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GEOSAT Radar Altimeter DEM Atlas of Antarctica North of 72.1 Degrees South

Summary

The atlas consists of 28 digital elevation maps which cover all of Antarctica north of 72.1 degrees south at a resolution of three kilometers. Each map contains surface elevations and coordinates for one atlas page covering 16 degrees of longitude. Data were acquired by the Geodetic Satellite (GEOSAT) Geodetic Mission (GM) from March 1985 through September 1986 and are available in both Universal Transverse Mercator (UTM) coordinates, and in latitude and longitude coordinates.

Data were mapped using the UTM projection in atlas form to decrease the distortion that usually occurs at the poles. Many features of the Antarctic Ice Sheet are shown in more detail than in previous digital elevation models, especially along the margin of the East Antarctic Ice Sheet. A geostatistical mapping technique (Herzfeld et al. 1993) improved the accuracy of surface elevations compared to previous GEOSAT elevation data sets. This atlas will facilitate the monitoring of changes in surface elevation that could indicate mass changes in the Antarctic Ice Sheet.

Citation

The following example shows how to cite the use of this data set in a publication. For more information, see our Use and Copyright Web page.

Herzfeld, U.C., and M.S. Matassa. 1999. GEOSAT radar altimeter DEM atlas of Antarctica north of 72.1 degrees south. Boulder, CO, USA: National Snow and Ice Data Center. Digital media.

Table of Contents

• 1 Data Set Overview
• 2 Applications
• 3 Theory of Measurements
• 4 Acquisition Materials and Methods
• 5 Preparation and Description
• 6 Notes and Plans
• 7 Products and Access
• 8 References
• 9 Glossary and Acronyms
• 10 Document Information

1. Data Set Overview

Discussion

Although the importance of Antarctica in the global system has long been recognized and discussed in the literature, data as basic as topographic maps of a resolution amenable to geophysical analysis are still lacking for large parts of Antarctica. This investigation adds to the knowledge of Antarctica by using the improved evaluation of satellite altimetry from the GEOSAT Geodetic Mission to build a more accurate atlas of Antarctica north of 72.1 degrees south.

The following samples are visual representations of several map pages. Click on the thumbnail version to view the full-sized map page.

>63 to 68 degrees south 97 to 113 degrees east	>67 to 72 degrees south 73 to 89 degrees east	>67 to 72 degrees south 97 to 113 degrees east	>67 to 72 degrees south 109 to 125 degrees east

Refer to the Data Format section for more detailed information on how files are stored.

Related Data Sets

SEASAT and GEOSAT Altimetry Data for the Antarctic and Greenland Ice Sheets

Investigator

Ute Christina Herzfeld

Geomathematik, Fachereich Geographie/Geowissenschaften, fachbereich 6 Universitaet Trier, D-54286 Trier, Germany OR Institute of Arctic and Alpine Research (INSTAAR) University of Colorado, Boulder, CO 80309-0450, USA

Contact Information

NSIDC User Services
National Snow and Ice Data Center
CIRES, 449 UCB
University of Colorado
Boulder, CO 80309-0449  USA
phone: +1 303.492.6199
fax: +1 303.492.2468
form: Contact NSIDC User Services
e-mail: nsidc@nsidc.org

2. Applications

The quality and detail of the maps in this data set makes them particularly useful to scientists for:

• documenting more ice margin details.
• showing ice drainage areas and troughs.
• identifying ice shelves.
• studying ice dynamics and changes in Antarctic ice-stream and ice-shelf systems.
• correcting satellite images for effects of slope and elevation.

The kriging method used in producing this atlas would also be applicable to future altimetry data and therefore will facilitate monitoring the changes in the ice streams and ice-surface elevation that are critical for a determination of mass balance and stability of the Antarctic Ice Sheet.

3. Theory of Measurements

Please see the section on Data Manipulations for more information on how data were processed.

4. Acquisition Materials and Methods

The data were collected during the GEOSAT Geodetic Mission by a radar altimeter.

5. Preparation and Description

Data Description

Data Source

The data originate from the GEOSAT Geodetic Mission. Refer to the radar altimeter document for more information about the instrument.

Data Format

The data for the UTM coordinates are stored a file called dtm.tar, which contains 28 files compressed by the UNIX gzip compression utility. Each file is named for the map area it covers. For example, the file named "m69e61_77n67_721.g.dtm.gz" contains data for the map area with central meridian of 69 degrees bounded east-west between 61.0 and 77.0 degrees and bounded north-south between -67.0 and -72.1 degrees.

The files are in ASCII format and contain the following 3 tab-delimited columns:

1		E-W UTM coordinate
2		N-S UTM coordinate
3		elevation value for grid node defined by columns 1 and 2

The data for latitude and longitude coordinates are stored in a file called dtm-11.tar. The files are in ASCII format and contain the following 3 tab-delimited columns:

1		Latitude
2		Longitude
3		elevation value for grid node defined by Columns 1 and 2

Grid Description

For the atlas, the Antarctic is divided into rows of map sheets (72.1 degrees to 67 degrees south; 68 degrees to 63 degrees south). Each of the 28 maps covers 16 degrees of longitude and overlaps two degrees with the maps on either side, so each map center is offset from the next one by 12 degrees of longitude. The map names (e.g., m69e61-77n721-67) give central meridian (69 degrees) and extent of the nominal map area (61 degrees to 77 degrees east, 72.1 degrees to 67 degrees). All the maps use the same projection with respect to the central meridian, and maps in the same row are the same size. For Antarctica, east coordinates always have six digits, while north coordinates have seven.

The files for the UTM coordinates are stored in the the dtm.tar file which expands to the dtm directory. The files for the latitude and longitude coordinates are stored in the dtm-ll.tar file which expands to the dtm-ll directory.

Data File Size (in compressed tar file format):

File with UTM coordinates (dtm.tar.gz)	8.0   MB
File with latitude and longitude coordinates (dtm-ll.tar.gz)	11.68 MB

Spatial Coverage

This data set covers all of Antarctica from 60 degrees south to 72.1 degrees south at a resolution of three kilometers.

Projection

The data were mapped using the Universal Transverse Mercator (UTM) Projection (Snyder, 1987). Data are stored as UTM coordinates and as latitude and longitude coordinates. The southern hemisphere UTM coordinates are negative numbers.

Temporal Characteristics

Temporal Coverage

The data points for this data set were collected by the radar altimeter on the GEOSAT satellite from March, 1985 to September, 1986.

Data Characteristics

Data Range

Unit of Measurement

UTM coordinates - meters [m]

Sample Data Records:

UTM version:

1		E-W UTM coordinate
2		N-S UTM coordinate
3		elevation value for grid node defined by columns 1 and 2

     233000.   -7.05900E+06   -1.30000   
     239000.   -7.05900E+06   -1.83622   
     242000.   -7.05900E+06   -1.97845   
     245000.   -7.05900E+06   -2.11752   
     248000.   -7.05900E+06   -2.11752   
     251000.   -7.05900E+06   -2.25113   
     254000.   -7.05900E+06   -2.33926   
     257000.   -7.05900E+06   -2.34469   
     260000.   -7.05900E+06   -2.64000   
     200000.   -7.06200E+06   -1.16000   
	 

Latitude/Longitude version:

1		Latitude
2		Longitude
3		elevation value for grid node defined by columns 1 and 2

  -63.559808       99.621068   -1.30000  
  -63.564277       99.741325   -1.83622  
  -63.566474       99.801469   -1.97845   
  -63.568646       99.861624   -2.11752   
  -63.570793       99.921789   -2.11752   
  -63.572914       99.981964   -2.25113   
  -63.575010      100.042149   -2.33926   
  -63.577081      100.102343   -2.34469   
  -63.579127      100.162548   -2.64000  
  -63.560204       98.954757   -1.16000   

Data Granularity

A general description of data granularity as it applies to the IMS appears in the EOSDIS Glossary. The data granularity for this data set is one map for every 16 degrees of longitude.

Data Manipulations

Derivation Techniques and Algorithms

Satellite radar altimeters measure the time it takes an electromagnetic signal to travel from the altimeter antenna to the ice sheet surface and back to the altimeter's receiver. This "range measurement" allows investigators to determine the satellite's height above the ice sheet. NSIDC's Radar Altimeter document describes the instrument and how it works.

Corrections are applied to the range measurements to account for the fact that returns are different over ice than over the ocean. The correction for interpreting the data over ice is called "retracking," and is described in papers by Davis and Zwally (1993), and Zwally, et al. (1983).

Over sloping terrain, the radar altimeter measurement needs to be corrected because the return comes from a point not directly below the satellite, but to the uphill side. The elevation indicated by the return time in this case is higher than that directly below the satellite. The data are slope-corrected to reduce the errors, using the slope correction algorithm from Brenner et al. (1983).

For this data set, elevations were derived from the slope-corrected data and grid values were calculated using a geostatistical technique (Herzfeld et al. 1993) and individual atlas pages were created using UTM coordinates. The Universal Transverse Mercator (UTM) Projection (Snyder 1987) compensates for the distortion of areas far away from the central meridian.

After the UTM coordinates were calculated, the data were gridded using ordinary kriging with search algorithms developed for geographical track-line data (Herzfeld et al. 1993 and Journel and Huijbregts 1978). The particular kriging algorithm used for this data are described in Herzfeld (1990).

Map sheets were overlapped by two degrees to ensure that each point of Antarctica is contained in at least one map.

The UTM coordinates were converted to latitude and longitude coordinates after the data was gridded.

Errors:

The maps are not accurate in mountainous terrain but are sufficiently accurate to determine surroundings and general shape of small glaciers.

6. Notes and Plans:

Limitations of the Data:

Orbital coverage of the GEOSAT Geodetic Mission did not extend to the poles.

Known Problems with the Data:

Some offshore contours are caused by ice shelves and ice islands. Contour lines that resemble ground-track patterns may be artifacts, or may occur when there is over-representation of isolated higher elevations in regions of generally low relief, such as islands or larger ice bergs in the sea.

7. Products and Access

Data Center Identification

National Snow and Ice Data Center (NSIDC) Distributed Active Archive Center (DAAC)

Contact Information

NSIDC User Services
National Snow and Ice Data Center
CIRES, 449 UCB
University of Colorado
Boulder, CO 80309-0449  USA
phone: +1 303.492.6199
fax: +1 303.492.2468
form: Contact NSIDC User Services
e-mail: nsidc@nsidc.org

Procedures for Obtaining Data

These data are available via ftp by contacting NSIDC User Services.

8. References

Brenner, A.C., R.A. Bindschadler, R.H. Thomas, H.J. Zwally. 1983. Slope-induced errors in radar altimetry over continental ice sheets. Journal of Geophysical Research 88:1617-1623.

Davis C.H., H.J. Zwally. 1993. Geographic and seasonal variations in the surface properties of the ice sheets by satellite radar altimetry. Journal of Glaciology 39:687-697.

Herzfeld, U.C. 1990. Quantitative spatial models of Atlantic primary productivity: an application of geomathematics. Journal of Geophysical Research 97:717-732.

Herzfeld, U.C., C.S. Lingle, L.-h. Lee. 1993. Geostatistical evaluation of satellite radar altimetry for high-resolution mapping of Lambert Glacier, Antarctica. Annals of Glaciology 17:77-85.

Herzfeld, U.C., and M.S. Matassa. 1999. An Atlas of Antarctica North of 72.1 degrees S from GEOSAT radar altimeter data. International Journal of Remote Sensing 20(2):241-258.

Journel, A.G., Ch. J. Huijbregts. 1978. Mining Geostatistics (London: Academic Press). 600 pp.

Snyder, J.P. 1987. Map projections - a working manual. U.S. Geological Survey Professional paper. #395, IX+383 pp.

Zwally, H.J., R.A. Bindschadler, A.C. Brenner, T.V. Martin, R.H. Thomas. 1983. Surface elevation contours of Greenland and Antarctic ice sheets. Journal of Geophysical Research 88(C3):1589-1596.

9. Glossary and Acronyms:

Glossary of Terms:

Please see the EOSDIS Glossary of Terms.

List of Acronyms:

Please see the EOSDIS Acronyms list for a general list of Acronyms. The following acronyms are used in this document:

DAAC:	Distributed Active Archive Center
DEM:	Digital Elevation Model
ftp:	file transfer protocol
INSTAAR:	INSTitute for Arctic and Alpine Research
GM:	Geodetic Mission
GSFC:	Goddard Space Flight Center
NASA:	National Aeronautics and Space Administration
NOAA:	National Oceanic and Atmospheric Administration
NSIDC:	National Snow and Data Center
UTM:	Universal Transverse Mercator

10. Document Information:

Document Revision Date:

July, 1999

Document Review Date:

June, 1999

Document Curator:

NSIDC Writers

Document URL:

http://nsidc.org/data/docs/daac/nsidc0075_geosat.gd.html