| Processing Steps
- 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 Coastal Services Center (CSC) 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. CSC 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. 6. The data were converted to LAZ format.
- The NOAA National Geophysical Data Center (NGDC) received lidar data files via ftp
transfer from the NOAA Coastal Services Center. The data are currently
being served via NOAA CSC Digital Coast at http://www.csc.noaa.gov/digitalcoast/.
The data can be used to re-populate the system. The data are archived in LAS or LAZ
The LAS format is an industry standard for LiDAR data developed by the American Society
of Photogrammetry and Remote Sensing (ASPRS); LAZ is a loseless compressed version
LAS developed by Martin Isenburg (http://www.laszip.org/). The data are exclusively
in geographic coordinates (either NAD83 or ITRF94). The data are referenced vertically
the ellipsoid (either GRS80 or ITRF94), allowing for the ability to apply the most
up to date geoid model when transforming to orthometric heights.