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FEMA BFE and Geoids

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(@clearcut)
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Am looking at the FIRM and FIS for a particular area and see that even though the documents are dated 2011, there is no mention of the geoid model. Only reference is to NAVD 88. Should one assume Geoid 09? There is a whole page in the FIS regarding vertical datum, but it only talks about the differences between NAVD 88 and NGVD 29. There is a pretty big vertical difference between some of the geoids here, the '03 and '09 are close to a half foot different.
Any thoughts?

 
Posted : July 26, 2016 1:29 pm
 ddsm
(@ddsm)
Posts: 2229
 

A FIS this new would have a series of Technical Study Data Notebooks (TSDN). These notebooks contain detailed metadata. Here is a link to the TSDN checklist to give you an idea of which TSDN to request.
http://www.fema.gov/media-library-data/1389195038850-bcc20558aafdd39eb6aed660fbfbf867/Procedure+Memorandum+62-Technical+Study+Data+Notebook+(TSDN)+Inventory+Checklist.pdf

Your local floodplain administration can help you with TSDN requests.

DDSM

 
Posted : July 26, 2016 5:26 pm
(@mightymoe)
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Be sure to use the basis of elevation stated, in this case NAVD88, which is not based on any GEOID model.

That is up to you, you have to make YOUR GPS work with NAVD88.

The FIRM may have taken many years to develop, the mapping might pre-date common use of GPS.

 
Posted : July 26, 2016 6:47 pm
(@jim_h)
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The purpose of geoid models is to get you to NAVD 88. The geoid model is not a datum. The models have improved over the years to better match NAVD 88 so it makes sense to use the latest one. That said, you should check into local benchmarks that have been leveled through and adjust if necessary.

 
Posted : July 26, 2016 7:56 pm