METHODOLOGY AND USE OF CLOSE RANGE PHOTOGRAMMETRY FOR GEOLOGIC STUDIES OF INACCESSIBLE OUTCROPS

Increasing need for quantitative data/information about geologic phenomena has led to development of tools to measure and describe geology in greater detail. Close range photogrammetry is a relatively low-cost method for collecting geologic information from inaccessible outcrops where elevations and locations are tied to real-world coordinates.

Ground control points and camera stations are surveyed at an outcrop or highwall using GPS and reflectorless total station technology. Control and check points are surveyed on the highwalls and annotated in the field. Identifiable marks or recesses in the surface, colored rocks, and prominent bends and intersections of joints are surveyed and noted. Numerous control and check points distributed throughout the area of interest is necessary. Camera stations are spaced 1/7 the distance between outcrop and tripod. Best photography is recorded under overcast conditions where there are no strong shadows.

A stereomodel is created by Sirovision software from a pair of overlapping photos using a single control point and multiple relative reference points. The program calculates the tilt of the photos (3D image may have errors if the tilt is >1 degree) and allows you to set up the extent of the 3D image (the shape should be close to a rectangle and should not be less than 1cm from the borders). Good 3D images have a matching correlation that is above 0.91.

Exported XYZ data from stereomodel can be recreated as a point cloud from which ArcScene can derive a TIN. Multiple 3D images can form a mosaic of a wall. A stereomodel can be imported into Sirojoint software to trace and export joint and bedding information such as dip and dip direction which can also be imported into ArcScene.

Although excellent stereomodels can be created, the results are also inconsistent. Some models can be corrected after reentry of control data. Nearly vertical walls, and offsets such as benches and trenches are troublesome. Most information can only be derived from within the Sirovision software. GIS-based software has difficulty or may not accurately display the 3-D data or derived information.

High resolution imagery of a dolomite quarry in Chicago showed that individual beds were not laterally continuous as perceived. Joints are not frequent, well developed, nor vertically continuous as expected.