Pixel map preparation using the HSV colour model

Abstract

Digital images are an invaluable aid to the study of large spatial data-sets. With the advent of the National Geoscience Mapping Accord (NGMA) between the Bureau of Mineral Resources, Geology and Geophysics (BMR), and the State Geological Surveys, large quantities of high-resolution airborne magnetic and gamma-ray spectrometric data, and gravity data, are being collected. BMR is now routinely releasing high-quality accurately projected pixel-image maps of such data. Images of the aeromagnetic data for NGMA 1:250 000 sheet areas, and the gravity of Australia, are produced in grey-scale shaded-relief and gradient-enhanced pseudocolour formats. The imaging is undertaken on an I2S system, and the final map is printed by a commercial digital graphic-art process. The colour images are constructed by converting histogram-equalised (HE) image grids in the Red, Green, Blue (RGB) colour model to the Hue, Saturation and Value (HSV) colour model. Pixel Hues are chosen from the natural colours ranging from blue to magenta, and are initially fully saturated and at maximum value. The Saturation and Value (or intensity) components are then modulated with the bright and dark portions of image data generated from a sun-angle routine applied to the original real grid data. This provides an artificial illumination to the colour image, giving it a natural effect in which gradients facing the 'sun' are whiter as the Saturation, is decreased, while shadow areas are darker as the Value is decreased. This image retains its brightness. If only the intensity (Value) component was modulated by gradient information, the resulting image for this colour model would always be darker than the original. Final airborne magnetic image grids are warped to a suitable map projection, and for both magnetic and gravity images separate high-resolution overlays are produced containing graticules and planimetric information. These are combined with high-quality typesetting of the map annotation. Map printing uses the subtractive Cyan, Magenta, Yellow and Black (CMYK) colour model.