FEMA Hazard Mapping in Google Earth...AND Elevation Cert ?

Download and review FEMA http://www.fema.gov/library/viewRecord.do?id=220 6">Appendix B: Guidance for Converting to the North American Vertical Datum of 1988. By seeing how the FEMA Mapping Partner converted from 29 to 88, you will have some insight into converting back to 29.

You might also try to find a copy the Mapping Partner's Technical Support Data Notebook (TSDN). This report will have the values computed, benchmarks used, methods used, etc.

Contact your local Floodplain Manager and have them request the TSDN for you.

DDSM

Thanks for the reply Dan,
My local counties are updated maps based on NAVD88...that's not the problem.
I'm concerned about the little nuances between the NAVD88 elevs. due to the different geoids (03, 09, now 12A). My quandry is when my Hays County map was issued (September 2005) was it based upon the 03 geoid? Is there an addendum to the process to account for the vertical shift due to geoid12A. Has anyone else resolved this?

When I posted the original questions, I was in the middle of downloading the NGS "vdatum 3.2" tool kit. My ISP is wireless out here in my portion of the country and was going to take 6 hours to complete. I aborted and downloaded through my 4G LTE phone (15 minutes). I'm going to calc the heights for my lat/long and determine the separation based upon the different geoids and determine my orthoelevations. If there is enough difference between the two, then i will start a new worry.

Thanks again.

First note that GEOID12A is not a datum, it is a model that allows you to estimate NAVD 88 orthometric heights from NAD 83 (2011) ellipsoid heights. In most areas I would be very cautious about using GPS to provide orthometric heights for flood certs. In the area of Hays County NGS estimates for the accuracy of GEOID12A are in the range of 4-6 cm (0.13 - 0.20 ft) (95% confidence) which when combined with the general uncertainty of an ellipsoid height of say 2-4 cm (.06 - 0.13 ft) would most likely not provide the height accuracy you need.

why not just occupy local NGS benchmarks that have published elevations on the datum you need to work in?

I suspect that the FEMA contractors used just the existing leveled NAVD 88 control in the area to produce the maps. As noted previously the various geoid models produced by NGS have been a way to estimate NAVD 88 from GPS observations. Those models work best in areas where they have a considerable number of NAVD 88 bench marks that have had GPS observations performed and submitted for inclusion into the National Spatial Reference System. The density of those observations is not consistent across the country. It has in the past been dependent on those who submit. Take a look at the map showing the density of control used in GEOID12A -- http://www.ngs.noaa.gov/GEOID/GEOID12A/maps/PDFs/CONUS.pdf you will see that for Texas it is pretty spares - hence the higher uncertainty that in other areas. You can now help to improve this by submitting GPS observations on 1st- or 2nd-Order NAVD 88 marks using OPUS-DB. For the first time with GEOID12A NGS used OPUS-DB data in the model.

Out here behind the Chicken Wire Curtain, the development within the last 20 years have destroyed most in not all marks. New subdivisions, r-o-w widening, etc. Occasionally you find one.

Only 1 listed within 5 miles: (US Ppwer squadron 1999-mark not found...yeah, yeah, yeah, I know, don't trust the US Power Squadron to really be able to find the marks.)
For the last few years, I have wanted to ask the landowner access to his property to search for the marker. It is a half-a-mile or so off of the nearest public road. I was going to use it for the national GPS network submittal. Just never followed through.

AY1785 ***********************************************************************
AY1785 DESIGNATION - HUGO
AY1785 PID - AY1785
AY1785 STATE/COUNTY- TX/HAYS
AY1785 COUNTRY - US
AY1785 USGS QUAD - DEVILS BACKBONE (1989)
AY1785
AY1785 *CURRENT SURVEY CONTROL
AY1785 ______________________________________________________________________
AY1785* NAD 83(1993) POSITION- 29 56 24.76751(N) 098 08 35.93279(W) ADJUSTED
AY1785* NAVD 88 ORTHO HEIGHT - 332. (meters) 1089. (feet) SCALED
AY1785 ______________________________________________________________________
AY1785 GEOID HEIGHT - -25.24 (meters) GEOID12A
AY1785 LAPLACE CORR - -3.55 (seconds) DEFLEC12A
AY1785 HORZ ORDER - FIRST
AY1785
AY1785.The horizontal coordinates were established by classical geodetic methods
AY1785.and adjusted by the National Geodetic Survey in February 1996.
AY1785.
AY1785.The orthometric height was scaled from a topographic map.

Thanks for the reply base9...
I've been toying with a "personal" monument I set in my yard. I have OPUS-RS, OPUS-S, RTN (Trimble VRS), and of course, self post-processed data for the last 5 years. Things get slow, I run a session for fun. I'm eat-up with this stuff. Differences between the values are significant between the RS vs. S files. ALL NAVD88, differences in GEOID.
A few of example files:

NAD_83(CORS96)(EPOCH2002.0000)(GEOID03)
OPUS-S RAPID
2012/01/27 (5 HR, 40 MIN) 94% OBS USED-100% FIXED AMBIGUITIES
ORTHO: 1129.50'
OPUS-RS RAPID (GEOID03)
2012/05/04 (49 min, 20 sec.) 42% OBS. USED
ORTHO: 1129.40'

NAD_83(2011)(EPOCH:2010.000) GEOID12 - (NOT 12A) This was before the issue of 12A
OPUS-S PRECISE
2012/01/27 (5 HR, 40 MIN) 94% OBS USED-98% FIXED AMBIGUITIES - SAME FILE AS ABOVE
ORTHO: 1129.84'

NAD_83(2011)(EPOCH:2010.000) GEOID12A
OPUS-S RAPID
2012/12/28 (5 HR, 150 MIN) 95% OBS USED-97% FIXED AMBIGUITIES
ORTHO: 1129.75'
OPUS-RS RAPID (GEOID12A)
2012/05/04 (1 HR 16 MIN) 52% OBS. USED
ORTHO: 1129.55'

NAD_83(2011)(EPOCH:2010.000) GEOID12A
POST PROCESSED with CORS data-PRECISE ON 2013/01/11
2012/12/28 (5 HR, 150 MIN)
ORTHO: 1129.78'

The County requires ECs for all dwellings affected (by even a portion of the property) by floodways if they are to begin new construction, or remodel (as in this case). I would imagine the vertical differences would be the same for this project, approx. 10 miles away from me. Plus, the subject dwellings appear to be 15' or more above the ZONE AE data. They actually sit on a river bank, separated by a roadway from the river. I won't know for sure until I run my observations. I am not worried by a few tenths. FEMA accepts OPUS files, and even RTN data, if named and included with the EC submittal.

I wish to thank everyone for their input. I was looking for anyone whom may have already run into/thought of this, and their perspective. Also, my post was to include the link to the Google Earth mapping of the FEMA data, which matches the FIRM maps in my area, and the digital flood data I have purchased from FEMA.

:good: 5 miles, 20 miles.....that's what I would do. Distance from current site is not necessarily a restrictive factor if the monument(s) are GPSable. Static them in and call it a day.