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IceBridge ATM L4 Surface Elevation Rate of Change, Version 1
This data set contains surface elevation rate of change measurements derived from IceBridge and Pre-IceBridge Airborne Topographic Mapper (ATM) widescan elevation measurements data (ILATM1B) for Arctic and Antarctic missions flown under NASA's Operation IceBridge (OIB) and Arctic Ice Mapping (AIM) projects.
|Temporal Resolution:||12 month|
|Platform(s)||C-130, DC-8, DHC-6, DHC-6, P-3A, P-3B|
|Data Contributor(s):||Michael Studinger|
|Metadata XML:||View Metadata Record|
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.Studinger, M. S. 2014, updated 2016. IceBridge ATM L4 Surface Elevation Rate of Change, Version 1. [Indicate subset used]. Boulder, Colorado USA. NASA National Snow and Ice Data Center Distributed Active Archive Center. doi: http://dx.doi.org/10.5067/BCW6CI3TXOCY. [Date Accessed].
Detailed Data Description
One application of laser altimetry data over ice sheets is to monitor the surface elevation change over time. The intent of this data product is to allow scientists to quickly view those surface elevation changes without having to download the entire collection of ATM data and perform the analysis individually. The ATM team has computed the ice surface elevation changes for all of the ATM-flown missions and made the results available for scientists to browse. Upon locating a region of interest, a scientist can download the data sets that contain the relevant data and perform a more detailed analysis. A set of maps in PNG format provides a graphic summary of the data set.
The surface elevation rate of change data files are Comma Separated Value (CSV) ASCII text in which each record (row) corresponds to a location where two ATM swaths from different years contain coincident measurements. For each such overlap location, a surface elevation rate of change value has been calculated. The first line in the file contains a title for each column.
Each CSV file is paired with an associated XML file and a PNG image. XML files contain point latitudes and longitudes, and file and campaign metadata. The image files show location of data.
Data are available on the FTP site in the
https://n5eil01u.ecs.nsidc.org/ICEBRIDGE/IDHDT4.001/ directory. Within this directory, the folders are organized by date, for example
Data files within each folder include information from multiple years.. For example, in the
/2014.04.01/ folder the
IDHDT4_2015-2014_ATM_dhdt_canada.csv data file includes dH/dt values from 2015 test data referenced to 2014 values. The folder is named
/2014/04/01/ to correspond with the earliest date referenced in the data file.
The surface elevation rate of change data files use the following naming convention and as described in Table 1:
Example file names:
||File name prefix indicating elevation rate of change data|
||Four-digit year of test campaign|
||Four-digit year of reference campaign|
||Airborne Topographic Mapper instrument|
||Surface Elevation Rate of Change|
||Region designation for campaign (e.g., Greenland, Antarctica, Canada, Alaska, Iceland, and Svalbard)|
||Indicates CSV, XML, or PNG file.|
The individual CSV surface elevation rate of change data files can range in size from approximately 2 KB to 68 MB, depending on the number of coincident elevation measurements.
The XML files range in size from approximately 11 KB to 84 KB.
The PNG image files range in size from approximately 1 KB to 54 KB.
The total volume of this data set is approximately 2.3 GB.
The spatial coverage for the surface elevation rate of change data set is bounded by:
Southernmost Latitude 90° S
Northernmost Latitude: 90° N
Westernmost Longitude: 180° W
Easternmost Longitude: 180° E
Each record of data in this data set corresponds to an area where two ATM lidar swaths have coincident measurements. This overlap can occur in one of two ways: when one swath intersects another swath (crossing), or when one swath reoccupies the same flight path as another swath (along-track). The amount of overlap may vary due to the aircraft altitude Above Ground Level (AGL), the angle of intersection in the case of crossing swaths, and the track separation in the case of along-track overlap. For the simplest case of two perpendicular crossing swaths at the nominal flight AGL of 450 m and 30 degree swath width, the area of overlap is a square with side equal to the width of the ATM swath; or a 240 m x 240 m square. For along-track comparisons, a difference is computed at regular intervals, typically 0.5 seconds of flight, which corresponds to 60-meter spacing at nominal 120 m/s flight velocity.
Projection and Grid Description
The surface elevation rate of change data are discrete points. Locations are given in geographic latitude and longitude defined in the WGS84 ellipsoid coordinate system.
Temporal coverage for this data set begins in June 1993 and is ongoing, currently through 15 May 2015.
The surface elevation rate of change measurements exist wherever there are coincident ILATM1B measurements. Therefore, the temporal resolution of this data set is seasonal, the same as for the ILATM1B data set, and includes every flight since 23 June 1993.
The parameter provided in this data set is the average rate of change of the surface elevation, between two measurement times, measured in meters per year.
Parameters contained in the ATM L4 Surface Elevation Rate of Change files are described in Table 2.
|Latitude||Average latitude of the coincident points extracted from the test pass||Degrees|
|Longitude||Average longitude of the coincident points extracted from the test pass||Degrees|
|Ellipsoid Elevation||Average elevation of the coincident points extracted from the test pass, relative to WGS84 ellipsoid||Meters|
|dH/dt||Surface Elevation Rate of Change (newer – older)||Meters/year|
|Test Date||Date when test trajectory passed over coincident location||YYMMDD|
|Test Time||Seconds of day when test trajectory passed over coincident location||Seconds|
|Ref Date||Date when reference trajectory passed over coincident location||YYMMDD|
|Ref Time||Seconds of day when reference trajectory passed over coincident location||Seconds|
|Trajectory Separation||Separation between test and reference trajectories||Meters|
|Number of Pairs Used||Number of matching pairs of ILATM1B elevation measurements used in the calculation of the dH/dt||Count|
|RMS||Root mean square of the individual height differences after compensating for the estimated dH/dt||Meters|
Sample Data Record
Below is an sample of the
IDHDT4_2013-1998_ATM_dhdt_greenland.csv data file.
IDHDT4_2013-1998_ATM_dhdt_greenland.csv.png image is shown below.
Software and Tools
The CSV data files may be opened using any software capable of reading ASCII text data.
XML files may be viewed in browser windows, or any XML reading software.
The PNG files may be viewed using any graphic software capable of reading Portable Network Graphics files.
An extensive quality assessment is performed on ILATM1B data, which provide the input to the change computation. Graphical presentations of the elevation change rate are created as color-coded geographic maps and as scatter plots (typically, surface elevation rate of change versus elevation). Visual inspection of these figures provides a qualitative assessment of consistency across sampling dates and other data sources, and correlation with geographic features. The RMS and Number of Pairs Used data fields also can inform regarding the quality of individual records. RMS is primarily dependent on surface roughness, although a high RMS or a limited number of points reduces the accuracy of the computed elevation change rate.
Data Acquisition and Processing
Surface Elevation Rate of Change is a derived product. Computation uses the point measurements in the IceBridge ATM L1B Qfit Elevation and Return Strength (ILATM1B) data set.
The surface elevation rate of change measurements are computed for each coincident location, found by comparing one year's flight trajectories against those of a different year. By comparing the flight trajectories for a combination of any two years, a single file is created that contains the data for all of the coincident locations.
For each location where there exist Airborne Topographic Mapper (ATM) widescan ILATM1B elevation measurements from two different campaigns, a surface elevation difference is computed and divided by the difference in times when the data were collected. This computation yields a rate of change of the surface elevation, which is presented in meters per year, with negative values indicating a decrease in the surface elevation over time.
The Surface Elevation Rate of Change is computed by comparing a test ILATM1B data set against a reference ILATM1B data set. A list of reference trajectories and the test trajectories are defined in a command file. For each pair of reference/test trajectories, the software determines the locations, if any, where the two trajectories overlap. Overlap is reached when the horizontal distance between a position in one trajectory and a position in the other trajectory is less than 200 meters. The trajectory timestamp for each overlap location is then used to find corresponding ILATM1B data for each pass. For each reference ILATM1B laser footprint, all the test ILATM1B footprints are found within a specified radius (typically ~2.5 meters). If enough ILATM1B test/reference pairs (typically more than 500) are found, then an elevation difference is calculated for each. Elevation differences greater than 300 meters are removed. The remaining elevation differences are averaged and divided by the elapsed date-time interval to yield a single surface elevation rate of change measurement for each overlap location. The RMS of these elevation differences is included in the output data record, where the RMS is the square root of the average squared deviation between the individual elevation differences and the overall mean. The rate of change is always presented in terms of the newer measurement minus the older measurement, regardless of which set is the test or reference.
Note: The "typical" numeric values in the description above have been experimentally determined to provide a reasonable compromise between accuracy of comparison points and number of comparison points, but are adjustable parameters that can be set in the command file. Height differences are computed by the numerical subtraction of the elevation found in two different years of ILATM1B data. The elevation for any given year is defined within a particular geodetic reference frame, which may differ between the two years being considered. Precise computation of surface elevation change using this data set must therefore incorporate an additional adjustment for changes in reference frames. This adjustment will be larger for earlier 1990s measurements than later 2010s. More information regarding reference frames can be found on the the International Terrestrial Reference Frame (ITRF) Web site. Note also that precise computation of change in ice volume should consider vertical crustal motion.
Data Collection Platforms 1993 to 2015
|Campaign||Platform Short name||Aircraft ID||Platform Long Name|
|1993 Arctic||NASA P-3B||N426NA||Lockheed P-3B Orion|
|1994 Arctic||NASA P-3B||N426NA||Lockheed P-3B Orion|
|1995 Arctic||NASA P-3B||N426NA||Lockheed P-3B Orion|
|1996 Arctic||NASA P-3B||N426NA||Lockheed P-3B Orion|
|1997 Arctic||NASA P-3B||N426NA||Lockheed P-3B Orion|
|1998 Arctic||NASA P-3B||N426NA||Lockheed P-3B Orion|
|1999 Arctic||NASA P-3B||N426NA||Lockheed P-3B Orion|
|2000 Arctic||Kenn Borek DHC-6||C-FSJB||de Havilland DHC-6 Twin Otter|
|2001 Arctic||NASA P-3B||N426NA||Lockheed P-3B Orion|
|2001 Antarctica||Kenn Borek DHC-6||C-FSJB||de Havilland DHC-6 Twin Otter|
|2002 Arctic||NASA P-3B||N426NA||Lockheed P-3B Orion|
|2002 Antarctica||Chilean Armada P-3||CA-408||Lockheed P-3A Orion|
|2003 Arctic||NASA P-3B||N426NA||Lockheed P-3B Orion|
|2003 Arctic||NASA P-3B||N426NA||Lockheed P-3B Orion|
|2004 Antarctica||Chilean Armada P-3||CA-408||Lockheed P-3A Orion|
|2005 Arctic||Kenn Borek DHC-6||C-FSJB||de Havilland DHC-6 Twin Otter|
|2005 Arctic||Twin Otter International DHC-6||N572AR||de Havilland DHC-6 Twin Otter|
|2006 Arctic||NASA P-3B||N426NA||Lockheed P-3B Orion|
|2006 Arctic||Kenn Borek DHC-6||C-FSJB||de Havilland DHC-6 Twin Otter|
|2007 Arctic||NASA P-3B||N426NA||Lockheed P-3B Orion|
|2007 Arctic||NASA P-3B||N426NA||Lockheed P-3B Orion|
|2008 Arctic||Kenn Borek DHC-6||C-GCKB||de Havilland DHC-6 Twin Otter|
|2008 Antarctica||Chilean Armada P-3||CA-408||Lockheed P-3A Orion|
|2009 Arctic||NASA P-3B||N426NA||Lockheed P-3B Orion|
|2009 Antarctica||NASA DC-8||N817NA||Douglas DC-8|
|2010 Arctic||NASA DC-8||N817NA||Douglas DC-8|
|2010 Arctic||NASA P-3B||N426NA||Lockheed P-3B Orion|
|2010 Antarctica||NASA DC-8||N817NA||Douglas DC-8|
|2011 Arctic||NASA P-3B||N426NA||Lockheed P-3B Orion|
|2011 Antarctica||NASA DC-8||N817NA||Douglas DC-8|
|2012 Arctic||NASA P-3B||N426NA||Lockheed P-3B Orion|
|2012 Antarctica||NASA DC-8||N817NA||Douglas DC-8|
|2013 Arctic||NASA P-3B||N426NA||Lockheed P-3B Orion|
|2013 Antarctica||NASA P-3B||N426NA||Lockheed P-3B Orion|
|2014 Arctic||NASA P-3B||N426NA||Lockheed P-3B Orion|
|2014 Antarctica||NASA DC-8||N817NA||Douglas DC-8|
|2015 Arctic||NASA C-130||N439NA||Lockheed C-130 Hercules|
Trajectory and Attitude Data
Trajectory and attitude data are incorporated in the ILATM1B data set from which the elevation change rate is computed.
The data provider performs the following processing steps.
- Generate a list of GPS trajectory files for every flight in a campaign
- Set up the command file with the trajectory list and the location of the ILATM1B data files for each campaign
- Run the processing software to compute the surface elevation rate of change measurements
Errors and Limitations
For details on error sources, see the documentation for the IceBridge ATM L1B Qfit Elevation and Return Strength (ILATM1B) data set.
References and Related Publications
Contacts and Acknowledgments
Cryospheric Sciences Laboratory
NASA Goddard Space Flight Center
Greenbelt, Maryland USA
The ATM project team would like to acknowledge the dedicated flight crews, whose efforts allowed the safe and efficient collection of this data over some of the most isolated and extreme regions on this planet.
Document Creation Date
18 April 2014
Document Revision Date
10 March 2015
07 August 2015
30 June 2016