| Processing Steps
- Acquisition. The LiDAR surveys utilized two different laser systems?the Leica ALS50
Phase II and the Optech 3100. Flight parameters were different for each system, resulting
in different native pulse densities (the number of pulses emitted by the LiDAR system
from the aircraft). 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. All study areas were surveyed with opposing
flight line side-lap of =50% (=100% overlap) to reduce laser shadowing and increase
surface laser painting. Both laser systems allow up to four range measurements per
pulse, and all discernable laser returns were processed for the output dataset.
- 1. Flight lines and data were reviewed to ensure complete coverage of the study area
and positional accuracy of the laser points. 2. Laser point return coordinates were
computed using ALS Post Processor software, IPAS Pro GPS/INS software, and Waypoint
GPS, based on independent data from the LiDAR system, IMU, and aircraft. 3. The raw
LiDAR file was assembled into flight lines per return with each point having an associated
x, y, and z coordinate. 4. Visual inspection of swath to swath laser point consistencies
within the study area were used to perform manual refinements of system alignment.
5. Custom algorithms were designed to evaluate points between adjacent flight lines.
Automated system alignment was computed based upon randomly selected swath to swath
accuracy measurements that consider elevation, slope, and intensities. Specifically,
refinement in the combination of system pitch, roll and yaw offset parameters optimize
internal consistency. 6. Noise (e.g., pits and birds) was filtered using ALS postprocessing
software, based on known elevation ranges and included the removal of any cycle slips.
7. Using TerraScan and Microstation, ground classifications utilized custom settings
appropriate to the study area. 8. The corrected and filtered return points were compared
to the RTK ground survey points collected to verify the vertical and horizontal accuracies.
9. Points were output as laser points, TINed and GRIDed surfaces
- The NOAA Coastal Services Center (CSC) downloaded topographic files in text format
from PSLC's website. The files contained lidar easting, northing, elevation, intensity,
return number, class, scan angle and GPS time measurements. Lake Roosevelt, John Day
River and Lower Okanogan data was received in UTM Zone 10 (in meters); Wenatchee and
Methow data were received in UTM Zone 11 (in meters); all datasets 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 All-Return ASCII txt files were parsed to LAS files.
2. The All-Return ASCII files were converted from txt format to las format using LASTools'
txt2las tool and reclassified to fit the CSC class list, N=1 (unclassified), G=2 (ground).
3. Bad elevations were removed the las files. 4. The las files were converted from
a Projected Coordinate System (UTM Zone 10/11) to a Geographic Coordinate system (NAD83).
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.