2011 FEMA Lidar: Chemung Watershed (NY) (AOI 2)

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This graphic shows the lidar coverage for the 2011 FEMA Chemung Watershed AOI-2 project in New York state.

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Acquisition

LiDAR data was acquired by Tuck Mapping Solutions, Inc. (TMSI) for the

Chemung Watershed and broken down into two AOIs based on the level of

processing performed on the dataset. AOI-1 covers approximately 709 square

miles, which is the full extent of the acquisition boundary plus a 100-meter buffer

around the perimeter of this boundary. AOI-1 was delivered as full-swath,

calibrated, and boresighted flight lines in LAS format. A subset of AOI-1,

delineated by a buffer around the major stream networks, covers approximately 308

square miles. This dataset (AOI-2) was processed by TMSI to classify bare-earth

ground points and hydro features and was delivered as a tiled, classified, point

cloud in LAS format using the following classification scheme:

Class 1 - Processed but unclassified

Class 2 - Bare-earth ground

Class 7 - Low points and noise

Class 9 - Water

Class 11 - Withheld

Processing

Processing of the LiDAR data begins with refinement of the initial boresight alignment parameter in the ALS Processor configuration file (.reg) delivered with the raw data. For projects that have more than one lift, the boresight for each lift has to be completed individually because it may differ slightly from lift to lift. Lift boresighting is accomplished using the tri-directional calibration flight lines over the project area. One calibration flight line is flown bi-directionally overlapping a project flight line within the lift. This bi-directional calibration will also be used as a parallel flight line with the adjacent flight line. There is a cross flight line collected perpendicular to both. All three lines along with the parallel project flight line are examined to ensure that they agree, within expected system tolerances, in the overlapping areas. The two bi-directional flight lines are used to diagnose Roll and Pitch. The two parallel flight lines are used to diagnose and correct Heading error. The two perpendicularly overlapping flight lines are used to examine Variable Scan Angle error. To begin lift boresight, the raw LiDAR data of the calibration flight lines will be processed with the initial boresight parameters determined from the LiDAR Sensor Calibration. Once the boresighting is done for the calibration flight lines, the adjusted settings will be applied to the complete lift and checked for consistency. For a well-maintained LiDAR system, functioning correctly under normal operating conditions, actual boresight angles can be considered constant throughout a single mission. Therefore, once the boresight angles have been adjusted based on the calibration flight lines, the same corrections can be applied to the entire lift. Under optimal circumstances, the boresight parameters determined for the calibration flight lines should be the same for all flight lines in the lift, but residual errors can occur. To correct for this, all of the overlaps between flight lines (side lap) and intersections of the project cross flight lines should be examined for internal consistency. If the results of the boresights start showing drift in the middle of the lift or the misalignment between flight lines starts exceeding project accuracy specifications, boresight parameters need to be adjusted to correct these errors.

Once boresight adjustments are completed for each individual lift, the technician checks and corrects the vertical misalignment of all the flight lines and also the matching between data and ground truth. This process includes calculating the zbias value for each flight line so that all flight lines are vertically aligned and the entire data set match to the ground control points within the project specified accuracy range. The technician will run a final vertical accuracy check after the z correction. The result will be analyzed against the project specified accuracy to

verify it meets the requirement.

Quality

Risk Assessment, Mapping, and Planning Partners (RAMPP) performed a limited review of the Chemung Watershed, New York dataset. 100% of the data was checked for completeness and 5% of the data was visually examined at the micro level for qualitative issues according to the scope of work. A vertical accuracy assessment was performed on the first return points in the Level 1-processed dataset (AOI-1) and the full point cloud in the Level 2-processed dataset (AOI-2). No major completeness or quality issues were identified. Both datasets meet the Federal Emergency Management Agency's (FEMA) vertical accuracy specifications.

Based on the limited qualitative and vertical accuracy assessments conducted by RAMPP on the data delivered, the Chemung Watershed, NY delivery meets the applicable project specifications as set forth by the IDIQ Subcontract # HSFEHQ-09-D- 0369-U005, Task Order HSFEHQ-10-J-0006, revised September 3, 2010.

The NOAA Office for Coastal Management (OCM) received AOI 2 of the Chemung Watershed project boundary. The dataset included topographic LAS files as part of a FEMA data delivery. The files contained lidar easting, northing, elevation, as well as ancillary collection attributes. The data were received in UTM 18 (meters) and were vertically referenced to NAVD88 using the "latest" geoid, which was assumed to be the Geoid09 model due to the provided collection dates (as GPS times were usable for this determination). OCM performed the following processing for data storage and Digital Coast provisioning purposes:

1. The LAS files were converted from feet to meters, removing erroneous elevations.

2. The LAS files were converted from orthometric (NAVD88) heights to ellipsoidal heights using the Geoid09 model but remaining in meters.

3. The LAS files were converted from a Projected Coordinate System to a Geographic Coordinate system with decimal degree units.