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North Carolina LIDAR Lessons Learned
Gary W. Thompson
North Carolina Geodetic Survey
David F. Maune
Dewberry & Davis LLC
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BACKGROUND
Hurricane Floyd
In North Carolina, heavy rainfall from Hurricane Floyd in 1999 revealed
limitations in the state’s flood hazard data and maps. The majority of the
state’s Flood Insurance Rate Maps (FIRMs) were at least 10 years old, with
many maps compiled from approximate studies in the 1970s without detailed
hydrologic and hydraulic (H&H) analyses. With the Federal Emergency
Management Agency’s (FEMA) limited budget, North Carolina received on
average only one updated flood insurance study for one county per year. After
Hurricane Floyd, it was clear that most of North Carolina needed to be
remapped digitally, consistent with FEMA’s map modernization plan, using
improved H&H modeling and analyses that define current flood risks with
greater accuracy. Approximately 50 counties needed to be remapped as soon
as possible.
Cooperating Technical State
As part of FEMA’s Cooperating Technical Partners (CTP) Program, North
Carolina entered into a Cooperating Technical State (CTS) agreement
whereby the state assumes primary ownership of, and responsibility for, the
National Flood Insurance Program (NFIP) maps for all North Carolina
communities, to include resurveying the state, conducting flood hazard
analyses, and producing updated, Digital FIRMs (DFIRMs). DFIRMs employ
the latest geographic information system (GIS) technology, facilitate updating
in the future, and allow on-line distribution. Furthermore, the new DFIRMs
include flood-hazard data overlaid on digital orthophotos, allowing
homeowners to clearly recognize their homes relative to floodplain
boundaries. The state provided most of the funding, something it had learned
is key to getting FEMA and many other federal agencies to sign teaming
agreements.
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North Carolina LIDAR Lessons
LIDAR
For the first phase of this project, the state advertised for firms that would
acquire high-accuracy digital elevation data and perform the flood studies for
six major watersheds. However, it did not specify what technology was to be
used. Nonetheless, all proposals were from firms that had both
photogrammetric and Light Detection and Ranging (LIDAR) capabilities, and
all proposed the use of LIDAR for the North Carolina Floodplain Mapping
Project. Two prime contractors were selected. The state specified that digital
elevation data should satisfy a vertical root-mean-square error (RMSE) of 20cm for coastal counties and 25-cm for inland counties. These accuracy
statistics were to be computed on the best 95% of the checkpoints, allowing
5% of the checkpoints to qualify as outliers because of dense vegetation or
uncleaned artifacts. Independent surveyors were hired by the North Carolina
Geodetic Survey to survey 120 checkpoints per county, 20 each in four land
cover categories (dirt/grass, weeds/crops, scrub, and urban areas) and 40 each
in forested areas.
LESSONS LEARNED: ISSUE PAPERS
The state prepared a series of issue papers that either prevented the
development of potential issues or addressed them as they became known.
These issue papers are available for review at the state’s website at
http://www.ncfloodmaps.com. As summarized below, the issue papers include
valuable information about the lessons learned from North Carolina’s LIDAR
experience.
Issue Paper No. 5 addresses quality control (QC) of the LIDAR elevation
data. It includes detailed procedures for the survey of independent QC
checkpoints, procedures for LIDAR accuracy assessment, procedures for
assessing errors to identify and correct systematic errors, and procedures for
final resolution of problems should the contractor disagree with the results of
the accuracy assessment reports.
Issue Paper No. 7 (IP-7) addresses writing the specifications for LIDAR
data acquisition, generation of bare-earth ASCII files, generation of breaklines
and Triangulated Irregular Networks (TINs), and development of Digital
Elevation Models (DEMs) in multiple file formats. The first IP-7 issue
involved the controversial requirement for calibration data to be collected
during each flight over a calibration course established at each airport. Both
LIDAR vendors subsequently learned the benefits of daily calibration and
prompt processing of calibration data to detect systematic errors.
Thompson and Maune
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A second IP-7 issue pertained to RMSE accuracy assessment. The state
learned that almost every county survey would have included erratic, skewed
results if it had used RMSE calculations on all 100% of the surveyed
checkpoints, instead of the best 95%. Based on lessons learned in North
Carolina, the National Digital Elevation Program recently adopted the policy
of using a 95-percentile alternative to RMSE calculations in forested areas.
A third IP-7 issue pertained to the designation of nominal point spacing.
The state did not specify the nominal point spacing of the raw LIDAR data,
leaving it up to the contractors to determine what point density was most
appropriate for generating elevation data with the specified vertical accuracy.
However, questions remain as to the cost effectiveness of denser datasets. The
state is currently working with NASA, FEMA, and Dewberry & Davis LLC
on a study to determine the impact on hydraulic modeling of using nominal
post spacings of one, two, three, four, five, or six meters.
A fourth IP-7 issue pertained to units. The delivery order for both
contractors required all data to be provided in North Carolina’s state plane
coordinates, with all horizontal and vertical coordinates in meters to three
decimal places. The DEMs were to have five-meter post spacing, and tile
sizes were 5,000-meter squares. However, one prime contractor was relying
on proprietary H&H modeling software that only works in U.S. survey feet.
When the state allowed one contractor to provide data in U.S. survey feet,
with 50-foot post spacings, and a 20,000-foot tiling scheme, a new issue was
created in which datasets provided to the public would not be standardized
unless the state paid extra for the conversions. This proved to be a costly
lesson learned.
A fifth IP-7 issue pertained to uncleaned artifacts in LIDAR data. The
state allowed 5% of the checkpoints to be discarded in the RMSE calculations
in order to account for uncleaned artifacts, implying that 95% clean is clean
enough. However, the LIDAR industry has no objective method for assessing
the cleanness of LIDAR datasets. Because cleaning the final few percent of
artifacts significantly increases the cost of a LIDAR project, the assessment
and treatment of uncleaned artifacts remains an unresolved issue for which the
LIDAR industry has not yet proposed a solution.
A sixth IP-7 issue pertained to data thinning. Because the Phase I LIDAR
data were collected by sensors generating between 4,000 and 50,000 laser
pulses per second, some datasets are much denser than others. The state
learned that vendors need to develop an intelligent thinning process in order
to prevent computers from being clogged with dense data where not needed,
while allowing dense data elsewhere to provide valuable information near
breaklines.
A seventh IP-7 issue pertained to generation of breaklines. For hydraulic
modeling, the two prime contractors used totally different methods for
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North Carolina LIDAR Lessons
generating either 2-D breaklines at shorelines or 3-D breaklines at the
tops/bottoms of stream banks. An analysis still needs to be performed to
determine whether one method is superior to the other. This lesson remains to
be learned.
Issue Paper No. 29 (IP-29) addresses the maintenance, archiving, and
dissemination of North Carolina’s LIDAR data. The state (and the LIDAR
industry) learned the need to identify a standard format in which the fullreturn LIDAR data would be archived by the Eros Data Center. A preferred
format— known as the .LAS format—is being finalized.
The state also learned the need to develop standard procedures for
archiving and disseminating bare-earth mass points and
breaklines—procedures that define responsibility for cleaning of artifacts;
standardize data format, reference grid, and units; and specify how mass
points can be provided to the public via the internet. The state learned that
TINs would not need to be archived and disseminated to the public because
they can be easily regenerated from the mass points and breaklines.
CONCLUSIONS
Of all the issues summarized above, the one that caused the most difficulty
pertained to the standardization of units. The state had originally indicated
that metric units would be used for horizontal and vertical coordinates, for
DEM post spacings, and for tile sizes. This decision was subsequently
reversed as the project was in process, allowing one contractor to use U.S.
survey feet. All other issues were resolved in an efficient manner through the
use of the cited issue papers, except where additional study is still needed.
North Carolina and FEMA still plan to determine the optimum nominal post
spacing of raw LIDAR data for hydraulic modeling, optimum procedures for
generation of breaklines, and procedures for QC to determine the cleanliness
of bare-earth surfaces from artifacts. Overall, this LIDAR project has been a
tremendous success.
The first new DFIRMs are now available on-line at
http://www.ncfloodmaps.com. Click on the top button that reads Digital Flood
Maps.