Structural Data Extraction from Stereographic or Equal Area Projections using Image Processing

Abstract

Structural orientation data from the field are commonly represented on stereographic or equal area projections, and the raw data are usually not accessible to other workers for integration into larger datasets or comparative study. We have developed a technique for retrieving raw planar structural data from the above projection types using image analysis techniques. The principle involves two steps: (i) conversion of the foliation pole plots in the image into Cartesian coordinates, and (ii) inversion of the data soobtained into a geographic reference frame. The process involves either scanning of the target projection, or cropping of the projection figure from a pdf file using image processing software, followed by tracing of the data points and generation of an image file that can be subsequently input into a MATLAB supported program, "StrucExtract". The results from the program are displayed in the form of dips and dip directions. The program has been tested with synthetic and natural examples, and may be invaluable for retrieval of structural data from older structural literature.