Radarsat Antarctic Mapping Project Digital Elevation Model Version 2

Summary

The high-resolution Radarsat Antarctic Mapping Project (RAMP) digital elevation model (DEM) combines topographic data from a variety of sources to provide consistent coverage of all of Antarctica. Version 2 improves upon the original version by incorporating new topographic data, error corrections, extended coverage, and other modifications.

This DEM incorporates topographic data from satellite radar altimetry, airborne radar surveys, the recently-updated Antarctic Digital Database (version 2), and large-scale topographic maps from the U.S. Geological Survey (USGS) and the Australian Antarctic Division. Data were collected between the 1940s and present, with most collected during the 1980s and 1990s. Although the RAMP DEM was created to aid in processing RAMP radar data, it does not utilize any RAMP radar data.

The 1 km, 400 m, and 200 m DEM data are provided in ARC/INFO and binary grid formats, and the 1 km and 400 m DEMs are also available in ASCII format. Data access is unrestricted, but users should register to receive e-mail notification of product updates and changes in processing.

Citation

We kindly request that you cite the use of this data set in a publication using the following citation. For more information, see our Use and Copyright Web page.

Liu, H., K. Jezek, B. Li, and Z. Zhao. 2001. Radarsat Antarctic Mapping Project digital elevation model version 2. Boulder, Colorado 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. Acronyms and Abbreviations
10. Document Information

1. Data Set Overview

Discussion

The RAMP DEM was developed by integrating a broad variety of available topographic source data in a GIS environment. By combining the comparative advantages of all available sources, the developers were able to fully exploit the most detailed and accurate topographic information in each data set. Error checking procedures included global statistical analysis, cross-validation methods, and creation of a synthetic stereo image for visualizing and detecting gross errors in the elevation data (Liu 1999). A new data integration technique allowed the developers to produce a DEM that is both seamless and geomorphologically consistent with ice-covered and ice-free terrain. The DEM captures details of geomorphology, ranging from small-scale mountain valleys to extensive ice sheet drainage basins.

Version 2 of the RAMP DEM incorporates diverse improvements over the original version, with effects in numerous regions of Antarctica, as summarized below (table adapted from Jezek et al. 1999):

Method Application areas
Increased accuracy and resolution using newly available data Coats Land, Theron Mountains, Berkner Island, Henry Ice Rise, and Korff Ice Rise
Extended DEM over islands and surrounding ocean surface in support of sea ice and oceanography studies South Shetland Islands, Latady Island, Weddell Sea, Amundsen Sea, Davis Sea, ocean around Queen Maud Land, and Shackleton Ice Shelf
Achieved better data selection and surface constraints using updated coastlines and grounding lines derived from the SAR mosaic coastline for the entire continent, particularly the ice margins of Ross Ice Shelf, Amery Ice Shelf, and Filchner-Ronne Ice Shelf; Filchner Ice Shelf and Roosevelt Island grounding lines
Removed artifacts in the DEM by adjusting interpolation parameters and densifying contours Crary Mountains, Sør Rondane Mountains
Corrected planimetric errors using SAR simulation and warping techniques Ellsworth Mountains

Related Data Sets

The Polar Ice Sheet DEMs and Topographic Data Web page contains a detailed listing of all the DEMs and topographic data archived at NSIDC.

Investigators

Dr. Hongxing Liu
Department of Geography
Texas A&M University
College Station, TX

Dr. Kenneth C. Jezek
Department of Geological Sciences and
The Byrd Polar Research Center
The Ohio State University,
Columbus, OH

Dr. Biyan Li
The Byrd Polar Research Center,
The Ohio State University,
Columbus, OH

Dr. Zhiyuan Zhao
The Byrd Polar Research Center,
The Ohio State University,
Columbus, OH

Contact Information

Please direct all inquiries to NSIDC User Services:

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 RAMP DEM was created for use in processing images for the RAMP AMM-1 SAR Mosaic of Antarctica (Jezek 1999). The DEM has potentially broad applicability to studies of ice sheet morphology and ice dynamics. Current and planned applications include flow line mapping, catchment area determination, balance velocity mapping, and atmospheric modeling. The DEM is not appropriate for direct detection of elevation change by comparison with other DEMs, unless those changes are very significant (> ± 15 m).

3. Theory of Measurements

Please see the section on Data Manipulations for more information on data processing methods.

4. Acquisition Materials and Methods

Data were collected from various sources and combined to produce the final product. Data sources included:

5. Preparation and Description

Data Description

Spatial Characteristics

Spatial Coverage

This data set covers all of Antarctica from 60 degrees south to 90 degrees south latitude, including the grounded ice, exposed land surface, and ice shelves. Version 2 features expanded coverage of the surrounding ocean and islands.

Spatial Resolution

Resolution of elevation data is in units of meters. Please refer to the Horizontal and Vertical Accuracy sections for more details.

Grid Description

Binary Grid
1 km 400 m 200 m
Rows 4916 12290 24580
Columns 5736 14340 28680
Cell Size 1000 m 400 m 200 m
Corner Point x -2868000 m1 -2868000 m1 -2868000 m1
Corner Point y -2458000 m1 -2458000 m1 -2458000 m1

1These values represent the outer edges of the upper left corner point, with the origin at the South Pole, x increasing to the right, and y increasing downward in the Scientific Committee on Antarctic Research (SCAR) projection. The SCAR projection is a polar stereographic projection from a WGS84 ellipsoid surface with the reference plane defined by -71 degrees (south) latitude.

ASCII grids contain fields for latitude, longitude, elevation relative to the WGS84 ellipsoid, and elevation relative to the OSU91A geoid. Coordinates are given in decimal degrees, from -180 degrees (west) to 180 degrees (east) longitude; and -90 to -60 (south) degrees latitude.

Projection

The RAMP DEM covers the entire Antarctic continent and its surrounding offshore ocean area in a polar stereographic projection with reference to the WGS84 ellipsoid. The projection latitude is 71 degrees south, with 0 degrees longitude oriented vertically at the top of the projection. The South Pole is the origin of the projection.

Temporal Characteristics

Temporal Coverage

Topographic source data were collected by various instruments, teams, and projects between the 1940s and 1990s.

Data Characteristics

Data Source

The data originate from various sources. The developers and data contributors compiled a comprehensive collection of digital topographic source data. The data used can be grouped into the following three categories:

Cartographic data include contours, spot height points, and surface structure lines digitized from paper topographic map sheets. Remotely sensed data consist of ERS-1 satellite radar altimetry data and airborne radar echo-sounding data. Survey data include ground-based survey data and satellite-based GPS measurements.

The following investigators contributed data:

Data Format

All DEM data are provided in ARC/INFO and binary grid formats, and the 1 km and 400 m DEMs are also available in ASCII format. Following is a chart that summarizes characteristics of the binary grid.

Binary Grid
1 km 400 m 200 m
Rows 4916 12290 24580
Columns 5736 14340 28680
Byte Order Big Endian Big Endian Big Endian
Bytes per Cell 2 2 2
Cell Size 1000 m 400 m 200 m
Bands 1 1 1
Row Bytes 11472 28680 57360

ASCII grids contain fields for latitude, longitude, elevation relative to the WGS84 ellipsoid, and elevation relative to the OSU91A geoid. Data are represented in decimal degrees, from -180 degrees (west) to 180 degrees (east) longitude; and -90 to -60 (south) degrees latitude.

ARC/INFO coverages of RAMP DEMs are organized into individual directories for each resolution (1 km, 400 m, and 200 m) and geoid/ellipsoid.

Unit of Measurement

Elevations for points in this data set are measured in meters [m] above both the WGS84 ellipsoid and the OSU91A geoid. (See the section titled Data Granularity for a list of files referencing these two models.)

While the WGS84 ellipsoid is based on an approximation of the Earth's shape using only an equatorial radius and a polar radius (or a radius and an eccentricity), the OSU91A geoid is a more complex surface representing mean sea level. The OSU91A geoid is reported as a height above or below the WGS84 ellipsoid. The relationship between the two for the RAMP DEM can be described algebraically as follows:

S = W - G

where:

W are the WGS84 elevations
G are the OSU91A elevations
S are mean sea level elevations of the OSU91A geoid, relative to the WGS84 ellipsoid

Data Range

Values are in meters for each grid.

Minimum Maximum
OSU91A 200 m 0 5022
WGS84 200 m -67 5008
OSU91A 400 m 0 5012
WGS84 400 m -67 4997
OSU91A 1 km 0 4982
WGS84 1 km -67 4968

Note: Maximum values decrease with increasing grid spacing because a larger region is averaged for each grid cell. Maximum elevation values are found in the Ellsworth Mountains near Vinson Massif. Zero values are at the coast; there are no points in the interior of Antarctica that are at or below sea level (i.e. with a geoid elevation of zero or less).

Sample Data Records

The following is sample output from a 1 km ASCII DEM file:


(   Lat       Lon     WGS  OSU  )

-78.99166  -23.45572 1124 1134
-78.99529  -23.41189 1122 1132
-78.99891  -23.36804 1119 1129
-79.00253  -23.32415 1116 1126
-79.00613  -23.28023 1113 1123
-79.00974  -23.23629 1109 1119
-79.01333  -23.19231 1105 1115
-79.01692  -23.14831 1102 1112
-79.02050  -23.10428 1099 1109
-79.02408  -23.06021 1095 1105
-79.02765  -23.01613 1092 1102
-79.03121  -22.97201 1089 1099
-79.03477  -22.92786 1086 1096
-79.03832  -22.88368 1081 1091
-79.04187  -22.83948 1075 1085
-79.04540  -22.79524 1069 1079
-79.04893  -22.75098 1062 1072
-79.05246  -22.70669 1055 1065
-79.05598  -22.66237 1048 1058

Data Organization

Data Granularity

A granule of RAMP DEM data (i.e. the smallest aggregation of data that is independently retrievable) includes coverage of the entire continent at a given resolution and geoid/ellipsoid model. Compressed and uncompressed file sizes are summarized below.

Format Granule File Name Compressed Uncompressed
ARC/INFO demosu1km_v2.tar.gz 11 MB 25 MB
ARC/INFO demwgs1km_v2.tar.gz 11 MB 25 MB
ARC/INFO demosu200_v2.tar.gz 136 MB 324 MB
ARC/INFO demwgs200_v2.tar.gz 141 MB 330 MB
ARC/INFO demosu400_v2.tar.gz 39 MB 112 MB
ARC/INFO demwgs400_v2.tar.gz 40 MB 114 MB
ASCII ramp1kmdem_wgsosu_v2.txt.gz 151 MB 424 MB
ASCII ramp400dem_wgsosu_v2.txt.gz 860 MB 2649 MB
Binary ramp1kmdem_osu_v2.bin.gz 15 MB 56 MB
Binary ramp1kmdem_osu_v2.hdr    
Binary ramp1kmdem_wgs_v2.bin.gz 15 MB 56 MB
Binary ramp1kmdem_wgs_v2.hdr    
Binary ramp200dem_osu_v2.bin.gz 270 MB 1410 MB
Binary ramp200dem_osu_v2.hdr    
Binary ramp200dem_wgs_v2.bin.gz 274 MB 1410 MB
Binary ramp200dem_wgs_v2.hd    
Binary ramp400dem_osu_v2.bin.gz 61 MB 352 MB
Binary ramp400dem_osu_v2.hdrr    
Binary ramp400dem_wgs_v2.bin.gz 64 MB 352 MB
Binary ramp400dem_wgs_v2.hdr    

Data Manipulations

Derivation Techniques and Algorithms

Interpolation of Satellite Radar Altimeter Data
The radar altimetry data sets for the RAMP Antarctic DEM had already been corrected for tracking and slope errors and preprocessed into evenly distributed points with a spacing of about 5 km, prior to being combined with the many additional data sets. (See NSIDC's Radar Altimeter document, Davis and Zwally 1993, Zwally et al. 1983, and Brenner et al. 1983.) The RAMP DEM development team used the Triangulated Irregular Network (TIN) Quintic interpolation method to further interpolate the satellite radar altimeter data (Liu, Jezek, and Li 1999).

Interpolation of Traverse Airborne Radar Data
Airborne radar data are densely sampled along flight lines but widely separated between flight transects. Most interpolation algorithms have difficulty resolving such a pattern. The development team for the RAMP Antarctic DEM used a procedure that combines the quadrant neighborhood-based Inverse Distance Weight (IDW) method to stabilize the interpolation result, with the TIN method to retain the topographic details present in the source data (Liu, Jezek, and Li 1999).

Interpolation of Contour-based Cartographic Data
Contour data are characterized by oversampling of information along contour lines and undersampling between contour lines, especially in low relief areas with widely spaced contours. It is the most difficult data type to interpolate with general-purpose interpolation techniques. The development team chose to use the TOPOGRID-based method (Hutchinson 1988; Hutchinson 1989; ESRI 1991; Gesch and Larson 1996) to interpolate the cartographic data in the RAMP Antarctic DEM. The team modified the TOPOGRID method slightly (Liu, Jezek, and Li 1999) to compensate for spurious sinks that occur in contour sparse areas corresponding to low slope areas like glacial valley floors (Bliss and Olsen 1996).

Determination of DEM Grid Spacing
The horizontal grid spacing of DEMs is an important parameter that needs to be specified during interpolation. In general, a small grid spacing is required to obtain an accurate representation of the surface details for a rugged and mountainous terrain, while a large grid spacing is sufficient for a low-relief terrain. For the satellite radar altimeter data and the airborne radar data, a post spacing of 1 km was used. For the contour data, 200 m grid spacing was used for rugged mountainous areas where the contour density is very high, while 400 m grid spacing was used for the sloped coastal area where the contours are relatively smooth and regularly, broadly spaced (Liu, Jezek, and Li 1999).

Data Integration
For mountainous and sloped coastal margins, the development team integrated the contour data, the spot elevation points, coastlines, grounding lines, and limited GPS data during the interpolation process. To avoid the edge effects, all the source data layers are merged into a number of overlapping blocks, and the interpolation extent at each time is set much smaller than that of input data. Individual DEM data sets are merged by using GIS logical "clipping" and "inserting" operations along coastlines and grounding lines, and by using a cubic Hermite blending function (S-shaped) along irregular buffer zones (Liu, Jezek, and Li 1999).

Accuracy

Horizontal (Spatial) Resolution
The real horizontal resolution of the DEM varies from place to place according to the density and scale of the original source data. The developers of the data set estimate that the horizontal resolution of the DEM is about 200 m in the Transantarctic Mountains and Antarctic Peninsula, and about 400 m in the sloped coastal regions. For ice shelves and the inland ice sheet covered by satellite radar altimeter data, the horizontal resolution is about 5 km, but where the airborne radar sounding data were used, the horizontal resolution is about 1 km. For the plateau inside 81.5 degrees south latitude, horizontal resolution is estimated at about 10 km (Liu, Jezek, and Li 1999).

Geolocation Accuracy
The accuracy of geolocation (i.e., the accuracy of the position of a given feature on the DEM) is governed by the accuracy of topographic data sources, and is generally better than the horizontal resolution of the DEM.

Vertical Accuracy
Vertical accuracy of the RAMP Antarctic DEM is ± 100 m over rugged mountainous areas, ± 15 m for steeply sloped coastal regions, ± 1 m on the ice shelves, ± 7.5 m for the gently sloping interior ice sheet, and ± 17.5 m for the relatively rough and steeply sloped portions of the ice sheet perimeter. For latitudes south of 81.5 degrees south, within the interior East Antarctic ice sheet and away from the mountain ranges, vertical accuracy is estimated to be ± 50 m (Liu, Jezek, and Li 1999).

Error Handling

Potential errors in the RAMP DEM include imperfections in the measuring instrument, faulty readings or recordings, calculation and execution faults, and digitizing errors. Errors were noted in the ARC/INFO contour coverages, with mislabeled contours and intersections of contours. In some cases, digitized contour lines deviated from their original position on the source map and often intersected one other, resulting in some positions having two or more conflicting values. Also, poor ground control and inaccurate navigation techniques used to acquire the original topographic data were noted. Some ground control points were assigned erroneously large values, due to data entry errors. The RAMP DEM development team employed a variety of techniques to detect and correct these errors (Liu 1999).

Global Statistical Analysis
Summary statistics were calculated from the ARC/INFO attribute tables where elevation data reside. These global statistics were used to identify extreme erroneous values, namely the elevation range, from prior knowledge about a specific region or from the frequency distribution of elevation measurements. Data points with elevation values outside the reasonable range were flagged, and erroneously large values and negative values for elevation were removed. Contour lines with irregular elevation values were detected and corrected according to the values of neighboring contours.

Cross-Validation
Cross-validation methods were used for multiple data sets that overlap in the same area. Spot height points were checked against corresponding contour coverages by first predicting the elevation values at the positions of spot points by interpolating between contours, then computing the differences between the interpolated values and spot height values. Points that had an absolute difference greater than one contour interval in the flat area and two times greater than the contour interval in highly variable areas were removed. Cross-checking was similarly conducted between contour data and satellite radar alimeter data.

Visual Inspection
Errors in elevation data were detected with a variety of interactive methods including perspective views, color sequence in contour lines, hill shading, and synthetic stereo display. In areas of question, contour lines were overlain with the source data to reveal errors in elevation values. When the DEM grid was rendered as a hill shaded image or synthetic stereo image, errors would appear as anomalous valleys or scars, especially when vertical exaggeration was increased or the illumination angle was adjusted.

Image Simulation
This method integrated a digital synthesis of a satellite image according to a DEM grid, with information about the satellite illumination angle and image geometry. In an area with homogenous land cover like that of Antarctica, a comparison and correlation analysis between the simulated image and real image can often reveal errors in the DEM.

Spatial Autocorrelation
Subtle errors in elevation values were detected using rigorous statistical methods, and by tracing errors back to the original source data after locating areas of spatial discontinuity. In checking the consistency and continuity of each data point relative to nearby points, erroneous data points are flagged as local outliers if they are inconsistent with neighboring points.

6. Notes and Plans

The developers of the RAMP DEM version 2 have established a GIS-based database to maintain all the topographic source data for each portion of the DEM. This database will allow for future updates to the DEM as new data become available. These data will be distributed by NSIDC, and registered users of the RAMP DEM data set will be automatically notified of any updates. The original RAMP DEM (version 1) is available from the NSIDC archives (contact NSIDC User Services).

7. Products and Access

Data Center Identification

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

Contact Information

Please direct all inquiries to NSIDC User Services:

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

Data are available via FTP.

8. References

Bliss, N.B., L.M. Olsen. 1996. Development of a 30-arc-second digital elevation model of South America. Paper presented at Pecora Thirteen Human Interactions with the Environment - Perspective from Space, Sioux Falls, South Dakota, August 20-22.

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.

ESRI. 1991. ARC/INFO User's Guide: Cell-based modeling with GRID. ESRI Inc.

Gesch, D.B., K.S. Larson. 1996. Techniques for development of global 1-kilometer digital elevation models. Paper presented at Pecora Thirteen Human Interactions with the Environment - Perspective from Space, Sioux Falls, South Dakota, August 20-22.

Hutchinson, M.F. 1989. A new procedure for gridding elevation and stream line data with automatic removal of spurious pits. Journal of Hydrology 106:211-232.

Hutchinson, M.F. 1988. Calculation of hydrologically sound digital elevation models. In Proceedings of Third International Symposium on Spatial Data Handling:117-133. Sydney, Australia.

Jezek, K.C., H. Liu, Z. Zhao, B. Li. 1999. Improving a digital elevation model of Antarctica using radar remote sensing data and GIS techniques. Polar Geography 23:185-200.

Jezek, K.C. 1999. Glaciological properties of the Antarctic ice sheet from RADARSAT-1 synthetic aperture radar imagery. Annals of Glaciology 29:286-290.

Liu, H., K. Jezek, B. Li. 1999. Development of Antarctic digital elevation model by integrating cartographic and remotely sensed data: A geographic information system based approach. Journal of Geophysical Research 104: 23,199-23,213.

Liu, H. 1999. Development of an Antarctic digital elevation model. Byrd Polar Research Center Report No. 19, 157 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. Acronyms and Abbreviations

The following acronyms and abbreviations are used in this document.

DAAC Distributed Active Archive Center
DEM Digital Elevation Model
ERS-1 European Remote Sensing Satellite-1
ftp File Transfer Protocol
IDW Inverse Distance Weight
NASA National Aeronautics and Space Administration
NSIDC National Snow and Ice Data Center
RAMP Radarsat Antarctic Mapping Project
SCAR Scientific Committee on Antarctic Research
TIN Triangulated Irregular Network
WGS84 World Geodetic System 1984

10. Document Information

Document Revision Date

01 November 2001

Document URL

http://nsidc.org/data/docs/daac/nsidc0082_ramp_dem_v2.gd.html