Mapping Methods and Data Sources
- Base Maps
- Hurricane Rita Preliminary High Water Marks
- Hurricane Rita Surge Inundation Limits
- Advisory Base Flood Elevations (ABFEs)
The Hurricane Rita Surge Inundation and Advisory Base Flood Elevation Maps (or “Rita Recovery Maps”) show a variety of information concerning the hurricane’s impact – information that was developed through numerous post-storm surveys and data analyses. The Rita Recovery Maps also show Advisory Base Flood Elevations (ABFEs), which are the best 1%-annual-chance (100-year) coastal flood elevations currently available to guide redevelopment along the Louisiana Gulf Coast.
The data sources and analysis methods used to develop the Rita Recovery Maps are described in the sections that follow. A more detailed discussion of the methods used to map the ABFEs in each parish is provided in a summary report, “Hurricane Rita Surge Inundation and Advisory Base Flood Maps - Summary of Methods” (PDF, 0.6 MB) (TXT, 51 KB).
The base map for the Rita Recovery Maps in Louisiana is pre-storm, digital orthophotography that was provided by U.S. Department of Agriculture’s National Agriculture Imagery Program.
Within days of the storm, field observers and survey crews were deployed by FEMA and various state and local governments to interview residents, find evidence of coastal high water levels, take digital photographs, and survey coastal high water marks (HWMs) from Hurricane Rita. These field crews collected detailed information about each HWM, including physical basis of the mark, such as a mud line inside the building, a mud line on the outside of the building, or debris. Wherever possible, crews also noted the coastal flooding characteristics captured by the coastal HWM, including storm surge, wave runup, and wave height. These designations were based on a combination of physical flood evidence and interviews with witnesses at the time of collection.
The survey crews used static Global Positioning System (GPS) methods to determine an accurate elevation for each coastal HWM. Coastal HWM locations were surveyed horizontally in North American Datum of 1983 (NAD 83), State Plane feet, and vertically in North American Vertical Datum of 1988 (NAVD 88), U.S. survey feet. (Note: In order to be consistent with the datum used in the effective FEMA flood map data, HWM elevations were converted to the National Geodetic Vertical Datum of 1929 for the recovery mapping.) Coastal HWM locations have been surveyed to within accuracies of 0.25 foot vertically and 10 feet horizontally, with a 95% confidence level. Results from FEMA’s HWM surveys are available on the Recovery and Mitigation Resources page.
Flood inundation limits were created for the coastal Louisiana parishes by mapping the coastal HWM elevations onto digital, pre-storm, topographic contour data developed from recent Light Detecting and Ranging (LIDAR) surveys. These inundation limits represent an estimate of the inland extent of flooding caused by Hurricane Rita’s storm surge. In areas where the coastal HWMs were close together but elevations differed significantly (more than 2-3 feet), engineering judgment was used to interpolate the inundation limit between coastal HWMs.
Please note: FEMA used the best data available at the time this project was completed to develop the Rita surge inundation limits, including eyewitness observations provided by some local officials and citizens. However, because of inherent variability in HWM elevations, the scarcity of available HWMs in some areas, and small-scale variations in topography, the extent of the surge inundation shown on the Rita Recovery Maps may over- or underestimate the actual coastal flooding that occurred. Thus, this information should be considered approximate and may be subject to change as additional data become available.
ABFEs were developed to provide communities with recommended building elevations for use in the reconstruction process until more detailed data become available. ABFEs are based on a new flood frequency analysis that takes into account Hurricanes Rita and Katrina,
as well as additional tide and storm data from other events that have occurred during the more than 25 years since the existing Flood Insurance Rate Maps (FIRMs) were developed. For more information on how the ABFEs were determined for each parish, see the Flood Recovery Guidance posted on the Recovery and Mitigation Resources page.