One of the natural events which takes place due to environmental conditions of each place, caused by different reasons, is mass movement of components of earth`s surface, and its occurrence leading to many environmental and economic consequences. Compared with other natural disasters, landslides are more controllable, hence, monitoring and detection of their variations and displacements has an important role in development planning. Remote sensing as a modern approach for investigating and monitoring natural disasters in extensive dimensions and short periods is a big progress in controlling natural disasters and decreasing their environmental effects.
Monitoring of unstable masses to evaluating displacement value of active landslides in different time periods is one of the important issues in studying sliding masses. It can be considered as the most important issue in constructing and retrofitting engineering projects. In this study, optical satellite images of ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) sensor have been used as a new instrument to detecting movements of sliding masses rather than radar images. Poshte-Ghaleh Sliding zone in Doroud is caused by activity of Doroud fault, therefore, considering the large extent of sliding zone, low-cost and short duration of this method, its behavioral monitoring using optical satellite images instead of other methods (such as earth`s surveying and radar interferometry) increases importance of this research. Thus, we can use this present technology, so survey the region in a shorter duration.
 
Material and Method
In present study we use the newly developed COSI-Corr (Co-Registration of Optically Sensed Images and Correlation) extension to assessing ground deformation. It is essential to check this newly developed method for its wider use in earth science’s related problems like detection of changes (caused due to different dynamic processes of the earth). The study of geological hazards using satellite data is a very practical and valuable method. Investigation the radar data is common, but in this method which is applied for the first time in Iran, the optical data of satellite sensors are used to detect the landslide. At this point a newly developed technique of sub-pixel correlation of optical images (from push broom sensors) before and after the event can definitely help for the measurement of land surface deformation. For this purpose, the level- 1A satellite images produced by the ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) satellite sensor before and after the occurrence of landslide at Posh-Ghaleh were applied. The imagery level 1A is required for carrying out sub-pixel correlationby COSI-Corr. Radiance of ASTER imagery is automatically done when ASTER imagery is opened with ENVI versions 4.2 and higher. The ASTER.hdf file format is adopted as input for COSI-Corr. ASTER band VNIR 3N (nadir viewing) is used (as recommended) for the estimation of ground deformation, though, any other band (VNIR, SWIR, TIR) can also be used. The ancillary file contains the positions, attitudes, and look directions of the satellite while acquiring the image, along with number of lines and columns, nominal ground resolution, and solar azimuth and elevation. This file was prepared from the given ASTER .hdf file for nadir looking VNIR (3N) band using COSI-Corr software. Therefore, the images were correlated using the COSI-Corr extension applied on ENVI 4/8 version and sub-pixel correlation technique.
Result and Discussion
The sub-pixel correlation of pre- and post-landslide ASTER images provided an image of displacements caused by landslide. Sub-Pixel correlation of two orthorectified pre - and post-landslide optical images affords horizontal ground displacements through an iterative, unbiased process of estimating phase plane in the Fourier domain. At first, the pre-processing of image data including the radiance correction was performed automatically using COSI-Corr extension. Then, co-registration and orthorectification of ASTER images before and after the landslide were performed. The image was prepared in orthorectification with optimization of ground control points to preserve the pixel value in resampling method. In order to investigating the displacement during the two time intervals, the two images were correlated by COSI-Corr, which was selected as an optimized method. Two correlation methods are available for sub-pixel correlation of optical imagery which are frequency and statistical. Frequency method works in Fourier domain while statistical is based on statistical approaches. For comparison of the performance of correlation methods in measuring ground displacement. The correlation was performed using sinc25 resampling, frequency correlation engines, and window size 32 with a step size 4 and threshold90. The output image include displacement in direction east-west, north-south and Signal to Noise Ratio that components, the image of regions as an output having the surface deformation between 2001 and 2008 .Finally, the displacement of landslide was detected as the decorrelation patches (pixel- based) in texture the correlated image (no pixel).
 
Conclusion
Sub-pixel correlation of optical images can give valuable information for terrain change monitoring and estimating ground deformation. The assessment of a newly established sub-pixel correlation technique (COSI-Corr) can give a better understanding of the process characterizing the change. The sub-pixel correlation of optical images technique measures horizontal displacement component in E-W and N-S direction, and its results are more precise at shorter wavelength, whereas the InSAR provide results in measuring the vertical component of the displacement. Both techniques can complement the information to have a complete characterization of landslide. The sub-pixel correlation of pre- and post-landslide ASTER images provided an image of displacements caused by landslide.