2002 Upper Texas Coast Lidar Point Data, Gulf of Mexico Shoreline in the Northeast 3.75-Minute Quadrant of the Lake Como 7.5-Minute Quadrangle: Post Fay Survey

This data set contains elevation data derived from a lidar survey approximately 300m wide of the Gulf of Mexico shoreline in the

Northeast Lake Como quarter-quadrangle on Galveston Island Texas. The geographic extent of the data set is equivalent to the

quarter-quadrangle plus 30 meters of overedge. The data is created by combining data collected using an Optech Inc. Airborne Laser

Terrain Mapper (ALTM) 1225 in combination with geodetic quality Global Positioning System (GPS) airborne and ground-based receivers.

The Bureau of Economic Geology, the University of Texas at Austin owns and operates an ALTM 1225 system (serial number 99d118).

This system is installed in a single engine Cessna 206 (tail number N4589U) owned and operated by the Texas State Aircraft Pooling

Board. The lidar data described by this document was collected on 18 September 2002 (26102) between 20:34 and 00:08 UTC

(actual data collection). Conditions on that day were low clouds at 335m Above Ground Level (AGL), haze, and occasional showers.

99d118 instrument settings for this flight were; laser pulse rate: 25kHz, scanner rate: 26Hz, scan angle: +/-20deg, beam divergence:

wide, altitude: 300-490m AGL, and ground speed: 70-106kts. Three GPS base stations, 2 Ashtech and 1 Trimble 4000SSI receivers (backup),

were operating during the survey. The three base stations were at the following locations: one 3.5km south of San Luis Pass, one at the

Scholes International Airport Galveston, and one on the seawall at Rollover Pass. This data set consists of 1687100 records of x,y,

and z values. The data set was generated from a larger data set and includes all valid points within the requested geographic bounds.

Original contact information:

Contact Name: Jim Gibeaut

Contact Org: Bureau of Economic Geology University of Texas at Austin

Title: Research Associate

Phone: 512-471-0344

Email: jim.gibeaut@beg.utexas.edu

The data described in this document are being used to create Digital Elevation Models of the Texas Coast. The DEMs are being

used to map the position of the shoreline along the Gulf of Mexico coast of Texas for determining rates of shoreline change following

Tropical Storm Fay (September 2002).

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The ALTM 1225 has the following specifications: operating altitude = 410-2,000 m AGL; laser pulse rate = 25 kHz;

laser scan angle = variable from 0 to +/-20 deg from nadir; scanning frequency = variable, 28 Hz at the 20 deg scan angle; and beam

divergence = 0.2 milliradian (half angle, 1/e). The ALTM 1225 does not digitize and record the waveform of the laser reflection,

but records the range and backscatter intensity of the first and last laser reflection using a constant-fraction discriminator and

two Timing Interval Meters (TIM). ALTM elevation points are computed using three sets of data: laser ranges and their associated scan

angles, platform position and orientation information, and calibration data and mounting parameters (Wehr and Lohr, 1999).

Global Positioning System (GPS) receivers in the aircraft and on the ground provide platform positioning. The GPS receivers record

pseudo-range and phase information for post-processing. Platform orientation information comes from an Inertial Measurement Unit (IMU)

containing three orthogonal accelerometers and gyroscopes. An aided-Inertial Navigation System (INS) solution for the aircraft's attitude

is estimated from the IMU output and the GPS information. Wehr, A. and U. Lohr, 1999, Airborne laser scanning - an introduction and

overview, ISPRS Journal of Photogrammetry and Remote Sensing, vol. 54, no.2-3, pp.68-82.

None planned, as needed

Any conclusions drawn from analysis of this information are not the responsibility of the Bureau

of Economic Geology or the University of Texas at Austin, the NOAA Office for Coastal Management or its partners.

This data can be obtained on-line at the following URL:

https://coast.noaa.gov/dataviewer

;

None

Users should be aware that temporal changes may have occurred since this data set was collected and some parts of this

data may no longer represent actual surface conditions. Users should not use this data for critical applications without a full

awareness of its limitations.

Selected portions from each lidar data set (last return only) were used to generate a

1m x 1m digital elevation model (DEM). Data estimated to have a horizontal accuracy of 0.01-0.03m from ground surveys using

kinematic GPS techniques were superimposed on the lidar DEM and examined for any mismatch between the horizontal position of

the ground GPS and the corresponding feature on the lidar DEM. Horizontal agreement between the ground kinematic GPS and the

lidar was within the resolution of the 1m x 1m DEM.

Ground GPS surveys were conducted within the lidar survey area to acquire ground "truth"

information. The ground survey points are estimated to have a vertical accuracy of 0.01-0.05m. Roads, which are open areas with

an unambiguous surface, were surveyed using kinematic GPS techniques. A lidar data set was sorted to find data points that fell

within 0.5m of a ground GPS survey point. The mean elevation difference between the lidar and the ground GPS was used to estimate

and remove an elevation bias from the lidar. The standard deviation of these elevation differences provides estimates of the lidar

precision. The 2002 lidar data set was determined to have an elevation bias of 0.06m when compared to ground truth. The 0.06m bias

was removed so that mean lidar elevations conform to NAVD88 with an RMSE of 0.103m.

Data were edited to remove outliers with ranges above 2200m and below -100m.

Not applicable