Issues and Answers in Quality Control of LIDAR DEMs for North Carolina DFIRMs

1. Issues and Answers in Quality Control of LIDAR DEMs forNorth Carolina DFIRMs Gary W. Thompson, RLS North Carolina Geodetic Survey David F. Maune, Ph.D., C.P. Dewberry & Davis LLC, Fairfax, VA

2. Hurricane Floyd — 1999 • Revealed limitations in the State’s flood hazard data and maps • Many maps compiled in the 1970s by approximate methods; no detailed H&H • Most of NC needed to be remapped digitally, consistent with FEMA’s Map Modernization Plan • Over 50 counties needed re-mapping immediately with new DFIRMs

3. Topography Flood Data Base + + = DFIRM DFIRM Components

4. Cooperating Technical State (CTS) • North Carolina, FEMA’s first CTS, is responsible for: • Re-surveying the State • Conducting flood hazard analyses • Producing updated DFIRMs • North Carolina Geodetic Survey (NCGS) serves as the State’s technical lead • Dewberry & Davis LLC serves as FEMA’s Map Coordination Contractor (MCC)

5. Phases I, II and III

6. Photogrammetry or LIDAR? • The North Carolina advertisement did not specify technologies to be used • Focus was on high-resolution andhigh-accuracy digital elevation data suitable for semi-automated H&H modeling • All firms proposed using LIDAR to generate the TINs and DEMs; but some proposed using photogrammetry to generate breaklines

7. Winning Teams • Watershed Concepts team includes: • EarthData International (LIDAR) • ESP Associates (ground surveying) • Greenhorne & O’Mara team includes: • 3Di EagleScan (LIDAR) • McKim & Creed (ground surveying) • Hobbs, Upchurch & Assoc. (ground surveying)

8. Delivery Order No. 1 • Task 1: LIDAR Data Acquisition • Vertical RMSE = 20 cm in coastal areas and 25 cm inland (equivalent contour interval of 2.16’ and 2.70’), the highest accuracy realistically achievable • This was a compromise from FEMA’s15-cm LIDAR standard, considered unrealistic based on prior studies • Daily calibration at local test site • Task 2: Generation of Bare-Earth ASCII files (randomly spaced)

9. LIDAR Laser Sensor • Laser scanner with mirror measures scan angles and distances for up to 50,000 pulses per second • Airborne GPS measures position • Inertial Measuring Unit (IMU) measures roll, pitch, heading • Record first/last returns

10. Issue: How best to perform LIDAR system calibrationCourtesy of EarthData International

11. Issue: How best to post-process LIDAR (These are “raw” images)Courtesy U.S. Army Topographic Engineering Center

12. Bare-earth data (post processed for vegetation/building removal)Courtesy U.S. Army Topographic Engineering Center

13. Delivery Order No. 1 (continued) • Task 3: Generation of Triangulated Irregular Network (TIN) and breaklines • Task 4: Development of 5m x 5m DEMs in ESRI GRID Float Format • Task 5: Development of DEMs in Three Additional File Formats • Task 6: Preparation of Project Report • Task 7: Production of Optional Digital Orthophoto Images

14. Digital Elevation Models (DEMs) • DEMs typically have uniform “post spacing” where x/y coordinates are evenly divisible by 5m, 10m, 30m, etc. • Interpolated from TIN data; e.g., LIDAR. • Neither TIN nor DEMpoints are clearly defined on the ground.

15. TINs — Superior for 3-D Surface Modeling; e.g., H&H Modeling • A TIN is a set of adjacent, non-overlapping triangles computed from irregularly spaced mass points with x,y coordinates and z values, plus breaklines. • Mass points can come from LIDAR or other source. • Best breaklines come from photogrammetry, then digital orthophotos.

16. Hydraulic Models Require “Representative” Cross Sections • Cross sections are carefully selected to be representative of reaches that are as long as possible, without permitting excessive conveyance change between sections. • Typically between 500’ and 2,500’ apart. • In addition to surveyed cross sections, others can be “cut” from the LIDAR data.

17. Issue: How best to generate Cross Sections

18. Watershed Concepts Surveyed cross sections at bridges Hydro-enforced stream centerlines Digital orthophoto breaklines at stream shorelines LIDAR models stream banks and overbank areas Greenhorne & O’Mara Surveyed cross sections at bridges Hydro-enforced stream centerlines Photogrammetric breaklines at tops and bottoms of stream banks LIDAR models overbank areas Issue: How best to generate Breaklines

19. Issue: How best to handle “obscured areas” and “artifacts”

20. Issue: How best to compute RMSEz of bare-earth TINs/DEMs Since TIN/DEM points not clearly defined: • Survey a minimum of 20 checkpoints in all 5 major land cover categories representative of the floodplain • Choose checkpoints on flat or uniformly sloping terrain; interpolate LIDAR points • Use no checkpoints in vegetation known to be too dense for LIDAR penetration • Discard 5% of “outliers”

21. Issue: Check points in such areas skew RMSE calculations • LIDAR has fewer areas than photogrammetry where the terrain is obscured. • One “bad” checkpoint in such areas will over-ride 1,000 “good” checkpoints elsewhere, and thus skew the results.

22. LIDAR Advantages Compared with Photogrammetry • LIDAR needs only a single line-of-sight to measure through/between trees • High-altitude LIDAR data are more accurate than from photogrammetry • LIDAR generates higher-density TINs/DEMs at lower costs • LIDAR acquires data both day and night (but not through clouds)

23. LIDAR Disadvantages Compared with Photogrammetry • LIDAR returns on water are unreliable • LIDAR is ill-suited for breaklines; e.g., 5-m point spacing could “jump” across a breakline • LIDAR is new technology; standards have not yet been developed • Contour lines are not as smooth • Streams are not automatically hydro-enforced, must be done manually

24. LIDAR contours not hydro-enforced(same problem with TINs/DEMs)

25. Conclusions • This project will demonstrate the do’s and don’ts of LIDAR for H&H modeling and serve as a model for years to come • This project will also be used to update FEMA standards

26. Issues and Answers in Quality Control of LIDAR DEMs forNorth Carolina DFIRMs QUESTIONS? ? ? ? ?