2013 USACE Topographic Lidar: Rio Puerto Nuevo (PR)

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This graphic shows the coverage of the 2013 USACE Puerto Rico lidar collection.

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All flights for the project were accomplished with a single-engine Cessna 206 which provided an ideal, stable aerial base for LiDAR acquisition. This platform has relatively fast cruise speeds that are beneficial for project mobilization and demobilization while maintaining relatively slow stall speeds which can prove ideal for collection of a high-density, consistent data posting.

IPAS-TC software was used to compute Inertial SOL file to process the final LiDAR LAS files. The method works by integrating Inertial Navigation Solution by processing IMU data and the simultaneously collected GPS data from SPAN System (Position and Orientation System/Airborne Vehicle) along with observables of locally positioned GPS base station on the ground. It computes a carrier phase GPS solution and then blends it with inertial data.

This project's data was processed in strip form, meaning each flight line was processed independently. Processing the lines individually provides the data analyst with the ability to QC the overlap between lines.

Each strip was imported into a project using TerraScan (Terrasolid, Ltd.) By creating a project the various flight lines are combined while breaking the dataset as a whole into manageable pieces. This process also converts the dataset from Geographic Coordinate System (NAD 83, 2011) to NAD83, 2011 Puerto State Plane. The ellipsoid height values will be converted to PRVD02, Meters, orthometric values using Geoid 12A, provided by National Geodetic Survey (NGS).

Breaklines are collected manually, based on the LiDAR surface model in TerraModeler version 013. The classification of points as either water or ground is determined based on a combination of factors in the data: point density, voids in data returns, and flatness of the surface, and intensity value. Auxiliary information, such as the imagery provided by USACE, as well as ESRI's Hydro layer is used as an additional aid in decision making.

Survey check points were collected in the following two areas: Rio de la Plata (West Area) and Puerto Nuevo Project (East Area). Spread across each area 15-20 survey check points for each of the following classes were collected. The five classes were:

DESCRIPTION - FEATURE CODE

a. Open/Low Grass - OLG

b. Tall Grass/Crops - TGC

c. Brush/Low Trees - BLT

d. Forest - FOR

e. Wetlands - WET

Overall, 201 points were collected as described in the table below (This tables includes landscape class points, but does not include the additional points collected for vertical and horizontal control). The additional points and accuracy calculations are provided in the accuracy assessment portion of this report.

The two landscape classes with dense tall grass vegetation (Brush/low trees and Tall Grass/Crops) tested at higher than the target accuracy. Based on the thick vegetation, the LIDAR points that got to the ground in these areas were minimal, and aggressive filtering and manual edits were required in order to remove the dense vegetation from the ground point classification. As a result, the ground points in these areas were sometimes tens to hundreds of meters apart. The distance of the ground points in these locations resulted in a triangulatedtest surface that may not represent the small terrain variations around the control survey points.

Although a particular LiDAR point cannot be tested, accuracy statements can be made about the performance of the ABGPS, IMU and LiDAR sensors. The ABGPS data are quality controlled by solutions from base stations. On this project, these solutions all agreed to better than 5 cm horizontally. The IMU sensor combines the post-processed GPS data with the raw inertial data to produce a best estimate of trajectory. Automated quality control checks will not allow the IMU solution to be of a lower accuracy than the provided input from the GPS solution. The altitude of the ALS70-CM sensor (S/N 7169) on this project was 1250-meters AGL providing a spot size of 29 cm in diameter. Each return is located somewhere within the spot on the ground, meaning the location of the point is located within 14.5 cm of the center of the spot.

The NOAA Office for Coastal Management (OCM) received the files in laz format from USACE . The files contained lidar elevation and intensity measurements. The data were in State Plane, NAVD88 (orthometric) heights in feet. OCM performed the following processing for data storage and Digital Coast provisioning purposes:

1. The data were converted from State Plane coordinates to geographic coordinates.

2. The data were converted from NAVD88 (orthometric) heights in feet to GRS80 (ellipsoid) heights in meters using Geoid 12a.

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