One of the main objectives of the present study is to develop a digital database of glaciers and glacial lakes using GIS. A digital database is necessary for the monitoring of glaciers and glacial lakes and to identify the potentially dangerous lakes. GIS is the most appropriate tool for spatial data input and attribute data handling. It is a computer-based system that provides the following four sets of capabilities to handle geo-referenced data:
  • data input,
  • data management (data storage and retrieval),
  • data manipulation and analysis, and
  • data output

Any spatial features of the earth’s surface are represented in GIS by the following:

  • area/polygons: features which occupy a certain area, e.g. glacier units, lake units, land-use units, geological units etc;
  • lines/segments: linear features, e.g. drainage lines, contour lines, boundaries of glaciers and lakes etc;
  • points: points define the discrete locations of geographic features, the areas of which are too small to illustrate as lines or polygons, e.g. mountain peaks or discrete elevation points, sampling points for field observations, identification points for polygon features, centres of glaciers and lakes etc, and attribute data refer to the properties of spatial entities.

The spatial entities described above can be represented in digital form by two data models: vector or raster models. In a vector model the position of each spatial feature is defined by a series of X and Y coordinates. Besides the location, the meaning of the feature is given by a ‘code’. In a raster model, spatial data are organized in grid cells or pixels, a term derived for a picture element. Pixels are the basic units for which information is explicitly recorded. Each pixel is assigned only one value.

For the present study, ENVI is used for precise geometric-correction, and Arcview3.0/Arcgis 8.0 for Windows is used for the spatial and attribute database development and analysis. ENVI is a special image-processing software, and Arcview3.0/Arcgis 8.0 for Windows is a geographic information system developed by ESRI (Environmental Systems Research Institute, Inc.). Analysis and modeling in a GIS requires input of relevant data. Eight scenes of TM images acquired in 1990 are the baseline to research the glacier change. The topographic maps on a scale 1:50,000 published in 1980s were used as reference to obtaining the spatial data of glaciers and glacial lakes. The ETM+ images acquired in 2000 were used as the base-map for second period. The list of topographic maps and RS images used for the study is given in Chapter 4. All the glaciers and glacial lakes were numbered and their attributes were inputted or computed. The details of the coding for the glacier and glacial lakes are given in detail in Chapter 4.

To compare change between different periods, we must co-register RS images supported by ENVI in order to correctly match each other. The correction precision is less than one pixel. Then, referenced with the corrected topographic maps, we choose the obvious objects with the same name in the RS images to correct it.

The most common method of entering spatial data is manual digitizing by using Arcview. It is always necessary to maintain the details, smoothness and accuracy of the input spatial data of all the glaciers and glacial lakes as in the maps of the given map scale. Before starting digitization one should know the map projection system. A map projection defines the relationship between the map coordinates and the geographic coordinates (latitude and longitude). The topographic maps are in Gauss-Kruger projection, and the Himalaya-China regions are situated in the 44th and 45th projection-zone.

All the polygons representing glaciers and glacial lakes are numbered as mentioned in Chapter 4. Label Points showing the location of glaciers and glacial lakes were set up in the Arcinfo. They were used later for identification of the polygons of the glaciers and glacial lakes. After digitization, the segments were checked and the glaciers and glacial lakes were numbered using point identifiers. But there is exception instance, some glaciers existed in the topographic maps have been divided into independent parts, so we numbered every sub-glacier by adding a numerical suffix to the original code (e.g., 5O191A0007-1, 5O191A0007-2).

In GIS, polygon maps with identifier domains of the objects have a related attribute table with the same domain. The domain defines the possible contents of a map, a table, or a column in a table (attribute). Some examples of ‘domain’ are class domain (a list of class names), value domain (measured, calculated, or interpolated values), image domain (reflectance values in a satellite image or scanned aerial photograph), identifier domain (a unique code for each item in the map), string domain (columns in a table that contain text), bit domain (value 0 and 1), etc. An attribute table is linked to a theme through its ID. An attribute table can only be linked to a theme with a unique identifier domain. An attribute table may contain several columns. Each column corresponds to a feature (such as point, line, polygon) in the theme.

The required attributes of the glaciers and glacial lakes were derived or entered in the attribute database in the GIS. Attributes such as area, location (latitude, longitude), length were derived from the spatial database. If other necessary digital spatial data layers, such as digital elevation models (DEM), are available, it is possible to generate terrain parameters such as elevation, slope, length as measuring units for glaciers and glacial lakes. Other attributes, such as orientation, elevation, map code, name were manually entered in the attribute database. Additional attributes, such as mean elevation, ice reserves were derived using logical calculations. Some of the attributes were also derived from the results of an aggregation in the same table or from another table using the table joining operations, such as glaciers associated with the glacial lakes, glacier length etc. The attribute database for glaciers and glacial lakes is given in the annexes.

The analysis for the change of glacier and the criteria for the identification of potentially dangerous glacial lakes are explained in Chapter 11. Using the logical calculation in the GIS, the activity of glacier and potentially dangerous glacial lakes were determined. To study the geomorphic characteristics of these potentially dangerous lakes, time-series of satellite images were used and the potentially dangerous glacial lakes were finally identified (Table 11.9).