Selection of Less Biased Threshold Angles for SAM Classification Using the Real Value–Area Fractal Technique

Abstract

The accuracy of classification of the Spectral Angle Mapping (SAM) is warranted by choosing the appropriate threshold angles, which are normally defined by the user. Trial-and-error and statistical methods are commonly applied to determine threshold angles. In this paper, we discuss a real value-area (RV-A) technique based on the established concentration-area (C-A) fractal model to determine less biased threshold angles for SAM classification of multispectral images. Short wave infrared (SWIR) bands of the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) images were used over and around the Sar Cheshmeh porphyry Cu deposit and Seridune porphyry Cu prospect. Reference spectra from the known hydrothermal alteration zones in each study area were chosen for producing respective rule images. Segmentation of each rule image resulted in a RV-A curve. Hydrothermal alteration mapping based on threshold values of each RV-A curve showed that the first break in each curve is practical for selection of optimum threshold angles. The hydrothermal alteration maps of the study areas were evaluated by field and laboratory studies including X-ray diffraction analysis, spectral analysis, and thin section study of rock samples. The accuracy of the SAM classification was evaluated by using an error matrix. Overall accuracies of 80.62% and 75.45% were acquired in the Sar Cheshmeh and Seridune areas, respectively. We also used different threshold angles obtained by some statistical techniques to evaluate the efficiency of the proposed RV-A technique. Threshold angles provided by statistical techniques could not enhance the hydrothermal alteration zones around the known deposits, as good as threshold angles obtained by the RV-A technique. Since no arbitrary parameter is defined by the user in the application of the RV-A technique, its application prevents introduction of human bias to the selection of optimum threshold angle for SAM classification.

Comparison of multi-temporal ASTER images for hydrothermal alteration mapping using a fractal-aided SAM method

“…The model has also been successfully applied to the classification of remote-sensing images (Cheng and Li 2002;Shahriari, Ranjbar, and Honarmand 2013;Shahriari et al 2014). Based on this model, background and anomalous values belong to different populations that have various power-law distributions.…”

“…We applied the SAM method (Kruse et al 1993) for image classification. Less biased threshold angles for SAM classification were determined by the real value-area (RV-A) fractal technique that is based on the concentration-area (C-A) fractal model (Cheng, Agterberg, and Ballantyne 1994;Shahriari, Ranjbar, and Honarmand 2013;Asl et al 2014;Shahriari et al 2014) in order to ascertain the reliability of the SAM classification. Finally, the effect of the acquisition properties (tilt angle and solar elevation) and the pre-processing level of ASTER products on the resulting hydrothermal alteration maps are discussed.…”

A Comparative Study of Landsat‐7 and Landsat‐8 Data Using Image Processing Methods for Hydrothermal Alteration Mapping

“…The presence of Cu–Mo–Au mineralization in the Central Iranian Cenozoic magmatic belt (CICMB; also known as the Urumieh‐Dokhtar volcanic belt, or UDVB) has made this Iran's most important Cenozoic volcanic belt. It is approximately 1800 km long and extends from the Azerbaijan province in the northwest to north of Makran in the southeast (Shahriari et al, ). The Arabian plate subduction beneath central Iran occurred during the Alpine orogeny, and led to the formation of these well‐known belts in Iran (Berberian & King, ; Hezarkhani, ; Shahriari et al, ).…”

“…It is approximately 1800 km long and extends from the Azerbaijan province in the northwest to north of Makran in the southeast (Shahriari et al, ). The Arabian plate subduction beneath central Iran occurred during the Alpine orogeny, and led to the formation of these well‐known belts in Iran (Berberian & King, ; Hezarkhani, ; Shahriari et al, ). In Iran, most porphyry copper deposits are located in the CICMB.…”

Application of Airborne Geophysical and ASTER Data for Hydrothermal Alteration Mapping in the Sar-Kuh Porphyry Copper Area, Kerman Province, Iran

“…Trial-and-error and statistical methods are commonly applied to determine threshold angles. The fractal-aided SAM method can determine less biased threshold angles for SAM classification of multispectral images [7], [8]. In this research, threshold angle for each reference spectra was determined based on RV-A fractal technique [7], [8].…”