VISUALIZATION OF FOLDS USING CUSTOM-GENERATED DIGITAL ELEVATION MODELS

Rendered digital elevation models (DEM's) of topographic surfaces provide a visual image that rivals direct observation of the landscape. Applied to surfaces of simple folds, DEM's provide means of visualizing structures that normally are either concealed in the subsurface or are eroded away. In this sense, the rendered DEM of a fold may provide greater benefits to students of map-scale structures than that of its more common application to landscapes.

While DEM's of topography are freely available, those of folded surfaces have to be custom generated and must be formatted for use by rendering software. This can be accomplished using a linear transition between two digitally described cross sections to produce intermediate profiles. The number of intermediate profiles is based on a horizontal spacing equal to that between elevations in digital descriptions of the two sections. The resulting fold DEM consists of elevation values at regular intervals of a square grid. The position of a particular value in the fold DEM matrix demarks the position on a map at which the elevation is modeled to occur on the folded surface. Appropriately oriented, columns of the matrix are north-south profiles along the length of the fold; rows of the matrix are east-west profiles parallel to end-member cross sections.

A terrain matrix file prepared from the fold DEM is a string of text (ascii) that contains space-delimited values. Values of elevation are read from the terrain matrix file by rendering software, such that they fill rows of a matrix. Values are ordered west to east within each row. Rows are ordered south to north. A header file contains specifications of the model including file title, data format (=ascii), coordinates of model boundaries, the number of rows and columns, units of elevation, and minimum and maximum values of elevation. The terrain matrix file and its header are associated by having the same name, but bear different tags, .txt and .hdr .

FOLD2DEM, software written in APL by the author, generates and formats fold DEM's from two digitally described cross sections. The author uses 3DEM, software by Richard Horne, to render DEM's, both of landscapes and of folded surfaces.

The rendered DEM of a folded surface and that of the associated landscape can be combined to demonstrate relationships between them in a single illustration.