2006 FEMA New Jersey Flood Mitigation Lidar: Middlesex County

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LiDAR data representing Middlesex County | Type of Source Media: Hard Drive

-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. All of the

lines were flown in a north/south direction for the project.

-Aircraft

A Piper Navajo, registration C-GPJT, was used for the survey. This aircraft has a flight range of approximately 4.5 hours and was

flown at an average altitude of 1550 meters AGL (Above Ground Level), thereby encountering flying altitudes of approximately 1550

meters above Mean Sea Level (MSL). The aircraft was staged from the Linden, NJ 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

A total of 4 missions were flown for this project with flight times ranging approximately 16.25 hours under good meteorological and

GPS conditions. 55 flight lines were flown over the project area to provide complete coverage. Lines were flown at 564 meter spacing

to achieve 50% lateral overlap.

-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 (13 degrees 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 5 cm average but no larger than 8 cm being recorded.

-Calculation of 3D laser points (raw data)

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

-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 425 tiles, as per the contract specifications. Once 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 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 New Jersey State Plane final projection system and US survey

feet using an in-house transformation software which utilizes Corpscon DLL.

The NOAA Office for Coastal Management (OCM) received the files in las format. The files contained Lidar elevation and intensity measurements.

The data were in projected in New Jersey State Plane (NAD83) coordinates, and referenced to the orthometric datum NAVD88 using Geoid 03.

OCM performed the following processing to the data to make it available within the Digital Coast:

1. The data were converted from New Jersey State Plane (NAD83) coordinates to geographic coordinates (NAD83).

2. The data were converted from NAVD88 (orthometric) heights to GRS80 (ellipsoid) heights using Geoid 03.

3. The LAS data were sorted by latitude and the headers were updated.