| Processing Steps
- The LiDAR data was collected between May 3, 2008 and April 25, 2009. The survey used
a Leica ALS50 Phase II laser system mounted in a Cessna Caravan 208B. The system was
set to acquire > or = 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 density
of > or = 8 points per square meter over terrestrial surfaces. The native pulse density
is the number of pulses emitted by the LiDAR system. Some types of surfaces (i.e.
dense vegetation or water) may return fewer pulses than the laser originally emitted.
Therefore, the delivered density can be less than the native density and lightly variable
according to distributions of terrain, land cover, and water bodies. The completed
areas were surveyed with opposing flight line side-lap of > or = 50% (> or = 100%
overlap) to reduce laser shadowing and increase surface laser painting. The system
allows up to four range measurements per pulse, and all discernible laser returns
were processed for the output dataset. During the LiDAR survey of the study area,
a static (1 Hz recording frequency) ground survey was conducted over monuments with
known coordinates. After the airborne survey, the static GPS data are processed using
triangulation with CORS stations checked against the Online Positioning User Service
(OPUS) to quantify daily variance. Multiple sessions are processed over the same monument
to confirm the antenna height measurements and reported position accuracy. Multiple
DGPS units are used for the ground real-time kinematic (RTK) portion of the survey.
To collect accurate ground surveyed points, a GPS base unit is set up over monuments
to broadcast a kinematic correction to a roving GPS unit. The ground crew uses a roving
unit to receive radio-relayed kinematic corrected positions from the base unit. This
method is referred to as real-time kinematic (RTK) surveying and allows precise location
measurement (sigma < or = 1.5 cm (0.6 in)).
- 1. Laser point coordinates are computed using the IPAS and ALS Post Processor software
suites based on independent data from the LiDAR system (pulse time, scan angle), and
aircraft trajectory data (SBET). Laser point returns (first through fourth) are assigned
an associated (x, y, z) coordinate along with unique intensity values (0-255). The
data are output into large LAS v. 1.1 files; each point maintains the corresponding
scan angle, return number (echo), intensity, and x, y, z (easting, northing, and elevation)
information. 2. These initial laser point files are too large to process. To facilitate
laser point processing, bins (polygons) are created to divide the dataset into manageable
sizes (< 500 MB). Flightlines and LiDAR data are then reviewed to ensure complete
coverage of the study area and positional accuracy of the laser points. 3. Once the
laser point data are imported into bins in TerraScan, a manual calibration is performed
to assess the system offsets for pitch, roll, heading, and mirror scale. Using a geometric
relationship developed by Watershed Sciences, each of these offsets is resolved and
corrected if necessary. 4. The LiDAR points are then filtered for noise, pits, and
birds by screening for absolute elevation limits, isolated points, and height above
ground. Each bin is then inspected for pits and birds manually; spurious points are
removed. For a bin containing approximately 7.5-9.0 million points, an average of
50-100 points are typically found to be artificially low or high. These spurious non-terrestrial
laser points must be removed from the dataset. Common sources of non-terrestrial returns
are clouds, birds, vapor, and haze. 5. The internal calibration is refined using TerraMatch.
Points from overlapping lines are tested for internal consistency and final adjustments
are made for system misalignments (i.e., pitch, roll, heading offsets and mirror scale).
Automated sensor attitude and scale corrections yield 3-5 cm improvements in the relative
accuracy. Once the system misalignments are corrected, vertical GPS drift is then
resolved and removed per flight line, yielding a slight improvement (<1 cm) in relative
accuracy. At this point in the workflow, data have passed a robust calibration designed
to reduce inconsistencies from multiple sources (i.e. sensor attitude offsets, mirror
scale, GPS drift) using a procedure that is comprehensive (i.e. uses all of the overlapping
survey data). Relative accuracy screening is complete. 6. The TerraScan software suite
is designed specifically for classifying near-ground points (Soininen, 2004). The
processing sequence begins by "removing" all points that are not "near" the earth
based on geometric constraints used to evaluate multi-return points. The resulting
bare earth (ground) model is visually inspected and additional ground point modeling
is performed in site-specific areas (over a 50-meter radius) to improve ground detail.
This is only done in areas with known ground modeling deficiencies, such as: bedrock
outcrops, cliffs, deeply incised stream banks, and dense vegetation. In some cases,
ground point classification includes known vegetation (i.e., understory, low/dense
shrubs, etc.) and these points are manually reclassified as non-grounds.
- The NOAA Coastal Services Center (CSC) received the files in las format. The files
contained LiDAR elevation and intensity measurements. The data were in Oregon Lambert
(NAD83), International Feet coordinates and NAVD88 (Geoid 03) vertical datum. CSC
performed the following processing to the data to make it available within the Digital
Coast: 1. The data were converted from Oregon Lambert (NAD83), International Feet
coordinates to geographic coordinates. 2. The data were converted from NAVD88 (orthometric)
heights to GRS80 (ellipsoid) heights using Geoid 03. 3. The vertical units of the
data were converted from International feet to meters. 4. The data were sorted by
latitude and the headers were updated.
- 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 format. 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 of 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 to the ellipsoid (either GRS80 or ITRF94), allowing for
the ability to apply the most up to date geoid model when transforming to orthometric