| Processing Steps
- Using a combination of Leica ALS60 and Optech Gemini LiDAR systems, 307 flight lines
of high density data, at a nominal pulse spacing (NPS) of 1.5 meter, were collected
over Maine (approximately 2327 square miles). Multiple returns were recorded for each
laser pulse along with an intensity value for each return. A total of twenty-one (21)
missions were flown April 18, 2012 May 12, 2012. Two airborne global positioning system
(GPS) base stations were used in support of the LiDAR data acquisition. 34 ground
control points were surveyed through static methods. The geoid used to reduce satellite
derived elevations to orthometric heights was Geoid09. The horizontal datum used for
this survey is North American Datum 1983 (NSRS2007), UTM19, and expressed in meters.
The vertical datum used for this survey is North American Vertical Datum 1988 (NAVD88),
and expressed in meters. Airborne GPS data was differentially processed and integrated
with the post processed IMU data to derive a smoothed best estimate of trajectory
(SBET). The SBET was used to reduce the LiDAR slant range measurements to a raw reflective
surface for each flight line. System Parameters: - LIDAR data was collected using
Leicas ALS60 and Optechs Gemini LiDAR Systems in Multi-Pulse mode. The ALS60 and Gemini
LiDAR Systems collect up to four returns (echo) per pulse, recording attributes such
as time stamp and intensity data, for the first three returns. If a fourth return
was captured, the system does not record an associated intensity value. The aerial
LiDAR was collected at the following sensor specifications: Nominal Post Spacing:
4.92 ft / 1.5 m, AGL (Above Ground Level) average flying height: 7,800 ft / 2,377
m (Leica ALS60), 6,800 ft / 2,072 m (Optech Gemini), MSL (Mean Sea Level) average
flying height: Varies by terrain, Average Ground Speed: 150 knots / 172 mph, Field
of View (full): 40 degrees, Pulse Rate: 99 kHz (Leica ALS60), 100 kHz (Optech Gemini),
Scan Rate: 38 Hz (Leica ALS60), 32 Hz (Optech Gemini), Side Lap (Minimum): 25%.
- The Leica ALS60 and Optech Gemini LiDAR system calibration and performance is verified
on a periodic basis using Woolpert's calibration range. The calibration range consists
of a large building and runway. The edges of the building and control points along
the runway have been located using conventional survey methods. Inertial measurement
unit (IMU) misalignment angles and horizontal accuracy are calculated by comparing
the position of the building edges between opposing flight lines. The scanner scale
factor and vertical accuracy is calculated through comparison of LiDAR data against
control points along the runway. Field calibration is performed on all flight lines
to refine the IMU misalignment angles. IMU misalignment angles are calculated from
the relative displacement of features within the overlap region of adjacent (and opposing)
flight lines. The raw LiDAR data is reduced using the refined misalignment angles.
- Once the data acquisition and GPS processing phases are complete, the LiDAR data was
processed immediately to verify the coverage had no voids. The GPS and IMU data was
post processed using differential and Kalman filter algorithms to derive a best estimate
of trajectory. The quality of the solution was verified to be consistent with the
accuracy requirements of the project.
- The individual flight lines were inspected to ensure the systematic and residual errors
have been identified and removed. Then, the flight lines were compared to adjacent
flight lines for any mismatches to obtain a homogenous coverage throughout the project
area. The point cloud underwent a classification process to determine bare-earth points
and non-ground points utilizing "first and only" as well as "last of many" LiDAR returns.
This process determined bare-earth points (Class 2), noise (Class 7), water (Class
9) ignored ground (Class 10), unclassified data (Class 1), overlap points (Class 11).
The bare-earth (Class 2 - Ground) LiDAR points underwent a manual QA/QC step to verify
that artifacts have been removed from the bare-earth surface. The surveyed ground
control points are used to perform the accuracy checks and statistical analysis of
the LiDAR dataset.
- Breaklines defining lakes, greater than two acres, and double-line streams, wider
than 100 feet (30.5 meters), were compiled using digital photogrammetric techniques
as part of the hydrographic flattening process and provided as ESRI Polyline Z and
Polygon Z shape files. Breaklines defining water bodies and streams were compiled
for this task order. The breaklines were used to perform the hydrologic flattening
of water bodies, and gradient hydrologic flattening of double line streams. Lakes,
reservoirs and ponds, at a nominal minimum size of two (2) acres or greater, were
compiled as closed polygons. The closed water bodies were collected at a constant
elevation. Rivers and streams, at a nominal minimum width of 100 feet (30.5 meters),
were compiled in the direction of flow with both sides of the stream maintaining an
equal gradient elevation. The hydrologic flattening of the LiDAR data was performed
for inclusion in the National Elevation Dataset (NED).
- The NOAA Coastal Services Center (CSC) received topographic files in LAS V1.2 format.
The files contained lidar elevation measurements, intensity values, scan angle values,
return information, and adjusted standard GPS time. The data were received in UTM
Zone 19N, NAD83 coordinates and were vertically referenced to NAVD88 using the Geoid09
model. The vertical units of the data were meters. CSC performed the following processing
for data storage and Digital Coast provisioning purposes: 1. Points in Class 11 (Overlap)
were changed to Class 12 (Overlap). 2. The topographic las files were converted from
orthometric (NAVD88) heights to ellipsoidal heights using Geoid09. 3. The topographic
las files were converted from a Projected Coordinate System (UTM Zone 19N) to a Geographic
Coordinate System (NAD83). 4. The topographic las files' horizontal units were converted
from meters to decimal degrees. 5. 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.