CFS Surveying Techniques & Data Documentation 3 — Questions and Answers
Question 1: A GPS survey for a flood study yields ellipsoid heights. To convert these to NAVD 88 orthometric heights, the surveyor should use:
- A constant conversion factor of -32 feet applied nationwide
- NOAA's GEOID model (e.g., GEOID18) to compute the geoid-ellipsoid separation (Correct answer)
- A simple subtraction of the WGS 84 ellipsoid radius
- The NGVD 29 to NAVD 88 conversion table only
Correct answer: NOAA's GEOID model (e.g., GEOID18) to compute the geoid-ellipsoid separation
NOAA's GEOID models (currently GEOID18) provide the geoid-ellipsoid separation (N) so that H (orthometric) = h (ellipsoid) − N.
Question 2: In a HEC-RAS hydraulic model, cross-section station values are measured from which direction?
- Left to right facing upstream (Correct answer)
- Left to right facing downstream
- From the channel centerline outward
- From the downstream end of the reach upstream
Correct answer: Left to right facing upstream
HEC-RAS cross-section stations are measured from left to right when facing upstream, following standard hydraulic engineering convention.
Question 3: Which field documentation element is critical when surveying a bridge opening for a flood study?
- The bridge paint color and material type
- Low chord elevation, high chord elevation, and roadway profile elevations (Correct answer)
- The bridge builder's name and construction date only
- Only the span length measured horizontally
Correct answer: Low chord elevation, high chord elevation, and roadway profile elevations
Low chord (soffit), high chord, and roadway profile elevations are essential bridge data for defining hydraulic opening and roadway overtopping potential.
Question 4: When a surveyor submits an Elevation Certificate, what does Section C document?
- Community information and FIRM panel data
- Building information including foundation type
- Flood zone and BFE determination (Correct answer)
- Surveyor certification and seal
Correct answer: Flood zone and BFE determination
Section C of the Elevation Certificate documents the flood zone determination and the Base Flood Elevation (BFE) applicable to the building.
Question 5: A surveyor performing topographic survey for a FEMA FIRM revision uses LiDAR data as supplemental information. What is a critical limitation of LiDAR in vegetated areas?
- LiDAR cannot measure horizontal distances accurately
- Vegetation canopy can prevent laser pulses from reaching the ground, causing elevated ground elevations (Correct answer)
- LiDAR only works in urban areas
- LiDAR requires full daylight and cannot be flown at night
Correct answer: Vegetation canopy can prevent laser pulses from reaching the ground, causing elevated ground elevations
In densely vegetated areas, LiDAR pulses may not penetrate the canopy to reach the ground surface, resulting in ground returns that are too high.
Question 6: What is the standard horizontal datum required for FEMA flood mapping spatial data?
- NAD 27 (North American Datum of 1927)
- NAD 83 (North American Datum of 1983) (Correct answer)
- WGS 84 geographic coordinates only
- State Plane coordinates without datum specification
Correct answer: NAD 83 (North American Datum of 1983)
FEMA requires all flood mapping spatial data to be referenced to NAD 83, the current standard horizontal datum in the United States.
Question 7: During differential leveling, a surveyor reads a backsight of 6.234 ft on a rod held at BM-A (elevation 452.18 ft). What is the Height of Instrument (HI)?
- 445.946 ft
- 452.18 ft
- 458.414 ft (Correct answer)
- 460.652 ft
Correct answer: 458.414 ft
HI = Benchmark elevation + Backsight reading = 452.18 + 6.234 = 458.414 ft.
CFS Surveying Techniques & Data Documentation 3
A GPS survey for a flood study yields ellipsoid heights.
To convert these to NAVD 88 orthometric heights, the surveyor should use: