Generation the coastline change map for Urmia Lake by TM and ETM+ imagery Next we compared two mentioned coastlines. With this comparison, the accuracy of the extracted coastline from this approach was evaluated 1.3 pixels (pixel size=30meters). Figures 2, 3 and 4 illustrate mosaicked images of Urmia Lake in 1989, 1998 and 2001 years respectively. These images are color composite RGB 543. Figure 5 illustrates the coastline change map for this lake. Figure 5 illustrate that the area of Urmia Lake in 1998 and 2001 respectively equaled to 5650 and 4610 square kilometers. Therefore area of this lake has been decreased approximately 1040 square kilometers from Aug-1998 to Aug-2001. Comparing the coastline map to Urmia Lake level variations For comparing the coastline map to Urmia Lake level variations, we utilized the obtained information from TOPEX/POSEIDON satellite. This satellite is in orbit at a height of 1330 km. this satellite can send back information that measures the average ocean height very accurately. So this can give information about where the ocean is a little higher or lower than other places. Figure 6 illustrate relative lake height variations computed from TOPEX/POSEIDON satellite data. Because the lack of TOPEX/POSEIDON data from 1989 to 1993, four points have been drawn in these years. These four points represent the height of water that has been measured by dipstick. This figure illustrates that the lake level variation equals 0.2 meters, from Jun-1989 to Aug-1998, which is small in comparison to seasonal level variations of this lake. But the lake level variation equals to 3 meters from Aug-1998 to Aug-2001, approximately.  Fig. 6 Urmia Lake Level variations from 1989 to 2001 It is necessary to say that the tide-range in Urmia Lake is inconsiderable. Therefore the lake level variations and coastline changes do not derive from tide. In fact, these changes derive from water balance of Urmia Lake. Summary and conclusion In this research, Urmia Lake coastlines extracted from ETM+ and TM imagery. These images have been acquired in 1989, 1998 and 2001 years. The coastline map that the coastline has small changes from 1989 to 1998 year. These small changes in Urmia Lake coastline are perpetual. But the coastline has been changed greatly from 1998 to 2001 year. These great changes have happened as the result of 3 meters decrease in height of water of Urmia Lake. It has caused approximately 1000 square kilometers decrease in area of Urmia Lake. Also, in this investigation we applied a new approach for coastline extraction, in which we utilized histogram thresholding and band ratio techniques together. To evaluate the accuracy of this approach, we required to compare the extracted coastline from this method with the extracted coastline from a ground truth map. Because the lack of a reliable ground truth map we utilized a ground truth image. This ground truth image was provided via fusing the ETM+ multispectral bands with ETM+ panchromatic band. Then we extracted coastline from this ground truth image via visual interpretation. Next we compared between two mentioned coastlines. With this comparison the accuracy of the extracted coastline from this approach was evaluated 1.3 pixels (pixel size=30meters). References
Page 3 of 3
| Previous | |