FirstFloorElevations _ncss

Slide 1: 

Flood Analysis and Floodplain Mapping: Where is the first floor?

Building Footprint Collection : 

Building Footprint Collection

Slide 3: 

Collect All structures for the state of NC over 1000 Sq feet. Approximately 7.5 million Structures

Slide 4: 

Vulnerabilities- Building Collection

Slide 5: 

Tasks Determine height of the structures Using data from All Return LiDAR dataset Determine lowest adjacent grade and highest adjacent grade for buildings From available data Create from TINs Develop methodology for determining first floor elevations Vulnerabilities- Building Collection

Slide 6: 

Vulnerabilities- Building Collection

Slide 7: 

LiDAR Derived 1st floor elevations

Slide 8: 

Issue need to have a method to determine Highest Adjacent Grade (HAG), Lowest Adjacent Grade (LAG) and First Floor Elevations (FFE) for all buildings in or near floodplains in NC Cost prohibitive to determine in field HAG, LAG and FFE

Slide 9: 

Proposed Solution Develop algorithm to perform terrain take-offs on building footprints to determine HAG and LAG Determine FFE by developing relationships by building type between HAG/LAG and FFE based on known data (FEMA Elevation Certificates, other locally available data) HAG, LAG and FFE

Slide 10: 

HAG/LAG Preliminary Results Case study performed in Mecklenburg County using 1200 known elevation certificates as a comparison points Building footprints takeoffs performed to obtain HAG and LAG using both TINs and DTM points Comparisons made between known values and terrain takeoffs HAG, LAG and FFE

Slide 11: 

HAG/LAG Conclusions HAG/LAG terrain takeoffs produce reasonable results when compared to known values HAG takeoffs compare better to known values TIN and DTM takeoffs produced similar results – use TIN takeoffs for efficiency HAG, LAG and FFE

Slide 12: 

FFE Preliminary Results Known HAG and LAG elevations were subtracted from known FFE values to obtain average difference by building type FFE Recommendations Use avg. known difference between FFE and HAG, add to HAG terrain takeoff elevation to estimate FFE for unknown buildings HAG, LAG and FFE

Slide 13: 

FFE Recommendations Use avg. known difference between FFE and HAG, add to HAG terrain takeoff elevation to estimate FFE for unknown buildings HAG, LAG and FFE

Hag/Lag/First Floor Elevation Issues : 

Hag/Lag/First Floor Elevation Issues This method is based on Elevation Certificates. To what level are we confident we can acquire a sufficient number of Elevation certificates Is there anything in the elevation certificates that will cause a hindrance to accurate calculations? Will this method calculate HAG and LAG with sufficient accuracy? Total destruction is often based on flooding of outlets. Building codes associate 18 inches with outlet level. Should the accuracy standards be 18 inches? Is that possible?

Slide 15: 

Better Resolution- How can we get it?

Slide 16: 

Problem: Key building attribute needed for flood risk assessment is finished floor elevations (FFE). Parcel databases do not contain this information IHRM team is researching available Elevation Certificate database FEMA Local Databases At best, only a small percentage of homes in or near the SFHA will have elevation certificates Goal – attribute all buildings within the 500-year floodplain with derived estimate for FFE within the remainder of the county HAG, LAG and FFE

Estimate Scale of Building Attribution : 

Estimate Scale of Building Attribution

Slide 18: 

HAG, LAG and FFE

Methodologies evaluated for pilot communities : 

Methodologies evaluated for pilot communities Traditional Survey LiDAR Derived HAG, LAG, FFE High Resolution Oblique Imagery Mobile LiDAR System Inclinometer Approach Reflector-less Field Survey HAG, LAG and FFE

Traditional Survey : 

Traditional Survey Pros: Most accurate / highly defendable Value added service to pilot communities Cons: Expensive – budget will not permit Time consuming – schedule will not permit Requires access and permission from home owner

LiDAR Derived : 

LiDAR Derived Pros: Fully automated Very little cost Leverages LiDAR Dataset Cons: For accurate elevation - foundation type is needed Post spacing and age of LiDAR will limit accuracy Initial pilot testing showed error ranges from 1.1 – 2.5 based on actual elevations dataset from Mecklenburg County

High Resolution Oblique Imagery : 

High Resolution Oblique Imagery Pros: Nationwide datasets available No field work required Cons: No apparent way to automate process Elevation data obtained from underlying DEM (believed to be NED) Initial pilot showed error ranges of 2-3 feet (oblique was low)

Mobile Ground Based LiDAR System : 

Mobile Ground Based LiDAR System Pros: Cost effective acquisition No property access Very accurate (0.049 ft) Cons: Large volume of data to process Line of sight from road required Equipment costs No automated processing system in place

Inclinometer Approach : 

Inclinometer Approach Pros: Remote quick acquisition of FFE No property access required Relatively good accuracy when pilot tested in Mecklenburg County Integration with GPS Cost effective in open urban and suburban areas Cons: Remote determination of FFE may be difficult for some structure types Field work involved

Reflector-less Field Survey : 

Reflector-less Field Survey Pros: No property access required Relatively good accuracy when pilot tested in Mecklenburg County Integration with GPS Cost effective in open urban and suburban areas Cons: Control and site setup required Remote determination of FFE may be difficult for some structure types Field work involved

Pilot Test Area – Griers Fork Drive – Charlotte Relatively Small Sample size – 14 structures : 

Pilot Test Area – Griers Fork Drive – Charlotte Relatively Small Sample size – 14 structures

Each Method Evaluated for Accuracy against Actual Elevation Certificates : 

Each Method Evaluated for Accuracy against Actual Elevation Certificates

Results of Initial Pilot Testing(Small Sample) : 

Results of Initial Pilot Testing(Small Sample)

Next Steps : 

Next Steps Determined that FFE were needed for all structures in the 500 year flood plain Decided upon the inclinometer as the approach Collection is underway All collection should be complete by August 2010

Export to Google Earth : 

Export to Google Earth

Questions? : 

Questions? Hope Morgan GIS manager Office of Geospatial & Technology Management North Carolina Emergency Management hmorgan@ncem.org 919-715-5711 ext: 104