A COMPARISON OF THE HOUGH TRANSFORM WITH MANUAL VISUAL INSPECTION OF DLG DRAINAGE PATTERNS AND THEIR CORRESPONDENCE TO STRUCTURAL CONTROLS BY THE UNDERLYING BEDROCK, CLARK COUNTY, ILLINOIS

Orthogonal north-south and east-west joint sets along the Rocky Branch of Big Creek in the Pennsylvanian Mattoon Formation of Clark County, Illinois (northeast of Clarksville; E1/2, Section 30, T12N, R12W) were mapped. These joint sets, as well as regional large-scale structural features (e.g., Charleston Monocline, Edgar Monocline, Marshall-Sidell Syncline, La Salle Anticlinorium, and eastern margin of the Illinois Basin) all exert some control on surficial drainage patterns even though Quaternary overburden ranges from a few meters to nearly 75 meters thick.

The Hough transform was applied to a Digital Line Graph (DLG) or Digital Elevation Model (DEM) from which stream intersections were identified. The Hough transform mapped all possible lines through each intersection point onto a plane described by r (distance, from the origin, of a normal to the line) and q (angle from horizontal of the normal) with the equation r=x*cos (q) + y*sin (q), where x and y are the coordinates of the stream intersection. Alignments are chosen as maxima in the accumulator matrix and mapped back to image space in order to get azimuthal orientations. Several maxima can be chosen above an arbitrary threshold.

A second technique was manual visual inspection of the DLG or DEM and recording a best-fit orientation of stream segments. Rose diagrams of stream orientations were then plotted.

A comparison of both techniques to joint orientations in the underlying Mattoon Formation indicates a relatively large degree of similarity. Similarities increased, as expected, with decreasing overburden thicknesses as well as with increasing dominance of underlying structure (e.g., major anticlinorium).