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Data Set ID:
IGBTH4

IceBridge Sander AIRGrav L4 Bathymetry, Version 1

This data set contains bathymetry of Arctic fjords and Antarctic ice shelves based on measurements from the Sander Geophysics Airborne Inertially Referenced Gravimeter (AIRGrav) system. The data were collected as part of Operation IceBridge funded aircraft survey campaigns.

This is the most recent version of these data.

Version Summary:

Initial release

Parameter(s):
  • Bathymetry/Seafloor Topography > Bathymetry
Data Format(s):
  • ASCII Text
Spatial Coverage:
N: 83, 
N: -72, 
S: 72, 
S: -75, 
E: -17, 
E: -90, 
W: -70
W: -135
Platform(s):DC-8, P-3B
Spatial Resolution:
  • Varies x Varies
Sensor(s):AIRGrav
Temporal Coverage:
  • 1 January 2010 to 31 December 2016
(updated 2018)
Version(s):V1
Temporal ResolutionVariesMetadata XML:View Metadata Record
Data Contributor(s):Kirsteen Tinto, Robin Bell, James Cochran

Geographic Coverage

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As a condition of using these data, you must cite the use of this data set using the following citation. For more information, see our Use and Copyright Web page.

Tinto, K., R. E. Bell, and J. R. Cochran. 2014, updated 2018. IceBridge Sander AIRGrav L4 Bathymetry, Version 1. [Indicate subset used]. Boulder, Colorado USA. NASA National Snow and Ice Data Center Distributed Active Archive Center. doi: https://doi.org/10.5067/DQVUVQCRYAM4. [Date Accessed].

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Detailed Data Description

Format

The IceBridge Sander AIRGrav L4 Bathymetry data files are in CSV format with associated XML files which contain additional metadata.

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File and Directory Structure

Data are available via HTTPS in the following directory:

https://n5eil01u.ecs.nsidc.org/ICEBRIDGE/IGBTH4.001/

Within this directory, the folders are named for each year, month, and day of the data collection, for example: /2010.01.01/.

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File Naming Convention

The data files have the naming convention shown below. File name variables are described in Table 1.

IGBTH4_20140207.csv
IGBTH4_20140207.csv.xml

IGBTH4_20140207.xyz

Where:

Table 1. Bathymetry Profile Data File Naming Convention
Variable Description
IGBTH4 File name prefix indicating Level-4 bathymetry data
YYYYMMDD Four-digit year, two-digit month, and two-digit day of data delivery date
.xyz Indicates ASCII .csv file or .xml file
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File Size

The .csv files range from approximately 800 KB to 5 MB. The entire data set is approximately 11 MB.

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Spatial Coverage

Spatial coverage for the IceBridge Sander AIRGrav L4 Bathymetry profiles data includes Arctic and Antarctic areas.

Greenland profiles:
Southernmost Latitude: 62° N
Northernmost Latitude: 83° N
Westernmost Longitude: 70° W
Easternmost Longitude: 17° W

Northwest Greenland grid: 
Southernmost Latitude: 72° N
Northernmost Latitude: 77° N
Westernmost Longitude: 67° W
Easternmost Longitude: 50° W

Antarctica: Abbot Ice Shelf profiles:
Southernmost Latitude: 74° S
Northernmost Latitude: 72° S
Westernmost Longitude: 104° W
Easternmost Longitude: 90° W

Spatial Resolution

For Greenland data, fjords are commonly represented by a single profile with points at 500 m intervals. The along-track resolution of inverted gravity is ∼4.4 km, corresponding to the 70 s filter and average survey speed of 125 m/s. Shorter wavelength features are from the radar-derived bed used to constrain the model. Profile lines approximate the fjord axis, but in parts were flown to the side of the midline and show shallower bathymetry corresponding to the fjord edges. Users are cautioned to check the flight track of profile data in order not to misinterpret this shallow bathymetry as an axial sill.

The northwest Greenland grid was flown at 5 km line spacing, with an average speed of 125 m/s giving an along-track resolution of 4.4 km. Data were gridded at 500 m cell size for inversion and then resampled along the flight lines.

The survey over Abbot Ice Shelf was flown as a series of nearly north-south parallel lines, 30 to 54 km apart (mean of 39.2 km), across the ice shelf and a single, nearly east-west line along the ice shelf axis. Free-air anomalies were filtered with a 70 s full wavelength filter, resulting in approximately 4.9 km half-wavelength resolution for a typical flying speed of 140 m/s (Cochran et al. 2014).

Projection and Grid Description

Greenland profiles: 500 m point spacing.
Projection: Polar Stereographic true at 70° N, origin 90° N, 45° W (EPSG 3413) bathymetry positive downward from WGS-84 ellipsoid.

Northwest Greenland grid: 500 m point spacing.
Projection: Polar Stereographic true at 70° N, origin 90° N, 45° W (EPSG 3413) bathymetry positive downward from WGS84 ellipsoid.

Abbot profiles: 200 m point spacing.
Projection: Polar Stereographic true at 71° S, 0 up, 180 down, bathymetry positive downward from GLO4C geoid (Forste et al, 2008).

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Temporal Coverage

01 January 2010 to 31 December 2016

Temporal Resolution

IceBridge campaigns are conducted on an annually repeating basis. Arctic and Greenland campaigns are conducted during March, April, and May; and Antarctic campaigns are conducted during October and November.

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Parameter or Variable

The IceBridge Sander AIRGrav L4 Bathymetry profiles data set contains profile lines, location in Polar Stereographic easting and northing, and bathymetry measured positively downwards.

Parameter Description

The bathymetry profile data file contains fields as described in Table 2. Profile data are preceded by a ten line header.

Table 2. Parameter Description and Units
Column Name Description Units
1 LINE

Greenland data:
Line ID: XX.YYZ
XX = Individual Glacier ID
YY = Year of flight
Z = Repeat tracks

Abbot data:
Line ID: abbotXX
XX = line number

n/a
2 FAG070_mGal Free Air Gravity Anomaly, measured mGal
3 FAG_calc_mGal Free Air Gravity anomaly, calculated mGal
4 LON Longitude on WGS84 ellipsoid Degrees
5 LAT Latitude on WGS84 ellipsoid Degrees
6 X X Coordinate (EPSG 3413) Meters
7 Y Y Coordinate (EPSG 3413) Meters
8 BATHY_m Modelled bathymetry from gravity inversion: positive down, with respect to WGS-84 ellipsoid Meters

Sample Data Record

Below is a sample from the IGBTH4_20140207.csv data file.

sample data record

Figure 1 illustrates the location of Greenland profiles.

glacier IDs
Figure 1. Greenland Glacier IDs

Table 3 lists the profiled Glacier IDs and names.

Table 3. Glacier IDs and Names
ID Name
5 Eqip Sermia
9 Store Gletscher
14 Kangerlussuup Sermersua
15 Rink Isbrae
16 Umiammakku Isbrae
17 Inngia Isbrae
18 Upernavik isstrom S
20 Unnamed near Upernavik
31 Alison Glacier
40 Sverdrup Glacier
44 Kong Oscar Glacier
48 Rink gletscher
63 Heilprin Gletscher
64 Tracy gletscher
70 Humboldt Glacier
72 Steensby gletscher
73 Ryder gletscher
75 C. H. Ostenfeld gletscher
78 Marie Sophie gletscher
79 Academy gletscher
80 Hagen brae
90 Morell Gletscher
92 Daugaard-Jensen
101 Sydbrae
102 Bredegletscher
104 Dendritgletscher
135 Ikertivaq N
140 Koge Bugt C
145 Graulv
147 A.P. Bernstorff Gletscher
149 Skinfaxe
151 Heimdal Gletscher
160 Kangiata Nunaata Sermia
161 Akullersuup Sermia
162 Narsap Sermia
193 Petermann Gletscher
194 Docker Smith Gl. W
200 Puisortoq N
207 Nordenskiaeld Gletscher
301 Unnamed near Upernavik
700 Puisortoq S
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Software and Tools

The data files may be opened by any ASCII text reader.

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Data Acquisition and Processing

Theory of Measurements

The gravity signal is extracted from an inertially based system in which a small mass is suspended within a magnetic field. Tiny variations in the acceleration of the gravimeter produce small electrical signals in the sensor as the mass moves within the magnetic field. The processed data from the AIRGrav instrument data consists of two data types: gravity and aircraft attitude.

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Data Acquisition Methods

The gravimeter is located as near the airplane center of mass as possible. Simultaneously acquired gravimeter output GPS data are recorded on hard disks on the plane. Following the flight this data is downloaded onto a PC for processing.

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Derivation Techniques and Algorithms

Inversion of the Greenland and Abbot gravity data for bathymetry was undertaken in two dimensions along individual flight lines using Geosoft GMSysTM software. The software performs iterative forward modeling using the technique of Talwani et al. (1959). The bed was kept fixed where it is observed in the radar data and the bathymetry in water-covered areas where the seafloor cannot be imaged with radar varied to obtain the best match to the observed gravity. The model is pinned to the observed gravity value at a location within the region where the bed can be observed.

Processing Steps

The following processing steps are performed by the data provider.

For all bathymetry data sets:

  1. The gravimeter data were filtered and decimated to 10 Hz to match the GPS data.
  2. GPS-derived accelerations were subtracted from the data.
  3. The gravity was corrected for the Eötvös effect.
  4. The expected gravity at the measurement latitude was subtracted.
  5. The resulting anomalies were decimated to 2 Hz and low-pass filtered to suppress noise.
  6. The free-air correction was applied.
  7. After evaluating the noise level with different filter lengths, a 70 s filter was used. The filter half-wavelength is approximately 5 km at the average flight speed of 275 knots (142 m/s). Features narrower than the half wavelength can be resolved, but have an attenuated amplitude.
  8. Horizontal accelerations associated with turns and other vigorous maneuvers disturb gravity measurements. A routine that examines horizontal accelerations was used to divide each flight into lines that are free of these perturbations.

For Greenland fjord and Abbot Ice Shelf data:

  1. A forward model of gravity was generated according to Talwani (Talwani, 1959) with ice density 0.915 g/cc, water density 1.028 g/cc and rock density 2.67 g/cc. Ice surface and base are taken from the NASA Airborne Topographic Mapper (ATM) (Krabill, 2010) and the CReSIS Multichannel Coherent Radar Depth Sounder (MCoRDS) (Allen, 2009).
  2. The model was pinned at a point where ice is grounded and nearby gravity variations are due only to variations in topography of the ice/rock interface. This establishes a DC shift to relate the anomalies calculated for the model space to the anomalies measured on Earth. Where multiple lines were flown along approximately the same track, the same DC shift was applied.
  3. A gravity inversion was performed on the water/rock interface under floating ice. This produced a model of the sub-ice shelf bathymetry responsible for the gravity anomaly.

For Northwest Greenland

  1. Data from survey lines were gridded by minimum curvature gridding with cell size of 500 m.
  2. Gravity data were upward continued from survey elevation to a constant elevation of 2000 m above the ellipsoid.
  3. A forward gravity model was built based on gridded data sets of surface, bed topography and seafloor bathymetry (Bamber et al., 2013). A DC shift was established to match gravity values calculated within the model space to observed values. The value of the DC shift was established from the mean misfit between calculated and observed gravity over large islands and peninsulas along the coast.
  4. The observed gravity was inverted for bathymetry in the software GMSys-3D, with algorithms based on Parker (1972).

Version History

On 11 February 2015, Version 1 was replaced by Version 1.1. The precision for lat and lon values in data file IGBTH4_20140207.csv was increased to 6 decimal places.

Error Sources

Errors range from ±50 meters up to ±200 meters in the Greenland data sets. The variability in uncertainty is largely due to profile length, with uncertainty increasing with distance from the pinning point usually near the grounding line. The main sources of error are from short-wavelength features, which are not modelled at full amplitude, and from the presence of either local variations in geology or long-wavelength regional geological variations. These have been accounted for in some cases, notably Petermann Glacier, where a regional correction to the observed gravity was applied in order to fit known bathymetric constraints at the end of the fjord (Johnston et al., 2011). Elsewhere, bathymetry inversion is not performed on gravity recovered from parts of fjords where magnetic anomalies indicate a significant change in rock type compared to the material at the point to which the model is pinned.

Errors are approximated as ±70 m for the Abbot data set, incorporating errors in gravity measurement, radar ice thickness, ATM surface elevation, 2-D model pinning point and some allowance for geological structures. Variations in bed density were incorporated utilizing rock outcrops as a guide. The model assumes the absence of sea floor sediments. If sea floor sediments are present the true bathymetry will be less deep than the model (Cochran et al., 2014).

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Sensor or Instrument Description

The gravity instrument is a Sander AIRGrav designed for airborne applications. The AIRGrav system consists of a three-axis gyro-stabilized, Schuler-tuned inertial platform on which three orthogonal accelerometers are mounted. The primary gravity sensor is the vertical accelerometer that is held within 10 arc-seconds (0.0028 degree) of the local vertical by the inertial platform, monitored through the complex interaction of gyroscopes and two horizontal accelerometers (Sander et al., 2004). An advantage of the AIRGrav system over other airborne gravimeters is that it has been shown to be capable of collecting high-quality data during draped flights (Studinger et al., 2008). The gravimeter records accelerations arising from variations in the Earth's gravity field and accelerations experienced by the airplane. These accelerations are recorded at 128 Hz. Aircraft accelerations are obtained utilizing differential GPS measurements.

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References and Related Publications

Contacts and Acknowledgments

Kirsteen Tinto, Robin E. Bell, James R. Cochran
Lamont-Doherty Earth Observatory
109C Oceanography
61 Route 9W - PO Box 1000
Palisades, New York 10964-8000 USA

Acknowledgments: 

Data collection was supported by NASA grants NNX13AD25A, NNX09AR49G, NNG10HP20C, and NNX10AT69G. Alexandra Boghosian assisted with the gravity inversions. We thank Stefen Elieff, Kevin Charles, Marianne McLeish, Joel Dube, Eric Seme, Sean O'Rourke, Eric Renaud, John Drolet and Craig McMahon from Sander Geophysics.

Document Information

Document Creation Date

16 October 2014

Document Revision Date

28 June 2018

No technical references available for this data set.

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