On many of the flood surveys I do the house is shown either right on or very close to the edge of the flood zone on FEMA's maps, and I've noticed a pattern in these cases: the vast majority of the time, the home is above the BFE and qualifies for a LOMA. And no, I'm not padding my numbers. Obviously an on-the-ground survey is going to give more accurate results than FEMA's topography, but if they're doing their mapping right you should end up with about a 50/50 success rate in these cases.
So here's my theory: their topography is based on LIDAR or photogrammetry, and these techniques leave a hole for several feet around any buildings since this area is blocked from the view of the airplane. To fill in the missing data, they simply average the grades surrounding the hole. In actuality, though, the last few feet next to a house is usually built up slightly to allow for drainage - nobody builds their home in a pit. Thus the unusually high success rate.
Has anyone else noticed the same pattern, or have their own theories?
In Other Words The Maps Are Cautious
Paul in PA
It's even worse than you imagine. FEMA & WV announced a couple years ago that they are going to remap the entire state and it's going to be based on 3 meter DTM's. I suspect that it is that type of mapping for the much of the country. If it was LIDAR it would be much better than that.
I was told many years ago by the Chief of Surveys for the Rock ISland District of the Corps of Engineers (covering the upper Mississippi River) that when the original field surveys were conducted by various public and private crews (mid to late '70's) the drainage basin sections were taken at roughly 1/2 mile intervals. USGS quadrangle maps were utilized to assist in interpolationg the data between sections, and with the contour interval being 10 feet on the quads....we end up finding actual ground elevations no better than 2 to 5 feet within FEMA's calculated BFE for any particular area.
I have found this to be the case in my work in NW IL.
Also, be prepared to see new maps that widen the current floodway to something as wide as the floodplain in larger river basins. I've seen a few maps of the upper Mississippi and it is surprising to to see the wider floodway, but lower base flood elevations.
I don't know what that may mean for folks previously removed from the floodplain who would be placed in the floodway. Revalidation lettes in the past have overlooked obvious conditions that should have been grandfathered in but were not.
I agree with jwabbitt's theory. I was told that FEMA used the best available information for their flood map updates. Sometimes, their best available information was the quad maps with 10' contours. During their calculations, they added half of the contour interval as a safety factor. I believe that is what surveyors are seeing in the field.
