Donghyun Jin

New Approach for Snow Cover Detection through Spectral Pattern Recognition with MODIS Data

Snow cover plays an important role in climate and hydrology, at both global and regional scales. Most previous studies have used static threshold techniques to detect snow cover, which can lead to errors such as misclassification of snow and clouds, because the reflectance of snow cover exhibits variability and is affected by several factors. Therefore, we present a simple new algorithm for mapping snow cover from Moderate Resolution Imaging Spectroradiometer (MODIS) data using dynamic wavelength warping (DWW), which is based on dynamic time warping (DTW). DTW is a pattern recognition technique that is widely used in various fields such as human action recognition, anomaly detection, and clustering. Before performing DWW, we constructed 49 snow reflectance spectral libraries as reference data for various solar zenith angle and digital elevation model conditions using approximately 1.6 million sampled data. To verify the algorithm, we compared our results with the MODIS swath snow cover product (MOD10_L2). Producer’s accuracy, user’s accuracy, and overall accuracy values were 92.92%, 78.41%, and 92.24%, respectively, indicating good overall classification accuracy. The proposed algorithm is more useful for discriminating between snow cover and clouds than threshold techniques in some areas, such as those with a high viewing zenith angle.

Retrieval of background surface reflectance with BRD components from pre-running BRDF

Many countries try to launch satellite to observe the Earth surface. As important of surface remote sensing is increased, the reflectance of surface is a core parameter of the ground climate. But observing the reflectance of surface by satellite have weakness such as temporal resolution and being affected by view or solar angles. The bidirectional effects of the surface reflectance may make many noises to the time series. These noises can lead to make errors when determining surface reflectance. To correct bidirectional error of surface reflectance, using correction model for normalized the sensor data is necessary. A Bidirectional Reflectance Distribution Function (BRDF) is making accuracy higher method to correct scattering (Isotropic scattering, Geometric scattering, Volumetric scattering). To correct bidirectional error of surface reflectance, BRDF was used in this study. To correct bidirectional error of surface reflectance, we apply Bidirectional Reflectance Distribution Function (BRDF) to retrieve surface reflectance. And we apply 2 steps for retrieving Background Surface Reflectance (BSR). The first step is retrieving Bidirectional Reflectance Distribution (BRD) coefficients. Before retrieving BSR, we did pre-running BRDF to retrieve BRD coefficients to correct scatterings (Isotropic scattering, Geometric scattering, Volumetric scattering). In pre-running BRDF, we apply BRDF with observed surface reflectance of SPOT/VEGETATION (VGT-S1) and angular data to get BRD coefficients for calculating scattering. After that, we apply BRDF again in the opposite direction with BRD coefficients and angular data to retrieve BSR as a second step. As a result, BSR has very similar reflectance to one of VGT-S1. And reflectance in BSR is shown adequate. The highest reflectance of BSR is not over 0.4μm in blue channel, 0.45μm in red channel, 0.55μm in NIR channel. And for validation we compare reflectance of clear sky pixel from SPOT/VGT status map data. As a result of comparing BSR with VGT-S1, bias is from 0.0116 to 0.0158 and RMSE is from 0.0459 to 0.0545. They are very reasonable results, so we confirm that BSR is similar to VGT-S1. And weakness of this study is missing pixel in BSR which are observed less time to retrieve BRD components. If missing pixels are filled, BSR is better to retrieve surface products with more accuracy. And we think that after filling the missing pixel and being more accurate, it can be useful data to retrieve surface product which made by surface reflectance like cloud masking and retrieving aerosol.

Long-term variability of Total PrecipitableWater using a MODIS over Korea

Abstract : Water vapor leading various scale of atmospheric circulation and accounting for about60% of the naturally occurring warming effect is important climate variables. Using the Total PrecipitableWater (TPW) from Moderate Resolution Imaging Spectroradiometer (MODIS) operating on both Terraand Aqua, we study long-term Variation of TPW and define relationship among TPW and climaticparameters such as temperature and precipitation to quantitatively demonstrate the impact on climatechange over East Asia focusing on the Korea peninsula. In this study, we used linear regression analysisto detect the correlation of TPW and temperature/precipitation and harmonic analysis to analyzechangeable aspects of periodic characteristics. A result of analysis using linear regression analysis betweenTPW and climate elements, TPW shows a high determination coefficient (R 2 ) with temperature andprecipitation (determination coefficient between TPW and temperature: 0.94, determination coefficientbetween TPW anomaly and temperature anomaly: 0.8, determination coefficient between TPW andprecipitation: 0.73, determination coefficient between TPW anomaly and precipitation anomaly: 0.69). Aresult of harmonic analysis of TPW and precipitation of two-year to five-year cycle, amplitude contributionratio of 3.5-year cycle are much higher and two phases are similar in 3.5-year cycle.Key Words : MODIS, TPW, Anomaly, Harmonic analysis요약 :

Experimental Verification of Characteristics of Magnetic Abrasive Polishing Combined with Ultrasonic Vibration

In this paper, we propose an ultrasonic magnetic abrasive polishing (US-MAP) technique to effectively machine a high-strength material, and we prove the efficiency of hybrid finishing. We use Taguchi`s experimental method to determine the influence of each parameter. Based on the results, US-MAP exhibited a higher polishing efficiency than traditional MAP, and a suitable frequency for hybrid finishing was 28 kHz. When investigating the effect of the parameters on the surface roughness, the ultrasonic amplitude had the greatest effect. However, when machining with 55-μm amplitude, the machining efficiency decreased as the magnetic flux density varied.

Study on Abrasive Adhesion and Polishing Effect in Wet Magnetic Abrasive Polishing

In a conventional magnetic abrasive polishing process, the polishing abrasives are mixed with ferrous particles and slight cutting oil to form a cluster of abrasives. However, when a tool rotates at a high revolution speed, most of the polishing abrasives are scattered away from it due to the increase in centrifugal force. This phenomenon directly reduces the polishing efficiency. The use of a highly viscous matter such as silicone gel instead of cutting oil for mixing is one method to solve this problem and increase abrasive adhesion. Another method to avoid high abrasive scattering is the application of wet magnetic abrasive polishing (WMAP). In WMAP, abundant mineral oil is preliminarily applied to the workpiece surface. This study experimentally evaluated the effect of WMAP on abrasive adhesion. The relationship between the amount of working abrasives and polishing conditions was characterized. Despite the lower adhesion ratio of polishing abrasives, the surface roughness was found to be significantly improved as the result of WMAP. § 이 논문은 2014 년도 대한기계학회 생산 및 설계공학부문 춘계학술대회(2014. 4. 24.-25., 라마다프라자호텔) 발표논문임 † Corresponding Author, jskwak5@pknu.ac.kr C 2014 The Korean Society of Mechanical Engineers