| Processing Steps
- The ABGPS, inertial measurement unit (IMU), and raw scans are collected during the
LiDAR aerial survey. The ABGPS monitors the xyz position of the sensor and the IMU
monitors the orientation. During the aerial survey, laser pulses reflected from features
on the ground surface are detected by the receiver optics and collected by the data
logger. GPS locations are based on data collected by receivers on the aircraft and
base stations on the ground. The ground base stations are placed no more than 40 km
radius from the flight survey area.
- The ABGPS, IMU, and raw scans are integrated using proprietary software developed
by Optech and delivered with the Optech System. The resultant file is in a LAS binary
file format. The LAS version 1.2 file format can be easily transferred from one file
format to another. It is a binary file format that maintains information specific
to the LiDAR data (return number, intensity value, xyz, etc.). The resultant points
are produced in the NAD83/2007 UTM 18 North Coordinate System, with units in Meters
and referenced to the NAVD88 datum. The LiDAR mass points were processed in American
Society for Photogrammetry and Remote Sensing LAS 1.2 format. The header file for
each dataset is complete as defined by the LAS 1.2 specification. The datasets were
divided into a 1500 meter by 1500 meter tiling scheme. The tiles are contiguous, do
not overlap, and are suitable for seamless topographic data mosaics that include no
"no data" areas. The names of the tiles include numeric column and row designations
and all files utilize the LAS file extension.
- The unedited data are classified to facilitate the application of the appropriate
feature extraction filters. A combination of proprietary filters are applied as appropriate
for the production of bare earth digital elevation models (DEMs). Interactive editing
methods are applied to those areas where it is inappropriate or impossible to use
the feature extraction filters, based upon the design criteria and/or limitations
of the relevant filters. These same feature extraction filters are used to produce
elevation height surfaces.
- Filtered and edited data are subjected to rigorous QA/QC, according to the Northrop
Grumman, Advanced GEOINT Solutions Operating Unit Quality Control Plan and Procedures.
A series of quantitative and visual procedures are employed to validate the accuracy
and consistency of the filtered and edited data. Ground control is established by
Northrop Grumman, Advanced GEOINT Solutions Operating Unit and GPS-derived ground
control points (GCPs) in various areas of dominant and prescribed land cover. These
points are coded according to land cover, surface material, and ground control suitability.
A suitable number of points are selected for calculation of a statistically significant
accuracy assessment, as per the requirements of the National Standard for Spatial
Data Accuracy. A spatial proximity analysis is used to select edited LiDAR data points
within a specified distance of the relevant GCPs. A search radius decision rule is
applied with consideration of terrain complexity, cumulative error, and adequate sample
size. Accuracy validation and evaluation is accomplished using proprietary software
to apply relevant statistical routines for calculation of Root Mean Square Error (RMSE)
and the National Standard for Spatial Data Accuracy (NSSDA), according to Federal
Geographic Data Committee (FGDC) specifications.
- The Bare Earth DEM was extracted from the raw LIDAR products and attributed with the
bare earth elevation for each cell of the DEM. Bare Earth DEMs do not include buildings,
vegetation, bridges or overpass structures in the bare earth model. Where abutments
were clearly delineated, this transition occurred at the junction of the bridge and
abutment. Where this junction was not clear, the extractor used their best estimate
to delineate the separation of ground from elevated bridge surface. In the case of
bridges over water bodies, if the abutment was not visible, the junction was biased
to the prevailing stream bank so as not to impede the flow of water in a hydrographic
model. Bare earth surface includes the top of water bodies not underwater terrain,
- 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 GPS week time. The data were received in UTM Zone 18N, 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 (Unknown)
wer changed to Class 14. 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 18N) 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.