Large Differences between Glaciers 3D Surface Extents and 2D Planar Areas in Central Tianshan

Abstract

Most glaciers in China lie in high mountainous environments and have relatively large surface slopes. Common analyses consider glaciers' projected areas (2D Area) in a two-dimensional plane, which are much smaller than glacier's topographic surface extents (3D Area). The areal difference between 2D planar areas and 3D surface extents exceeds −5% when the glacier's surface slope is larger than 18 • . In this study, we establish a 3D model in the Muzart Glacier catchment using ASTER GDEM data. This model is used to quantify the areal difference between glaciers' 2D planar areas and their 3D surface extents in various slope zones and elevation bands by using the second Chinese Glacier Inventory (CGI2). Finally, we analyze the 2D and 3D area shrinking rate between 2007 and 2013 in Central Tianshan using glaciers derived from Landsat images by an object-based classification approach. This approach shows an accuracy of 89% when it validates by comparison of glaciers derived from Landsat and high spatial resolution GeoEye images. The extracted glaciers in 2007 also have an agreement of 89% with CGI2 data in the Muzart Glacier catchment. The glaciers' 3D area is 34.2% larger than their 2D area from CGI2 in the Muzart Glacier catchment and by 27.9% in the entire Central Tianshan. Most underestimation occurs in the elevation bands of 4000-5000 m above sea level (a.s.l.). The 3D glacier areas reduced by 30 and 115 km 2 between 2007 and 2013 in the Muzart Glacier catchment and Central Tianshan, being 37.0% and 27.6% larger than their 2D areas reduction, respectively. The shrinking rates decrease with elevation increase.

Topography-Related Glacier Area Changes in Central Tianshan from 1989 to 2015 Derived from Landsat Images and ASTER GDEM Data

“…The glacier outlines in this study were corrected by comparing the images acquired in different years, seasons and time, on a 3D surface view for shade verification and on GoogleEearth. The clean ice accuracy (90.8%) against the high resolution GeoEye01 image in 2013 is similar to that reported in the literature, e.g., 89.3% in the Muzart Glacier Catchment [20], 93% for clean ice in the Mount Manaslu in Nepal using object-based classification from Landsat images [35], 95% for clean ice by the band ratio method in European Alps and Chugach Mountains, Alaska [49]. The relatively larger difference (~10%) in this study is mainly attributed to the coarser Landsat image (30 m) against the higher spatial resolution of the GeoEye01 image (4 m), which can detect more small glaciers, glacier edges and terminus than the former.…”

“…Although there are large areas of debris-covered glaciers (729 km 2 in 2007) in the Central Tianshan (Southern Inylchek, Tomur, and Koxkar glacier) and their surface thinning is considerable, their areas were relatively stable and only shrank slightly by 1.7 ± 4.8% from 1975 to 2008 [27,31]. Therefore, this study only focuses on clean ice mapping and change analysis from Landsat images using the object-based classification approach, which makes full use of the image reflectance, texture and topographic information and is efficient in manual correction after the initial classification [20].…”

“…The classified images are manually corrected by visual comparison with images acquired in different years. Finally, the classified objects are merged and exported to vector polygons for further visually checking and manual edition in ArcMap, eliminating misclassified pro-glacial water and shadow areas by overlaying with DEM data and Google Earth images [20]. The unidentified and cloud-blocked areas (~3% of total glacier areas) in any time period are excluded for the area change analysis for the four periods.…”

A Review on Applications of Remote Sensing and Geographic Information Systems (GIS) in Water Resources and Flood Risk Management

“…Water resources mapping and management Glaciers mapping Landsat, ASTER GDEM, GIS, TIN 3D model. [36] Soil moisture detection GPR, CMP, FO, GIS spatial analysis [16] Groundwater and subsidence analysis GIS spatial analysis, GPS [37] Irrigation planning UAV, HTM for video image classification, GIS visualization [32] Rainfall measurements and design storm…”

“…In the high Tianshan Mountains, most glaciers lie on steep slopes, and their actual surface extent (3D area) may be much larger than the 2D area. Wang et al [36] in this special issue establish a 3D model to quantify glaciers' 3D and 2D area differences in the Muzart Glacier catchment and in Central Tianshan using ASTER GDEM data, CGI2 and Landsat images. They found that glaciers' 3D areas was 34.2% larger than their 2D areas in the Muzart Glacier catchment and by 27.9% in the entire Central Tianshan, where glaciers' 3D areas reduced by 115 km 2 between 2007 and 2013, being 27.6% larger than their 2D area reduction.…”