FIRM

Where are the benchmarks????
Does FEMA want us to use GPS (NAVD 1988 Datum) or NGVD 1929 datum and Vertcon with Google Earth Longitude & Latitude to convert to NAVD 1988 datum???

In my experience here on the Maine coast, all the Firm's I've seen have been on NGVD 1929.  Not all maps have references to BM's in the area, but those that do reference 1929.  Hope this helps. 

As for using Google Earth, I assume you're referring to the field on an elev. cert. that asks for a Lat and Long for the site.  I've used Google Earth in this case, as they are only looking for an overall position of the site, and not for sub-centimeter precision control. 

Newtons Apple

WOW - It is my understanding that All Flood maps after 9/11/09 (last revision) are using 88 datum. If the base flood elev. is AE (El 10) is shown on the map this elevationn is based on 88 datum. In my area (Long Island-New York) all elevations on surveys prior to 9/11/09 were 29 datum. Now all elevations that were based on 29 datum must be lowered 1.1' +/- depending on Vertcon equations. The elevations of first floor, garage, corner of buildings must be based on 88  datum on the elevation certificate.

         Carlton 

Mr. Bausch,

I think it matters a great deal where you are working.  The flood maps here (new since the State took over the mapping process from the Feds) list the datum and a conversion from the NAVD 29 to the NAVD 88 elevations.

If you want to see an example of what I'm talking about look on the left hand side of the map I attached.

Hope that helps.

Larry P
NCPLS

Larry

Big Help. I thought all FEMA maps in the USA had an effective date 9/11/09. I can enlarge your map on my PC to read the detail notes. Very interesting. Thanks.
I had to have a solo surveyor with GPS equipment  take a shot for  an Elevation Certificate NAVD 1988 datum on the site. The variation from 29 datum to 88 datum is minus 1.1' +/-  in this area depending on Longitude Latitude from Google Earth and Vertcon conversion equations. I needed a check after using a 29 data bench to run elevations. Elevations are critical in this area with insurance premiums the big issue.


    Carlton  

Flood Insurance Rate Maps can contained information in NGVD29 or NAVD 1998 and it will depend on the effective date of the map. Not all communities get maps at the same time, and maps do not get updated at the same rate all over the US. The vertical Datum and Benchmark information of older maps is posted on the legend (right panel). In newer maps these items might not be on the legend but are always within the Flood Insurance Study (FIS) which is the technical document from where a FIRM is produce. FIRM panels and FIS data are available in the FEMA Map Service Center (http;//msc.fema.gov) under Product Catalog (top left) option of the homepage. For an Elevation Certificate, NGVD 29 and NAVD 1988 are widely accepted and can be used without preferrence, it is only when another Datum is used that a conversion will need to be provided on the certificate.

Apologies - it's easy for me to forget there are local differences for the flood maps.  Here in midcoast Maine, I haven't seen any updated flood maps beyond 1986.  

We're a bit behind up here... 

Guarioo

In my area - Long Island N.Y. the conversion factor of minus 1.1' +/- from 29 to 88 datum. If the base flood elevation is AE (el 10) on the FIRM revised 9/11/09) then this must be on 88 datum since elevations shown on EC must conform to 88 datum and all elevations on a survey with 29 datum must be lowered to conform to 88 datum or a note that all elev on survey are based on 29 datum if the survey is to be enclosed with the EC.


     Carlton