Is Google Earth sufficient for a Lat/Long on the FEMA NFIP Elevation Certificate? I mean, if it's not, can someone suggest a site or method (non-GPS) that one can use?
I hope that I'm not being careless or silly in thinking that this is sufficient and accepted practice. The EC form states that it must be with in 66 feet of the front center of the building. There is no other dwelling/building within 100 feet of my subject dwelling.
Any suggestions and comments are appreciated.
Carl
It's good enough as long as you're willing to sign for it. That is until they don't have it georeferenced quite right.
For what it's worth, I can put in the location of my office desk to .01" and GE nails it. Would I trust it everywhere - NO. You'd be better off with a hand held, WAAS enabled receiver that will cost you a hundred bucks or so.
I set a point in the parking lot at our office and tied it to CORS. The point is at the end of a parking stripe. When I put those coordinates in google earth, it was dead on the end of the stripe. I've checked some other points around town, too, and the google earth georeference looks to be right on the money around here. Check a couple of points. You can even check some NGS points, like if you know there is point (with good Lat and Long) on a bridge or some such thing that you can easily identify and gauge it's accuracy.
Have used Google Earth before. I have also scaled lat/lon off a quad map before. I think you just need to be close enough so they can find it on the map.
Seriously, if you had to be dead-on, then where on the property would you take the reading? Every property corner would have a different lat/lon.
> Have used Google Earth before. I have also scaled lat/lon off a quad map before. I think you just need to be close enough so they can find it on the map.
>
> Seriously, if you had to be dead-on, then where on the property would you take the reading? Every property corner would have a different lat/lon.
I can see the house in question very clearly on Google Earth, and the form says in item A5: "Provide latitude and longitude coordinates for the center of the front of the building". Use either decimal degrees
(e.g., 39.5043°, -110.7585°) or degrees, minutes, seconds (e.g., 39° 30’ 15.5”, -110° 45’ 30.7”) format. If decimal degrees are used, provide coordinates to at least 4 decimal places or better. When using degrees, minutes, seconds, provide seconds to at least 1 decimal place or better. The latitude and longitude coordinates must be accurate within 66 feet. When the latitude and longitude are provided by a surveyor, check the “Yes” box in Section D and indicate the method used to determine the latitude and longitude in the Comments area of Section D. If the Elevation Certificate is being certified by other than a licensed surveyor, engineer, or architect, this information is not required. Provide the type of datum used to obtain the latitude and longitude. FEMA prefers the use of NAD 1983
I think that I'm well within those parameters.
> > ... and the form says in item A5: "Provide latitude and longitude coordinates for the center of the front of the building".
Hmmm. I guess I need to read more closely.
Thanks.
> It's good enough as long as you're willing to sign for it. That is until they don't have it georeferenced quite right.
>
> For what it's worth, I can put in the location of my office desk to .01" and GE nails it. Would I trust it everywhere - NO. You'd be better off with a hand held, WAAS enabled receiver that will cost you a hundred bucks or so.
Dave,
What would your datum with WAAS be? NAD 1927 or NAD 1983? The Elevation Cert only "allows" for these two datums (which is silly). Google Earth is based on WGS84. I'm learning, but some of this is more than I'm used to dealing with.
Carl
Yup. That's more than adequate. Every piece of data with which we work is subject to some kind of error. Don't sweat the small stuff.
Someone, somewhere decided to go with the accuracy specified for some unknown reason. Rounding to the nearest ten-thousandth of a degree will be a much different distance at the Equator than in Nome, Alaska. I think it has more to do with some techie a thousand miles distant being able to know "about" where you are working.
Most hanhelds that I am familiar with allow you to select your datum of choice. And even if they don't there is always Corpscon.
Maptech has an interactive screen that lists lat and lon. I don't know how that compares to Google Earth, but here is the website.
http://mapserver.mytopo.com/homepage/index.cfm?CFID=6412677&CFTOKEN=62785971
> Maptech has an interactive screen that lists lat and lon. I don't know how that compares to Google Earth, but here is the website.
>
>> http://mapserver.mytopo.com/homepage/index.cfm?CFID=6412677&CFTOKEN=62785971br >
It came up with the same, or awful darn close.
Thank you.
There are places where Google Earth is off more than 66 feet, so you would need to check how well it matches some points around the point of interest. A spot of interest to me has a discontinuity in the roads of 33 ft. See 41.601, -93.1351 I'm not sure which photo is closer to right, but wouldn't trust either one to be within 66.
ddd.dddd in latitude gives about 36 ft increments, less distance in longitude. If that is good enough for them, then it really is just enough to identify the house.
Recreational grade GPS units with WAAS are virtually always within 66 feet IF they have a good constellation, clear sky, and have been tracking long enough to settle the clock errors and elevation estimate. An averaged waypoint (say 2 minutes) on 3 different days and times of day will have (my guess) 95% chance of being within 10 ft. I once averaged an hour on 5 days and came within 2 ft of a position that had been referred to good control. The internal computation accuracy seems to be about 2 or 3 ft regardless of how much you average.
All rec grade units I've examined store their data in WGS84 and have a table of conversions to other datums. However, the table is probably the old one from DMA that says WGS84 and NAD83 are identical. They never change even a least digit when switching between those datums. Since there really is typically a meter difference you can trivially improve the results by using HTDP, but there is no need if you are rounding to 4 place degrees.
close enough for me. i use it often.
You are welcome.:-)
NO!
WAAS enabled handheld is close enough.
SJ
Google Earth is based on a spherical Earth, not an ellipsoidal Earth. Therefore, depending on where the optical center of a particulare image is, your place of interest could be accurately positioned near the center of the lens or awfully off based on an off-axis position of a satellite image. Accuracy varies from good to terrible. You're willing to "stamp" for that uncertainty?
Very interesting about the spherical shaped earth on G.E.
We just did an elevation cert yesterday and although I am more than certain G.E. is good enough, we setup the GPS. It just doesnt seem like you can certify to a web based program.
That said, I always bring my aerial control points into G.E. and create a kml for my photogrammetrist and the results are amazing. The horizontal is always sub-foot and lately the vertical seems to be around a foot as well.
The elev cert is only looking for 66 foot accuracy.
Also, don't you have to state how the lat.-lon. was determined?
:coffee:
Take a look at a Letter of Map Revision Determination Document from FEMA. I have one from April 08, 2010 that says:
APPROXIMATE LATITUDE & LONGITUDE OF PROPERTY 34.XXX, -92.XXX
SOURCE OF LAT & LONG: ESRI: FEMA GEOCODE/GOOGLE MAPS DATUM:NAD83
You will also note that they will re-write your certified Legal Description to put it into the official FEMA Metes and Bounds format. The only thing wrong with the FEMA format is that it strips out the Bounds and leaves only the bearings and distances.
The only saving grace is that neither the Surveyor's name nor the Owner's name appear anywhere on the official determination document.
DDSM
> Also, don't you have to state how the lat.-lon. was determined?
>
Not really. When you state your Lat and Long you have to state your Horizontal Datum (NAD 1927 or NAD 1983).
And then in Section D you have to state to: "Were latitude and longitude in Section A provided by a licensed land surveyor"
So you don't really have to state your procedure for how it was determined.
Carl
