[time-nuts] question about Thunderbolt geo acuracy

Randy D. Hunt randy_hunt960 at yahoo.com
Fri May 11 17:23:15 UTC 2012


On 5/11/2012 6:46 AM, Jim Lux wrote:
> On 5/11/12 5:23 AM, swingbyte wrote:
> s disappointing!
>>
>> I need to measure the height of my house floor to be above the flood
>> plane contour. I might have a look at some dted from work. Might have to
>> pay a real surveyor to measure the height datum.
>> Thanks for all the info though guys
>>
>
> for that, you need a real surveyor who can provide a "legally 
> accepted" measurement.  Someone who can
> a) know from the flood level definition what vertical datum they are 
> using (probably NOT something normal in the geodesy world)
> b) knows the legalities of establishing the difference
>
> The mechanics of surveying (leveling in this case) are straightforward 
> to learn.  The legalities and local practices in documentation are 
> not.  This is what getting a Land Surveyor's license is all about.
>
> There's also a question of what the legal height of your house is, 
> relative to the property (from a flood insurance standpoint).  They 
> might have some arbitrary offset in the rules. Sort of like how 
> baseline electrical power consumption is actually about 2/3 of the 
> expected minimum consumption in the area for a given size house and 
> appliances (e.g. nobody is likely to consume less than baseline)
>
> There are some mortgage servicers, by the way, who take property 
> addresses that have been geolocated and FEMA flood plain definition 
> maps to determine whether you definitely don't, definitely do, or just 
> might need flood insurance.  The maps change (as does the 
> geolocation). From what I understand, about 3-5% of the properties 
> scanned require some sort of manual intervention (maybe the address 
> doesn't geolocate, or it's right on the line, or)
>
>
>
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Actually, The percentage can be higher.  The scale of the FEMA flood 
panels are usually around 1=2000.  Some of the older panels were 
1=4000.  The newest panels can be around 1=1000 (approx 5" to the 
section).  Horizontal scale is not the problem, it's the vertical 
scale.  Also how the stream bed profile was established (surveyed).  
There can be a lot of change in the real world compared to was gets 
plotted on the panel and in the profile.  When there is an obvious 
discrepancy between the two (mapped profile and real world) a registered 
surveyor or engineer must be called in to reconcile the difference.  The 
cost for doing this might seem high, but when compared to the cost of 
flood insurance paid over the life of a mortgage, it's very cheep.

Just my 2 cents worth. . .


Randy Hunt, retired Engineering Technician, Flood Plain Administrator 
(32years)



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