Resource: con_base

This resource is used to specify the contour base value.  This has several applications depending on the application.

Value Ranging

Value ranging is the method of specifying what range of values to actually contour. This is specified with the con_base resource with the following syntaxes:

• >min -- this specifies the minimum valid value to contour/color.  This also sets the contour base value (see below).
• <max -- this specifies the maximum valid value to contour/color

For example, if the wind speeds above 100 knots are to be shown, use:

   upcalc .. -va=spd[knt] -cb=>100

Both a minimum and maximum can be specified if separated by commas.  

Color Fill Contouring

When contouring a grid, the first color in the color fill array is linked to the lowest value range in the grid. Since the minimum value will change from plot to plot, the colors in the color fill will switch the values they represent. The con_base resource locks a particular value to the lowest color range. For example:

   sfccalc ... -in=2 -cb=0

will lock magenta into the range of 0-2 and light magenta to 2-4 and so on. These color ranges will not shift from plot to plot.

Negative Dash Line Contouring

When a negative dash line contour type is used, the breakoff point between line contours and the dashed contours is 0. Line contours exist at this value and above. Dashed contours appear below this value. The con_base resource is used to set the break point if it is not 0.

Wind Vectors

Wind vectors are scaled to the highest wind speed prior to plotting. When looping wind vector plots, it should be noted that since the vector lengths for the same wind speed could change from one frame to the next. In order to lock a specific length to a specific speed, use the con_base resource. The contour base value sets a wind vector of that speed to one grid spacing, thus locking in a length to speed relationship. Remember to pick a value close to the maximum speed or the vectors may be too large or too small.

Streamlines

The number of  streamlines within a grid box is controlled by the con_base resource. The streamline algorithm automatically doubles the grid for this computation. The default is one streamline per grid box but this is roughly 2 since the grid is doubled. Setting the contour base to 2 would double the number of streamlines on the plot.

Data Plotting

When a contour interval is specified during data plotting, the plotted data will be colored based on the value. The minimum value for which the colors in the fill attribute arrays are linked to is 0 or the value of the con_base resource.