| Processing Steps
- Acquisition. The LiDAR survey utilized a Leica ALS50-II sensor in a Cessna Caravan
208B. Depending on acquisition day, weather, and terrain, the Leica systems were set
to acquire 105,000 laser pulses per second (i.e.105 kHz pulse rate) and flown at 900
meters above ground level (AGL), capturing a scan angle of +/- 14 degrees from nadir.
These settings were developed to yield points with an average native pulse density
of 8 pulses per square meter over terrestrial surfaces. It is not uncommon for some
types of surfaces to return fewer pulses than the laser originally emitted. These
discrepancies between ?native? and ?delivered? density will vary depending on terrain,
land cover, and the prevalence of water bodies. All areas were surveyed with an opposing
flight line side-lap of more than 60% (less than 100% overlap) to reduce laser shadowing
and increase surface laser painting. The Leica laser systems allow up to four range
measurements (returns) per pulse, and all discernable laser returns were processed
for the output dataset. To accurately solve for laser point position (geographic coordinates
x, y, z) the positional coordinates of the airborne sensor and the attitude of the
aircraft were recorded continuously throughout the LiDAR data collection mission.
Aircraft position was measured twice per second (2 Hz) by an onboard differential
GPS unit. Aircraft attitude was measured 200 times per second (200 Hz) as pitch, roll
and yaw (heading) from an onboard inertial measurement unit (IMU). To allow for post-processing
correction and calibration, aircraft/sensor position and attitude data are indexed
by GPS time.
- 1. Resolved kinematic corrections for aircraft position data using kinematic aircraft
GPS and static ground GPS data. Software - Waypoint GPS v.8.10, Trimble Business Center
2.6 2. Developed a smoothed best estimate of trajectory (SBET) file that blends post-processed
aircraft position with attitude data. Sensor head position and attitude were calculated
throughout the survey. The SBET data were used extensively for laser point processing.
Software - IPAS TC v.3.1 3. Calculated laser point position by associating SBET position
to each laser point return time, scan angle, intensity, etc. Created raw laser point
cloud data for the entire survey in *.las (ASPRS v. 1.2) format. Software - ALS Post
Processing Software v.2.74, Corpscon 6 4. Imported raw laser points into manageable
blocks (less than 500 MB) to perform manual relative accuracy calibration and filter
for pits/birds. Ground points were then classified for individual flight lines (to
be used for relative accuracy testing and calibration). Software - TerraScan v.12.004
5. Using ground classified points per each flight line, the relative accuracy was
tested. Automated line-to-line calibrations were then performed for system attitude
parameters (pitch, roll, heading), mirror flex (scale) and GPS/IMU drift. Calibrations
were performed on ground classified points from paired flight lines. Every flight
line was used for relative accuracy calibration. Software - TerraMatch v.12.001 6.
Position and attitude data were imported. Resulting data were classified as ground
and non-ground points. Statistical absolute accuracy was assessed via direct comparisons
of ground classified points to ground RTK survey data. Data were then converted to
orthometric elevations (NAVD88) by applying a Geoid03 correction. Software: TerraScan
v.12.004, TerraModeler v.12.002 7. Bare Earth models were created as a triangulated
surface and exported as ArcInfo ASCII grids at a 3?foot pixel resolution. Highest
Hit models were created for any class at 3-foot grid spacing and exported as ArcInfo
ASCII grids. Software - TerraScan v.12.004, ArcMap v.10.0, TerraModeler v.12.002
- The NOAA Coastal Services Center (CSC) downloaded topographic files in LAZ format
from PSLC's website. The files contained lidar easting, northing, elevation, intensity,
return number, class, scan angle and GPS time measurements; the data was received
in state plane Washington (in feet) and vertical coordinates were referenced to NAVD88
in feet using the Geoid03 model. CSC performed the following processing for data storage
and Digital Coast provisioning purposes: 1. The All-Return LAZ files were checked
for bad elevations 2. The laz files were converted from a Projected Coordinate System
(SP 4602) to a Geographic Coordinate system (NAD83) 3. The laz files were then converted
to ellipsoidal vertical units in meters using the geoid03 conversion.
- 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.