Table of Contents

1. Contacts and Acknowledgments
2. Detailed Data Description
3. Data Access and Tools
4. Data Acquisition and Processing
5. References and Related Publications
6. Document Information

Citing These Data

The following example shows how to cite the use of this data set in a publication.

Larsen, Chris. 2010, updated current year. IceBridge UAF Lidar Scanner L1B Geolocated Surface Elevation Triplets, [list dates of data used]. Boulder, Colorado USA: NASA Distributed Active Archive Center at the National Snow and Ice Data Center. Digital media. http://nsidc.org/data/ilaks1b.html.

Overview

Platform	Piper Supercub (PA-12)
Sensor	UAF Glacier Lidar system
Spatial Coverage	Alaska
Spatial Resolution	Approximately 20 cm
Temporal Coverage	19 August 2009 to present
Temporal Resolution	Annual
Parameters	Elevation
Data Format	LAS 1.2 sequential binary
Metadata Access	View Metadata Record
Data Access	FTP

1. Contacts and Acknowledgments

Investigator(s) Name and Title

Chris Larsen
Geophysical Institute
University of Alaska Fairbanks
903 Koyukuk Drive
Fairbanks, Alaska 99775, USA

Technical Contact

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

Acknowledgements

This program has been supported by grants from NASA, NOAA and NSF. Current support comes from NSF Arctic Natural Sciences grant ARC-0612537 and from NASA's Operation Ice Bridge, Earth Science Project Office, Grant NNX09AP54G.

2. Detailed Data Description

Format

The UAF Lidar Scanner Level-1B Geolocated Surface Elevation Triplets data files are in LAS 1.2 sequential binary format. The LAS file format is a public file format for the interchange of 3-dimensional point cloud data between data users (ASPRS Standards Committee LASer (LAS) File Format Exchange Activities).

File and Directory Structure

Data are available on the n4ftl01u.ecs.nasa.gov FTP site in the /SAN2/ICEBRIDGE_FTP/ directory. Within this directory, there are two subdirectories as described in Table 1.

Table 1. Directory Description

Directory	Description
/ILAKS1B_UAFscan_v01/2009_AK_UAF/	2009 UAF Lidar Scanner Level-1B Geolocated Surface Elevation Triplets data
/ILAKS1B_UAFscan_v01/2010_AK_UAF/	2010 UAF Lidar Scanner Level-1B Geolocated Surface Elevation Triplets data
/ILAKS1B_UAFscan_v01/2011_AK_UAF/	2011 UAF Lidar Scanner Level-1B Geolocated Surface Elevation Triplets data
/ILAKS1B_UAFscan_v01/2012_AK_UAF/	2012 UAF Lidar Scanner Level-1B Geolocated Surface Elevation Triplets data

File Naming Convention

File Size

Data files range from approximately 1 MB to 3 GB.

Volume

The total data set volume is approximately 75 GB.

Spatial Coverage

Spatial coverage for the IceBridge UAF Lidar Scanner Level-1B Geolocated Surface Elevation Triplets campaigns:

Southernmost Latitude: 55° N
Northernmost Latitude: 72° N
Westernmost Longitude: 156° W
Easternmost Longitude: 130° W

Spatial Resolution

The footprint on the ground of the laser shot points is on the order of 20 cm in diameter. Average spacing along path and perpendicular to the flight path is roughly 1 meter by 1 meter, subject to Height Above Ground (HAG) flown and attitude of the aircraft. Preferred HAG is 500 to 600 m. Optimal conditions result in a swath 500 to 600 m wide with 1 meter by 1 meter density.

Projection and Grid Description

The data are provided with Universal Transverse Mercator (UTM) eastings and northings. Easting and northing values are in UTM zones 6, 7, and 8. Geoid values are not included.

Temporal Coverage

These data were collected as part of NASA Operation IceBridge funded campaigns from 19 August 2009 to the present.

Temporal Resolution

IceBridge campaigns are conducted on an annual repeating basis. Alaska campaigns are conducted during May, June, July, August, and September.

Parameter or Variable

The UAF Lidar Scanner Level-1B Geolocated Surface Elevation Triplets data set contains elevation measurements with UTM easting and northing.

Parameter Description

3. Data Access and Tools

Data Access

Data are available via FTP.

Software and Tools

LAS files can be opened using tools available from the ASPRS Lidar Data Translation Toolset Web site. See also the LAStools Web page for various tools for converting, filtering, viewing, processing, and compressing LAS format lidar data.

4. Data Acquisition and Processing

The UAF altimetry data are comprised of a series of point measurements on the surface of glaciers recorded from an aircraft. Each point is derived from a pulsed laser range measurement combined with aircraft Global Positioning System Inertial Measurement Unit (GPS/IMU) positioning and orientation measurements. The footprint on the ground of the laser shot points is on the order of 20 cm in diameter. Coordinates and elevation for each point are referenced in the International Terrestrial Reference Frame (ITRF00) and are accurate to within +/- 30 cm. Longitude/Latitude values are derived by projecting the ITRF coordinates into WGS 84. Easting and northing values are in UTM zones 6, 7, and 8. All vertical data are in HAE.

Data Acquisition Methods

The GPS records the position of the aircraft every second as it flies over a glacier. The laser continually measures the distance between the aircraft and the glacier surface, and the gyroscope measures the direction the laser is pointing.

The Riegl scanner has a 60 degree beam sweep, range up to 650 m, and measures 20,000 data points per second. The IMU measures the aircraft attitude (pitch, roll, yaw, and rates about those axes) and several other measurements, all at 100 times per second. The Trimble GPS records raw, dual frequency data which is post-processed after the survey against similar data recorded at a fixed GPS base station to provide precise positioning of the aircraft. At typical aircraft speeds and heights data are collected on a roughly 1 meter by 1 meter grid along a 500 meter wide swath (UAF Glacier Lidar System Web page).

Derivation Techniques and Algorithms

Processing Steps

The following processing steps are performed by the data provider.

1. All GPS processing of the aircraft position uses L1 frequency 1575.42 MHz and L2 frequency 1227.6 MHz data recorded at 5 Hz, processed with the TRACK GPS differential phase kinematic positioning program, a module of the GAMIT/GLOBK software programs from the Department of Earth Atmospheric and Planetary Sciences, MIT. For further information on TRACK, see http://geoweb.mit.edu/~tah/track_example.
2. GPS base station coordinates are found using Online Positioning User Service (OPUS). For further information on OPUS, see http://www.ngs.noaa.gov/OPUS/. The kinematic processing and the laser shot point coordinates are referenced to these base station coordinates.

Version History

On 21 September 2012, the 2009 and 2010 data were replaced with V01.1 reprocessed data. The reprocessing involved:

1. Identifying and correcting infrequent timing shifts wherein the laser scanner data and IMU/GPS data were misaligned by 1 or 2 integer seconds. This only concerned a small handful of files, but the effect can introduce errors on the order of 10's to 100's of meters in horizontal and vertical coordinates of the point cloud data.
2. Applying corrected boresight angles, that is, the small angular offsets between the IMU/GPS reference frame and the laser vector. These corrections were applied to all data. The previous, incorrect boresight angles introduced errors on the order of meters to 10's of meters in the horizontal and vertical coordinates of the point cloud data.
3. Cleaning of spurious data points that did not represent real targets or surfaces. These appear in the previous versions of these data as random, individual points far away from the surfaces being imaged.

Sensor or Instrument Description

The UAF lidar scanner is a laser altimetry system used for measuring surface elevation changes of glaciers throughout Alaska and western Canada. The altimetry system consists of a highly accurate GPS receiver, a laser, and a gyroscope.

The main components of the UAF lidar scanner are a Riegl LMS-Q240i scanning laser altimeter, an Oxford Technical Solutions Inertial+2 inertial measurement unit, a Trimble R7 geodetic GPS receiver, and a small form factor PC for data logging.

5. References and Related Publications

Related Data Collections

Related Web Sites

6. Document Information

Acronyms and Abbreviations

The acronyms used in this document are listed in Table 4.

Table 4. Acronyms and Abbreviations

Acronym	Description
ASCII	American Standard Code for Information Interchange
ASPRS	American Society for Photogrammetry and Remote Sensing
CIRES	Cooperative Institute for Research in Environmental Science
FTP	File Transfer Protocol
GPS	Global Positioning System
HAE	Height Above Ellipsoid
HAG	Height Above Ground
IMU	Inertial Measurement Unit
ITRF	International Terrestrial Reference Frame
L1B	Processing Level-1B
L1	L1 is a civilian-use GPS signal broadcast on the 1575.42 MHz frequency, that currently contains the course acquisition signal used by all current GPS users. Also known as L1C.
L2	L2 is a civilian-use GPS signal broadcast on the 1227.6 MHz frequency. Also known as L2C.
LAS	Laser file format for lidar point cloud data
NASA	National Aeronautics and Space Administration
NOAA	National Oceanic and Atmospheric Administration
NSF	National Science Foundation
NSIDC	National Snow and Ice Data Center
UAF	University of Alaska Fairbanks
URL	Uniform Resource Locator
UTM	Universal Transverse Mercator
WGS 84	World Geodetic System 1984

Document Creation Date

19 December 2011

Document Revision Date

08 February 2012
16 July 2012

Document URL

http://nsidc.org/data/docs/daac/icebridge/ilaks1b/index.html