Apply Log Contours

Use the Map Tools > Contour > Log Contour menu option (LOGCONT GX) to draw contours on a map using a specified grid in logarithmic mode.

You need to supply a grid file that has been gridded logarithmically.

Log Contour dialog options

Grid file

The grid file name.

To draw log contours, the grid file must be in logarithmic format. (On the gridding dialog, click the Advanced> button and select the log option.)

Script Parameter: LOGCONT.GRID

Number of contours between decades

Select "automatic", "decades only", "1 contour", "3 contours" or "5 contours" as an option for contour levels (Default is decades only). The "automatic" option determines the range in log space, and selects a reasonable divisor. Note that if the dynamic range of the grid is smaller than that implied by the number of contours per decade, then no contour levels will be plotted (e.g. if you select "decades only" and the grid data range is 4.2 to 4.5, then nothing will get plotted because only integer values 2, 3, 4, 5 etc. would be contoured).

Script Parameter: LOGCONT.OPT

Suppression gap (mm)

Suppresion gap in mm (Default is 0.0).

Script Parameter: LOGCONT.GAP

Application Notes

The logarithmic method lets you set up a contouring scheme using a grid that has been gridded logarithmically, using log option 1. The grid values are assumed to be log(Z).

The log contours for the Number contours between decades parameter:

1 contour   - 0.5
3 contours - 0.25, 0.5, 0.75
4 contours - 0.2, 0.4, 0.6, 0.8
7 contours - 0.125, 0.25, 0.375, 0.5, 0.625, 0.75, 0.875

Troubleshooting - Labels Display as Linear Equivalents

If the labels of the log contours display as their linear equivalents this means the values in the grid have been stored as linear. To display the log contour labels as true log values, the grid will need to be recreated in logarithmic format. Gridding of the channel needs to be performed again by setting the Log option (in the Advanced gridding parameters) as Log gridding, store log.