# Automatically Extracted Antarctic Coastline Using Remotely-Sensed Data: An Update
## 6 Conclusions  
In this study, we have presented an updated Antarctic coastline based on high-resolution multi-source satellite imagery. The coastline of the Antarctica is defined as the boundary between the ocean and the ice sheet, including ice shelves. Hence, fast ice is not considered part of the glacial Antarctic ice sheet. Our procedures involve an automatic coastline extraction based on the Canny edge detection algorithm with modification to use a multi-component digital procedure. The crucial step in the modification is to automate the thresholds calculation. We examined the bimodality of the intensity histogram of each image and automatically calculated the clusters for the required low and high values. The maximum of lower gradient magnitude clusters is considered to be the local low threshold, and the minimum of higher gradient magnitude clusters is considered to be the local high threshold. Based on the modification, the thresholds were generated according to the local characteristics of each image and applied in the edge detection. After conducting the post-processing step, we presented the derived Antarctic coastline products for three different years--namely, 2005, 2010, and 2017. Results showed that our algorithm is not sensitive to different spatial resolution of images, and accurately extracted the coastlines. A previous study indicated that coastline errors resulting from different resolution images are small, although coastline length could be influenced by resolutions according to the fractal geometry theory [42]. Therefore, it was not necessary to resample the pixel spacing of the high-resolution Landsat imagery, and we were able to connect the edge segments extracted from different types of imagery into a single polyline. The spatial resolution of our coastlines was 30-75 m in 2005 and 2010, and 30 m in 2017. Consequently, the skill and the precision of largely automated spatial analysis of the Antarctic coastline have improved. In order to verify the location accuracy of our extraction results, we visually lined up the extracted coastlines and the original images. Visual inspection shows that the automatically extracted coastlines are highly consistent with the visual maps. In order to confirm the accuracy of the extracted coastline, we compared the results in 2005 against the 2004 MOA. To do so, the coastline was divided into five sectors: WS, IO, WP, RS, and BAS. Data were obtained from a sample of 10,000 points in each section. The comparison showed that most of the deviations were less than 5 km. Many points with identical coastline positions for the two data sets were identified. From the MOA in 2009, the length of the Antarctic coastline was 39,608 km. Compared with the length for each sector from our results in 2010, the length in 2009 was always longer except for the coastline in the WS. Additionally, nine areas of extracted coastline in 2010 were selected and compared to manually extracted coastline based on visual interpretation of Landsat images. We chose 1000 points separated by 60 m and computed their distance to the manually extracted coastline. We found that 92.3% of the differences were within 100 m.  
From the extraction results, we found that the length of the Antarctic coastline increased by 167 km from 2005 to 2010, and then increased by 391 km from 2010 to 2017, with an overall increase of 558 km from 2005 to 2017. However, the area of the Antarctic continent decreased by \(8.1\times 10^{4}\) km\({}^{2}\) from 2005 to 2010, then increased by \(1.2\times 10^{5}\) km\({}^{2}\) from 2010 to 2017, yielding a net increase of \(3.9\times 10^{4}\) km\({}^{2}\) from 2005 to 2017. This means that the length change does not necessarily follow the same trend as area change. The changes of both parameters were mainly caused by the expansion or retreat of some ice shelves. Therefore, we also calculated the variation of the area of six ice shelves with significant area change from 2005 to 2017. We found that the area of these ice shelves increased by 8069 km\({}^{2}\) over that time.