2006 URS Corporation Bare Earth Topographic Lidar: Shawsheen River, Massachusetts

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This graphic shows the lidar coverage for Essex and Middlesex Counties, Massachusetts.

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Terrasurv was tasked to perform a geodetic control survey in support of LIDAR mapping of the Shawsheen river area in Essex and Middlesex Counties,

Massachusetts. The Global Positioning System (GPS) was used in a static differential mode to measure the interstation vectors of the network. The

National Spatial Reference System (NSRS) was used to provide control for the network. Continuously Operating Reference Station WMTS was used, along

with two ground stations of the NSRS. Two Trimble dual frequency receivers were used on day 361 of 2006. A base receiver was set up near on C 35, at

the Lawrence Municipal Airport. This point was also used by the flight crew during the aerial data acquisition phase. The two northerly LIDAR points

(SR-1 and SR-5) were surveyed using this station as a base. The other three LIDAR stations, and benchmark V 34, were surveyed using the CORS as a

base. The horizontal datum was the North American Datum of 1983, 1996 adjustment (NAD 1983 1996), and the vertical datum was the North American

Vertical Datum of 1988 (NAVD 1988) Geoid03.

URS contracted EarthData International, Inc. (EarthData) to collect and deliver high quality topographic elevation point data derived from multiple

return, light detection and ranging (Lidar) measurements for an area of interest totaling approximately 82 square miles in Essex and Middlesex

Counties, Massachusetts. Data was collected at a nominal three meter (3) meter post spacing between points at an altitude of 2438 meters (8,000 feet)

above mean terrain. This data was used to produce a bare-earth surface model and hydro-enforced breaklines for the project. The aerial acquisition

was conducted on 16 December, 2006 using and aircraft (tail number N2636P). Lidar data was captured using an ALS-50 Lidar system, s/n ALS036,

including an inertial measuring unit (IMU) and a dual frequency GPS receiver.

EarthData has developed a unique method for processing lidar data to identify and remove elevation points falling on vegetation, buildings, and

other aboveground structures. The algorithms for filtering data were utilized within EarthData's proprietary software and commercial software written

by TerraSolid. This software suite of tools provides efficient processing for small to large-scale, projects and has been incorporated into ISO 9001

compliant production work flows. The following is a step-by-step breakdown of the process.

1. Using the lidar data set provided by EarthData, the technician performs calibrations on the data set.

2. Using the lidar data set provided by EarthData, the technician performed a visual inspection of the data to verify that the flight lines overlap

correctly. The technician also verified that there were no voids, and that the data covered the project limits. The technician then selected a

series of areas from the data set and inspected them where adjacent flight lines overlapped. These overlapping areas were merged and a process

which utilizes 3-D Analyst and EarthData's proprietary software was run to detect and color code the differences in elevation values and profiles.

The technician reviewed these plots and located the areas that contained systematic errors or distortions that were introduced by the lidar sensor.

3. Systematic distortions highlighted in step 2 were removed and the data was re-inspected. Corrections and adjustments can involve the application

of angular deflection or compensation for curvature of the ground surface that can be introduced by crossing from one type of land cover to

another.

4. The lidar data for each flight line was trimmed in batch for the removal of the overlap areas between flight lines. The data was checked against

a control network to ensure that vertical requirements were maintained. Conversion to the client-specified datum and projections were then

completed. The lidar flight line data sets were then segmented into adjoining tiles for batch processing and data management.

5. The initial batch-processing run removed 95% of points falling on vegetation. The algorithm also removed the points that fell on the edge of

hard features such as structures, elevated roadways and bridges.

6. The operator interactively processed the data using lidar editing tools. During this final phase the operator generated a TIN based on a desired

thematic layers to evaluate the automated classification performed in step 5. This allowed the operator to quickly re-classify points from one

layer to another and recreate the TIN surface to see the effects of edits. Geo-referenced images were toggled on or off to aid the operator in

identifying problem areas. The data was also examined with an automated profiling tool to aid the operator in the reclassification.

7. The point cloud data were delivered in LAS format.

3-D breaklines were created for the creation of a completely new hydrology dataset specifically tailored to meets the needs of the users of terrain

data.

1) Breaklines were generated for all streams draining greater than approximately 1 square mile.

2) Two-dimensional lines defining the centerline and banks of those streams were manually digitized into Microstation format from the available 2002

source digital aerial imagery using lidar hillshades as an ancillary reference.

3) Breaklines were collected, unbroken through closed water bodies and culverts, as well as under roads, railroads, and bridges, in order to maintain

proper stream network connectivity.

4) The entire breakline dataset was checked to ensure integrity of the linework with respect to topologic structure, connectivity, and positive downhill

stream flow.

5) Single line streams were collected with the following criteria: Any area of drainage that did not meet the criteria to be collected as a double banked

stream or closed water body.

6) Double-banked streams were collected with the Any area of drainage that was at least 40 feet wide for a distance of greater than 540 feet, and

excluding closed water bodies. Double banked streams were delivered as linear features.

7) Artificial path lines were collected as the center line for all double banked streams and closed water bodies. Artificial paths were also created for

closed water bodies where no flow path was delineated coming into or leaving the water body, and/or where the closed water body existed as the origin

source of a flow path.

8) Artificial paths that are drawn in the two instances above go through the center of the water body and stop halfway through it. Artificial paths were

only drawn for water bodies that fell on a streamline.

9) Any islands found within water bodies were collected in instances where trees were visible on them (indicating that these are permanent features). These

islands were delivered as separate polyline features if present.

10) Any and all closed water bodies were collected, regardless of size, excluding such features as swimming pools, for the entire project area.

11) Water bodies were delivered as polygon features.

12) Single line streams, double banked streams, artificial paths and closed water bodies have unique attribution such that one feature type can be easily

distinguished from another.

13) Linework was delivered in ESRI shape file format.

14) The 3D Hydro Breaklines were developed for the sole purpose of supporting flood mapping and should not be used to generate contours

The NOAA Office for Coastal Management (OCM) received topographic files las V1.1 format. The files contained lidar elevation measurements, Class 2 Points, return

information, scan angle, intensity values and GPS Week Time. The data were received in Massachusetts State Plane Mainland Zone 2001, NAD83 coordinates

and were vertically referenced to NAVD88 using the Geoid03 model. The vertical units of the data were meters. OCM performed the following processing for

data storage and Digital Coast provisioning purposes:

1. The GPS Week Time was converted to Adjusted Standard GPS Time.

2. The las files were changed from V 1.1 to V 1.2.

3. The las files were converted from orthometric (NAVD88) heights to ellipsoidal heights using Geoid03.

4. The las files were converted from a Projected Coordinate System (MA SP Mainland) to a Geographic Coordinate System (NAD83).

5. The las files' horizontal units were converted from meters to decimal degrees.