Add Contours to Maps

Oasis montaj contouring functionality is designed to help you make contour maps from a data model represented by a grid file. Contour maps are commonly used to display Earth Science data collected in plan over a given map area. For example, they might be used to present the results of a ground magnetic survey in which each contour line represents the same magnetic reading.

One characteristic of Earth Science data and in particular geophysical and geochemical data is that the information being contoured commonly has a very large dynamic range. In presenting such data, the contours must be able to show details in both low and high relief areas, as well as clearly indicate the direction of the relief in active areas. Oasis montaj contouring is optimized for this type of data. The main capabilities are:

• Suppression of lower-level contours as gradients become too large to cleanly draw the contours

• Indication of depressions by drawing hachure marks on the lowest closed contours

• Annotation of extreme highs and lows, and/or map edge annotation of contour values

• Indication of gradient direction by orienting contour labels up-gradient

• Up to 32 contour levels may be defined with optional contour labelling, line styles and data ranges. The levels may be defined either as a set of intervals or as fixed, individual contours

• Optional trimming of the contour map to a polygonal shape for irregularly shaped maps

• Automatic generation of default values for most control parameters

• Two trace-smoothing options to improve cosmetic appearance of traces

Create a Grid File for Contouring

Oasis montaj basic contouring methodology is to thread contour lines trough constant levels defined in a gridded data file. A grid is a collection of points along rows and columns that define a two-dimensional rectangular area on some plan, usually a ground plan.

Gridded data files are usually created by using one of the Oasis montaj gridding methods, or may be imported from other software systems that support the Geosoft Exchange Format (*.GXF). The grid file can contain any number of points. Contours are added directly to the map you are working with.

It is important that the grid cell size (the distance between nodes of the grid) is appropriate for contouring. We recommend that you use a cell size that is 5 mm at map scale when plotted. Grids with cell sizes less than 1 mm will require more processing time than it is necessary to make a good contour map, while grid cells larger than 3 mm can produce contours that appear jagged. If the grid contains important information that is only honoured at cell sizes less that 1 mm, you should probably not be contouring the data, or you should be plotting the data at a smaller scale (make the same area on the ground larger on paper).

Contouring Options

There are a number of options for contouring: