STcorr: An IDL code for image based normalization of lapse rate and illumination effects on nighttime TIR imagery
Highlights
► Nighttime TIR imagery may require retrieval of altitude, aspect and slope effects. ► Stcorr is an array-oriented, rapid, image based code to normalize these effects. ► Code is tested with nighttime TIR data of Nemrut volcano. ► Effect of lapse rate and illumination on nighttime TIR imagery was demonstrated.
Introduction
Thermal infrared (TIR) imagery has become a widely used tool in earth sciences during the last decade and is useful for; the detection of geothermal fields/spots (e.g. Mongillo, 1994, Hellman and Ramsey, 2004, Lombardo and Buongiorno, 2006), environmental studies (e.g. Wentz and Schabel, 2000), soil moisture assessment (e.g. Price, 1980, Wetzel et al., 1984), mineral mapping (e.g. Vaughan et al., 2005, Rowan et al., 2006) and volcano monitoring (e.g. Pieri and Abrams, 2005, Pugnaghi et al., 2006). The temperature of an object on the surface of the earth depends on the summation of radiative, conductive and latent heat transfer between the object and its surroundings over the diurnal period (Warner and Chen, 2001). On daytime images, topography is typically the dominant expression because of the differential solar heating and large shadow area (Sabins, 1978, Sahu, 2008). It was stated that the most significant topographic factors in determining temperatures are the elevation, slope and aspect (Florinsky et al., 1994). The main causes for these topographic factors are (1) the lapse rate and (2) the illumination (aspect and the slope) effects.
The lapse rate is the negative change of actual temperature with altitude (−dT/dz), it changes from region to region and season to season (Singh, 1991, Jacobson, 2005, Jain et al., 2008). With all other factors equal, both air and surface temperatures generally decrease as the elevation increases and this effect is more readily discernable at night (Eneva and Coolbaugh, 2009). The lapse rate generally observed in nature that assumed to be suitable in most cases is −6.5 °C/km (Warner and Chen, 2001, Eneva and Coolbaugh, 2009). The lapse rate has a substantial effect on the TIR imagery especially where there is considerable difference in altitude in the scene. It is relatively easy to detect high thermal anomalies around the summit zone of an active volcano, where the decrease in temperature with respect to the overall height causes an evident contrast with the thermal anomalies. However around the basal zones of the volcano, lack of this contrast may mask possible anomalies.
The absorbed solar radiation is a function of the surface albedo and the illumination effects (Watson, 1975). The illumination depends on the relative orientation of the pixel towards the sun's actual position (Minnaert, 1941, Smith et al., 1980, Teillet et al., 1982, Mayer et al., 1993), which can be modeled with the aspect and the slope of the pixel. As a result of the aspect effect, the sun-facing slopes appear brighter than the opposite slopes in daytime images. The slope effect basically refers to the fact that the amount of the exposure to the sun differs according to the slope. For an arbitrarily oriented surface, angle of incidence is the angle between normal to the surface and the sun–earth vector, in degrees (Iqbal, 1984). The intensity of solar radiation is largely affected by the angle of incidence and by definition; incidence is a single dimension controlled by both aspect and slope. Incidence angle is an effective value for illumination correction of daytime images where it is already defined by the time-of-acquisition (e.g. in cosine correction). In nighttime images however, there is no time-of-acquisition input for solar incidence angle. Therefore, there is not a solar angle of incidence value for the illumination correction in nighttime images.
In many remote-sensing applications, topographic effects are not desirable and need to be corrected before image interpretation (Gu et al., 1999). Thermal effects due to the differential solar heating and shadowing are greatly reduced on nighttime images (Sabins, 1978). Nevertheless, depending on the acquisition time, these effects can also be visible in the nighttime thermal images. Different surface materials with specific physical properties (i.e. thermal inertia, albedo, emissivity, and moisture content) respond differently to solar radiation, resulting in various surface temperatures along 24-h of a day (Elachi, 1987, Watson, 1973, Coolbaugh et al., 2007). Even during predawn hours, significant differences in temperature persist due to the differential heating effects of the sun of the previous day (Coolbaugh et al., 2007). Where topographic slopes are relatively steep and variable, such as mountainous terrain, it can be difficult and tedious to distinguish thermal anomalies from strong false anomalies caused by warmer sun-facing slopes in uncorrected nighttime images (Coolbaugh et al., 2007).
Section snippets
Methodology of STcorr: topographic correction of satellite imagery
Quantitative theoretical modeling of the physical variables to predict surface temperatures involves differential equations and LaPlacian transformations which utilize iterative numerical solutions (Elachi, 1987, Kahle, 1977, Watson, 1973, Coolbaugh et al., 2007). The main methods for correcting the topographic effects that depend on the bidirectional reflectance distribution are: cosine method (Smith et al., 1980, Teillet et al., 1982), Minnaert correction (Minnaert, 1941, Smith et al., 1980,
Code design
IDL, the Interactive Data Language, is an array-oriented language for data analysis, visualization, and cross-platform application development; it has powerful graphical display techniques. In array-oriented languages, any operation on an array (a vectoral set, a matrix or higher rank matrices), without using a loop, is performed on all elements of the array; therefore the resulting code is simple and fast (e.g. Iverson, 1980). STcorr is an IDL (version 6.3 for Microsoft Windows© OS) code that
Example: correction of nighttime TIR image of Mt. Nemrut volcano
Correction of nighttime TIR image using STcorr algorithm has been applied to Mt. Nemrut, an active dormant volcano situated in the eastern Turkey (Ulusoy et al., 2008, Çubukçu et al., 2012). It has a summit caldera with dimensions of 8.5×7 km (Fig. 3). The western half of the caldera is filled by a freshwater lake (Nemrut Lake) and a small lake with hot springs locates at the northern part (Fig. 3). Bathymetrical surveys exhibit that the maximum depth of the larger lake reaches to 176 m, while
Results
Scatter plots derived from the different steps of the method applied to Nemrut volcano are shown in Fig. 4. Linear relationship between altitude and surface temperature is evident (Fig. 4a); it shows that the decrease of temperature with elevation is the dominant trend. For the altitude correction, regression equation generated by Stcorr can be defined as follows:where “a” defines the lapse rate coefficient and the y-intercept “b” defines the theoretic sea level temperature. In
Discussion
We have used basic statistical analyses to show the quality of the produced images when compared to the initial image. Calculations can be grouped according to (1) geological features in the selected areas (Table 1: analysis – 1), (2) areas defined by pixel values greater than mean temperature (Table 1: analysis – 2), and (3) directional response. For geological approach, two known hydrothermal areas (Mazik dome and intra-caldera land area) and a hydrogeological area, where pure hydrogeological
Conclusion
For the cases where topographic variations affect the geothermal anomalies and for volcano monitoring, retrieval of altitude and aspect effects may be essential in nighttime TIR imagery applications.
Most of the illumination correction methods, used in correction of the daytime TIR images, utilize hill shade images, produced with a fixed sun azimuth angle and direction, instead of aspect correction. Upon the appropriate pre-processing aimed to reduce daytime effects which were not described in
Acknowledgments
This work benefited from a research grant of Hacettepe University Scientific Research Foundation (Project no: 01 01 602 021). It was also financially supported by Blaise Pascal (Clermont-Ferrand II) University, CROUS, UMR-CNRS 6524 and benefited 3 integrated PhD grants from French Ministry of Foreign Affairs. French embassy in Turkey and Hacettepe University were supported several scientific expert missions. The authors would like to acknowledge the thorough and stimulating reviews and comments
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