contour value.
Maximum contour
Enter the maximum contour value.
Contour interval
Enter the contour interval.
Enter R in the Define contours by (I)nterval or (R)ange? field to display:
Contour range
Enter the contour range.
Smooth contours?
Enter N to extract raw unsmoothed contours or Y to apply a cubic spline smoothing operation to the raw contours.
Enter Y in the Smooth contours? field to display the following two fields:
Minimum points in contour
Enter the minimum number of points in a contour for smoothing. Contour segments must have at least 5 points to be able to be smoothed by the spline process. Contour segments with less than 5 points are copied unsmoothed into the new contour layer unless this option is used to delete them. Having smoothed and unsmoothed contours in the same layer is not recommended as it can lead to contour crossing. You are therefore advised to use the default value of 5 for this option.
Number of lines per segment
Enter the number of lines per segment for the smoothing. The smoothing process has the effect of adding extra points to the raw contours. The value in this field is the number of new points that will be inserted between two adjacent points in a raw contour line. The default value of three is recommended to give the best results in the vast majority of cases.
Constrain contours?
Enter N if you wish to extract contours from the whole grid. Enter Y if you wish to define spatial constraints, for example inside string or above a certain x value, to define a subregion of the grid for contour extraction.
Enter Y in the Constrain contours? field to display the
ENTER CONSTRAINTS form.
A constraint is a way of describing a region of space for use with the Grid Model. Constraints may be used for such applications as selecting data to be used in an interpolation, selecting regions of a grid to be filled by an interpolation, or selecting regions of a grid for contour extraction. Valid constraints within the Grid Model are: string constraints (closed segments), x (easting) value constraints, y (northing) value constraints, and line constraints. Note that these constraints are applied in the xy plane, this means that the z coordinate is ignored for string constraints.
This form has a number of features to make generating constraints easier. At the top are a set of buttons, each of which gives a form showing an example of a constraint. Below that is a scrolling region to be used as a reference when constructing different sorts of constraints specifications, showing what is required for the different constraint types.
The scrolling region containing the input fields is the general constraint specification entry form and values for the fields will differ depending on the type of constraint being specified. The Reference region shows the permissible values.
Logical Expression
The operators available for the various objects depend on the nature of those objects. The logical `NOT' operator is used so as to reduce the number of operators necessary to cover all situations (for example NOT ABOVE is equivalent to BELOW).
STRING
Valid operators for a string constraint are `INSIDE' and `NOT INSIDE'. The string location, id and string and segment ranges as well as the direction in which the string is to be extended is given, if the string and segment ranges are not specified, all strings and segments in the specified file and/or string are used. All strings and segments specified must be closed.
Y VALUE
This specification is for data relating to a certain y (northing) value and is defined by the y value. Valid operators are 'ABOVE' and 'NOT ABOVE'.
X VALUE
This specification is for data relating to a certain x (easting) value and is defined by the x value. Valid operators are 'ABOVE' and 'NOT ABOVE'.
LINE
A line in the xy plane may be specified as a constraint. The line is of the form Ax + By + C = 0. Valid operators are `ABOVE' and `NOT ABOVE' with ABOVE being defined as all values x, y (ie easting, northing) for which Ax + By + C > 0. For example, constraining to above an x value of 100 would use the line equation 1x+0y-100=0 (i.e. x=100) and the constraint specification would be ABOVE line 1 0 -100.
Constraint combination
All constraints specified, by default, are combined into a logical expression with the AND operator. Alternatively, any other logical expression involving AND, OR and the specified constraints identified by their tags (a-z) may be given. The expression may include parentheses which may be nested.
Complete the ENTER CONSTRAINTS form and choose Apply to commence processing the constraint.
Example of Constraints
A number of constraints examples are described below. These examples include the purpose of the constraint, each of the constraints specifications and the constraint expression.
Constrain to parts of the grid inside a lease boundary
This example constrains the grid to those that lie within a mining lease boundary. The lease boundary is represented by string 9 segment 2 in the file ml1.str. If the string and segment numbers had been left blank then all strings and segments from the file would have been used.
Constraint Specification
a) INSIDE STRING ml 1 9 2
Constraint Expression
(a)
Constrain the grid to those parts below a certain y value (northing)
This example constrains the grid to those parts whose y value (northing) lies below 500. Note that there are two ways in which this y value may be expressed. Y value 500 (Y=500) is equivalent to the line equation 0x + 1y - 500 = 0. The more general form of the y value constraint is also shown.
Constraint Specification
EITHER
a) NOT ABOVE Y VALUE 500
OR
| xcoeff | ycoeff | const | ||||
| a) | NOT | ABOVE | LINE | 0 | 1 | -500 |
Constraint Expression
(a)
Complex constraint by combining constraint specifications
This example shows how constraints may be combined using logical expressions.
Constraint Specification
| a) | INSIDE | STRING | sec | 20000 | 1 | |
| b) | ABOVE | Y VALUE | 19050 | |||
| c) | NOT | ABOVE | Y VALUE | 20050 | ||
| d) | INSIDE | STRING | sec | 20100 | 1 | |
| e) | ABOVE | Y VALUE | 20050 | |||
| f) | NOT | ABOVE | Y VALUE | 20150 |
Constraint Expression
((a AND b AND c) OR (d AND e AND f))
Select Apply in the EXTRACT CONTOURS FROM A GRID form to display the CONTOUR DRAWING STYLES form.
CONTOURS:
Colour
Enter the colour to draw the contours.
Thickness
Enter the thickness to draw the contours.
Pattern
Enter the pattern in which to draw the contours.
Draw contour value?
Do you want to draw labels on the contours? (Y/N)
Enter Y in the Draw contour value? field to display the following two fields:
Frequency
Enter the contour label frequency (in terms of distance along the contour).
Size
Enter the contour label size.
Create index contours?
Do you want index contours to be drawn in a different style? (Y/N). Index contours allow you to highlight specific intervals in your contours, that is you can choose to create contours every 2 metres but have the contour at each 10 metre interval drawn in a distinct style (by defining an index interval of 10).
Enter Y in the Create index contours? field to display a group of fields relating to the index contours:
INDEX CONTOURS:
Interval
Enter the index contour interval. Index contours allow you to highlight specific intervals in your contours. For example if you chose to create contours every 2 metres you can have the contour at each 10 metre interval drawn in a distinct style (by defining an index interval of 10).
Colour
Enter the colour to draw the index contours.
Thickness
Enter the thickness to draw the index contours.
Pattern
Enter the pattern in which to draw the index contours.
Draw contour value?
Do you want to draw labels on the index contours? (Y/N)
Enter Y in the Draw contour value? field to display the following two fields:
Frequency
Enter the index contour label frequency (in terms of distance along the contour).
Size
Enter the index contour label size.
SAVE THE NEW DRAWING STYLES
Drawing styles file
Enter the name of the styles file for saving the drawing styles. The drawing styles must be saved to a new styles file. This is so that when you save the contours to a string file and later recall the string file, the drawing styles are consistent.
Processing
While the contours are being calculated the following progress feedback bar is displayed:
Calculating contours
If contour smoothing was requested the following progress feedback bar is displayed while the contours are being smoothed:
Smoothing contours
The following message is displayed while the contours are being drawn on the screen:
Drawing contours, please wait...
Results
The contours are drawn on the screen in the graphics layer defined by the user. The contour graphics layers can be saved to string files using the SAVE FILE button on the GRID menu. You can also toggle their display using the LAYERS Status button on the GRID menu.
Warning
Maximum number of contours has been exceeded
The maximum number of contours that may be extracted in one pass is 1000.