2006 FEMA Lidar: Rhode Island Coastline

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This graphic shows the lidar coverage for the 2006 FEMA project for the coastal regions of Rhode Island in Washington County.

Date that the source FGDC record was last modified.

Converted from FGDC Content Standards for Digital Geospatial Metadata (version FGDC-STD-001-1998) using 'fgdc_to_inport_xml.pl' script. Contact Tyler Christensen (NOS) for details.

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- Airborne GPS Kinematic

Airborne GPS kinematic data was processed on-site using GrafNav kinematic On-The-Fly (OTF) software. Flights were flown with a minimum of 6 satellites in view (13o above the horizon) and with a PDOP of better than 4.5. Distances from base station to aircraft were kept to a maximum of 35 km, to ensure a strong OTF (On-The-Fly) solution. For all flights, the GPS data can be classified as excellent, with GPS residuals of 5cm average but no larger than 12 cm being recorded.

- Calculation of 3D laser points (raw data)

The post-processing software to derive X, Y, Z values from roll, pitch, yaw, and range is Optech's Realm.

Data

- Classification and Editing

The data was processed using the software TerraScan, and following the methodology described herein. The initial step is the setup of the TerraScan project, which is done by importing client provided tile boundary index encompassing the entire project areas. The 3D laser point clouds, in binary format, were imported into the TerraScan project and divided in 325 tiles, as per the contract specifications. tiled, the laser points were classified using a proprietary routine in TerraScan. This routine removes any obvious outliers from the dataset following which the ground layer is extracted from the point cloud. The ground extraction process encompassed in this routine takes place by building an iterative surface model. This surface model is generated using three main parameters: building size, iteration angle and iteration distance. The initial model is based on low points being selected by a "roaming window" with the assumption is that these are the ground points. The size of this roaming window is determined by the building size parameter. The low points are triangulated and the remaining points are evaluated and subsequently added to the model if they meet the iteration angle and distance constraints. This process is repeated until no additional points are added within an iteration.

A second critical parameter is the maximum terrain angle constraint, which determines the maximum terrain angle allowed within the classification model. The data is then manually quality controlled with the use of

hillshading, cross-sections and profiles. Any points found to be of class vegetation, building or error during the quality control process, are removed from the ground model and placed on the appropriate layer. An integrity check is also performed simultaneously to verify that ground features such as rock cuts, elevated roads and crests are present. Once data has been cleaned and complete, it is then reviewed by

a supervisor via manual inspection and through the use of a hillshade mosaic of the entire project area.

- Projection Transformation

The data was processed in the native UTM zone in meters and then transformed to the Rhode Island State Plane final projection system and US survey feet using an in-house transformation software which uses the Coorpscon DLL.

- General Overview

The Airborne LiDAR survey was conducted using an OPTECH 3100EA flying at a nominal height of 1550 meters AGL with a total angular coverage of 40 degrees. Flight line spacing was nominally 564 meters providing overlap of 50% on adjacent flight lines. Lines were flown in east/west and north/south orientated blocks to best optimize flying time considering the layout for the project.

- Aircraft

A Piper Navajo, registration C-FQQB was used for the survey. This aircraft has a flight range of approximately 6.5 hours and was flown at an average altitude of 1550, thereby encountering flying altitudes of approximately 1550 meters above sea level (ASL). The aircraft was staged from the East Haven CT Airport, and ferried daily to the project site for flight operations.

- GPS Receivers

A combination of Sokkia GSR 2600 and NovAtel DL-4+ dual frequency GPS receivers were used to support the airborne operations of this survey and to establish the GPS control network.

- Number of Flights and Flight Lines

For both Connecticut and Rhode Island Coastline Sites; a total of 6 missions were flown for this project with flight times ranging approximately 31 hours under good meteorological and GPS conditions. 33 flight lines were flown over the Connecticut Coastline site to provide complete coverage.

Quantitative vertical positional accuracy assessment:

Independent accuracy testing was performed by Dewberry & Davis using 20 high accuracy quality control checkpoints distributed throughout the project in four major land cover types (open terrain, weeds/crops, forest, urban) . Dewberry used testing procedures consistent with those specified in the National Standard for Spatial Data Accuracy (NSSDA) as well as LiDAR accuracy testing guidelines and specifications published by the Federal Emergency Management Agency (FEMA), National Digital Elevation Program (NDEP), and American Society for Photogrammetry and Remote Sensing (ASPRS).

The NOAA Office for Coastal Management (OCM) received topographic LAS files as part of the USGS CLICK elevation data shipment. The files contained lidar easting, northing, elevation, as well as ancillary collection attributes. The data were received in Rhode Island State Plane Zone 3800, NAD83 coordinates and were vertically referenced to NAVD88 using the Geoid03 model. The vertical and horizontal units of the data were in U.S. feet. 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 Geoid03 model.

3. The LAS files were converted from a Projected Coordinate System (RI Zone 3800h) to a Geographic Coordinate system with decimal degree units.