GLIMS: Global Land Ice Measurements from Space
Monitoring the World's Changing Glaciers
GLIMS objectives are to establish a global inventory of land ice, including surface topography, to measure the changes in extent of glaciers and, where possible, their surface velocities. This project is designed to use primarily data from the ASTER (Advanced Space-borne Thermal Emission and Reflection radiometer) instrument, and the monitoring activities are expected to continue through the life of the ASTER mission. This work will also establish a digital baseline inventory of ice extent for comparison with inventories at later times.
Since the time of the initial selection of the American members of the ASTER team, this plan has been developed to be compatible with the design of ASTER hardware. (See Endnote 1.) We estimate that our goals can be achieved with access to less than one percent of ASTER's downlink capability (although this depends on what the overall system efficiency turns out to be). This project is organized around the use of ASTER data; however, the database is designed to accommodate results from other imaging systems, such as Radarsat or Landsat 7.
The number of glaciers in the world is not well known. Two large digital inventories (World Glacier Monitoring Service (WGMS) and Eurasia at the National Snow and Ice Data Center (NSIDC)) have been combined and total about 80,000 glaciers; these inventories include latitude, longitude, an estimate of glacier area, and for some glaciers a large number of scalar parameters describing the size and condition of the glacier. The objective of the GLIMS project is a GIS database capable of measuring changes in individual glaciers. To our knowledge, this type of GIS does not exist, except for limited regions. For ice masses of 0.1 square kilometers or larger, existing inventories are probably incomplete by a factor of several.
The basic plan is image acquisition on an annual basis, using an average of 3 to 5 imaging attempts per year for all GLIMS targets. Highly automated software to detect and map ice margins and surface feature velocities is being developed by the GLIMS group. The international glaciological community has been asked to collaborate in this development and in mission planning and data analysis. The derived products will be curated by the NSIDC.
GLIMS' targets consist of all permanent land ice except the ``uniform'' interiors of Antarctica and Greenland. A preliminary global set of several hundred observation data acquisition requests (DARs), in the form of polygonal areas, has been submitted to the ASTER mission planning group. (See Endnote 2.) These DARs specify nominal seasons for minimal snow cover, nominal gain settings based upon season and latitude, an include several imaging attempts to deal with the likelihood of cloud cover. This set will be refined as initial coverage is obtained and updated on about a 3 month basis. The shape of geographic polygons describing GLIMS targets will become more efficient, and the number of DARs is expected to increase. GLIMS Regional Centers will refine seasonal timing based upon local knowledge of seasonal snow coverage and cloud behavior.
Nadir viewing is preferred with a minimum of cross-track pointing. The primary data for analysis is band 3N (nadir). The most common data requests will be for the stereo data stream (Bands 3N and 3B (aft-looking)), with SWIR requested about one-third of the time. Stereo coverage will be requested until it is obtained once (repeated stereo is expected to be useful primarily for surge events on large glaciers); thereafter, the swath extension required for Band 3B overlap will not be required. (See Endnote 3.)
In order to base GLIMS processing on the highest spatial resolution from ASTER possible, we intend to use Level 1A data. Inter-telescope registration by the ASTER data processing system is not required as precision registration will be done within GLIMS analysis.
No automated cloud mask algorithm is known for Band 3 data only. We anticipate that assessment of cloud coverage will be done as soon as Level 1A data are available at the EROS Data Center (EDC) by a person there trained in distinguishing surface ice and snow from clouds. When this assessment is uncertain, the regional centers will be asked to make an assessment. If a reliable Band 3 plus SWIR cloud algorithm is developed, it will be used. It is hoped that cloud assessment turnaround will be accomplished on the average in less than 24 hours after data are received at EDC. This information, on a sub-scene basis if desired, could be transferred immediately to the ASTER Mission Planning Group; the most effective method of this transfer has yet to be determined. One possibility is to modify the DAR polygon where cloud-free coverage is obtained.
GLIMS will include a coordination center in the USGS, and a number of Regional Centers (RCs) which adopt mission planning and data analysis activities for significant geographic regions containing land ice; it is anticipated that many RCs will incorporate ``Stewards'' for subsets of glaciers in their region, down to individual glaciers. Stewards will not interact directly with ASTER operations nor input analysis products to NSIDC; these will go through their RC.
As Regional Centers are established, their electronic address will be associated with the relevant DARs so that they are automatically informed of availability of Level 1A data for their area. RCs will access ASTER data directly from EDC (or from the ASTER data center in Japan).
The GLIMS group will develop basic analysis algorithms, procedures, and software that are compatible with common PC systems and at least one workstation platform; the non-proprietary software will be distributed to each RC. GIS and DEM algorithms will be implemented in connection with some existing software packages, probably commercial, for which GLIMS would provide ``cookbook'' guides to supplement their manuals. RCs may use any software they wish for generation of products for NSIDC, as long as the results are delivered in the standard format. RCs will return their analysis products in standard form to NSIDC for inclusion into the GLIMS database. The GLIMS database will include glacier IDs, names, cross-references to images and other databases, scalar glaciological parameters, polygons of ice, snow and image registration regions, ice displacement vectors, transforms between image coordinates, northing and easting offsets from a local reference point, and geodetic coordinates, uncertainties in all of these values, and descriptions of the source instruments.
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This plan was first presented to the international community at the Annual Meeting of the International Glaciological Society in Columbus, Ohio in August, 1994, and has been actively discussed in that community since.