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Incorrect Benchmark?


jxavierf

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Hi all. Have a conundrum. I have an elevation certificate from a local surveyor here in Mount Pleasant, SC. The The benchmark used is from 1929 and the findings indicate that the base of my house is 4 feet below sea level. What is interesting, is that if I cross reference the following sources, 1. http://www.daftlogic.com/sandbox-google-maps-find-altitude.htm

2. Google Earth

3. http://www.gpsvisualizer.com/elevation

 

I get an elevation between 0' and 1'. So the surveyor says -4', yet the above three sources indicate between 0' and 1'......could the benchmark be wrong? And if so, how do I challenge this...who do I contact.

 

TIA!

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jxavierf,

 

If you have a question then you should call the surveyor and ask him how he created the certificate. You need to be using the correct datum to begin with; NGVD29 or NAVD88. Those two datums are approximately 2-4 feet different in many parts of the country.

 

Without running a level circuit or a very precise GPS elevation to your house; you are just guessing what your elevation really is. More than likely you do not have that type of equipment or expertise to perform an elevation survey on your own, so you would need to contact somebody who does. The techniques that you used are probably in the neighborhood of 20+ feet in accuracy and are very unreliable.

 

Give him a call and he should be able to answer your questions.

 

Kurt

 

Hi all. Have a conundrum. I have an elevation certificate from a local surveyor here in Mount Pleasant, SC. The The benchmark used is from 1929 and the findings indicate that the base of my house is 4 feet below sea level. What is interesting, is that if I cross reference the following sources, 1. http://www.daftlogic.com/sandbox-google-maps-find-altitude.htm

2. Google Earth

3. http://www.gpsvisualizer.com/elevation

 

I get an elevation between 0' and 1'. So the surveyor says -4', yet the above three sources indicate between 0' and 1'......could the benchmark be wrong? And if so, how do I challenge this...who do I contact.

 

TIA!

Edited by CallawayMT
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Even in a coastal city, it strikes me as strange that a house would be below sea level. Are you sure the surveyor's certificate says it is -4 with respect to sea level and not from the Base Flood Elevation (BFE)?

 

In stating elevations with any precision, it is necessary to specify which datum is used, usually NAVD88 but in some places still NGVD29. The one to be used will be specified by the local Flood Insurance Rate Map (FIRM) data base. The datum is independent of the date the bench mark was set - there could be values in each datum for the same mark. There may be as much as a few feet difference between them in some places.

 

You didn't tell us which bench mark was used, so there isn't much we can say about that. Any data you find from on-line mapping sources is going to be approximate. Using only one bench mark is a risky operation because things do happen to them, but perhaps the surveyor knows that it has been checked against others in the area recently.

 

I don't suppose there is much you can do about it except get another surveyor to check the first one's work.

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Bill 93 - the datum that was used is NGVD29 [is it possible to have them reshoot it at the NAVD 1988?]. And yes it striked me odd that in 1975 someone would build a house 4 feet below sea level, with a 2 foot crawl space...so that the first floor would be one/two foot below sea level.....why not at least build a 3 or 4 foot crawl space. Anyway....yes the BFE is 14'....the "lowest adjacent grade at lowest elevation" is 10' 3". But this is being intrepreted by the insurance company as - 4 foot below sea level. Today I used an I-Phone with an elevation app and got .5 feet as elevation....yes I know that these likely are not too accurate...but this is then the forth source telling me that the base elevation is btwn 0' - 1'. Any additional insight would be appreciated! TIA!

 

Even in a coastal city, it strikes me as strange that a house would be below sea level. Are you sure the surveyor's certificate says it is -4 with respect to sea level and not from the Base Flood Elevation (BFE)?

 

In stating elevations with any precision, it is necessary to specify which datum is used, usually NAVD88 but in some places still NGVD29. The one to be used will be specified by the local Flood Insurance Rate Map (FIRM) data base. The datum is independent of the date the bench mark was set - there could be values in each datum for the same mark. There may be as much as a few feet difference between them in some places.

 

You didn't tell us which bench mark was used, so there isn't much we can say about that. Any data you find from on-line mapping sources is going to be approximate. Using only one bench mark is a risky operation because things do happen to them, but perhaps the surveyor knows that it has been checked against others in the area recently.

 

I don't suppose there is much you can do about it except get another surveyor to check the first one's work.

Edited by jxavierf
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>I-Phone with an elevation app

It doesn't measure elevation. It crudely measures the location and looks it up in one of the approximate data bases. None of them will be accurate to the foot level. The surveyor says your ground is 10 ft above sea level, and your other sources are more pessimistic than that. Give up on that approach.

 

>is it possible to have them reshoot it at the NAVD 1988?

 

As I said, the datum used in an area will be specified by the flood insurance organization. It doesn't matter which datum is used because if they used the other datum both the expected flood elevation and your house elevation would change by the same amount. It is the house elevation relative to the flood that is important.

 

>BFE is 14'

 

That is the NGVD29 elevation (a height above mean sea level if we don't get technical) that is how high they expect a 100-year flood to get.

 

>intrepreted by the insurance company as - 4 foot below sea level.

 

This would be below FLOOD level, not mean sea level. It's flood level that counts.

 

If the house isn't high enough, then the insurance will be expensive. I'm not expert on interpreting the flood insurance rules regarding what part of a house needs to be how high above BFE to reduce the risk.

 

There might be things that can change what point on the house is critical. If there isn't anything important in the crawlspace, and if it is enclosed so they counted it as a portion of the house that would be damaged, then it's worth asking. Maybe installing approved flood vents (so water pressure wouldn't collapse walls) would change the critical height from the crawlspace to the finish floor level. You'll have to review with your surveyor and insurance person to see if any mitigation is possible. If the finish floor is below BFE, then I wouldn't have a lot of hope.

Edited by Bill93
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jxavierf,

 

You have a problem, so you will have to bite that proverbial bullet and seek the help of those professionals that are there where you live. You have not let anyone here know where you live or what that benchmark that concerns you is. If you would get that info (the numbers and letters from the disk), more talk could be a little bit more helpful to you. But, still you need to get the help of those close to you to be of help with your insurance company.

 

Here is a link to The American Surveyor Blog: that has this information that pertains to the age of your home.....

 

In the mid-1980s, FEMA published a new flood insurance rate map, but with different measurements. It recalculated the base flood level based on a new study showing that the Beaumont-area flood plain had actually shifted about three feet since the federal government's first measurements in the 1950s.

 

You stated that your home was built in 1975, and then in the mid-1980's FEMA published a new flood insurance rate map. You definitely need to go talk with your insurance agent. They are there to help you and let you know what this all means to you. You probably do not want to hear that, but you just have to go and get help somewhere else than here.

 

There are many places throughout the country that has flood plains and several have had housing developments built on them. One such development was in Bloomington, Utah. It was a flood plain for the Virgin River. Several years after they were built a hundred year flood went through and peoples places were flooded and damaged. They sued the people who built them but. their insurance rates for flood damage is now very high.

 

I hope you go and get local help......

 

Shirley~

Edited by 2oldfarts (the rockhounders)
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The online sources you used would use data sources that would not have the kind of elevation accuracy you would need. As for your iPhone, I've seen a post by someone asking why their iPhones (including a 4S) were 100 feet off. GPS based elevation (assuming that's what your iPhone is using) has a rather limited accuracy that may be worse then it's positional accuracy. Even the altimeters used by aircraft aren't perfect since they have to be adjusted to the current barometric pressure.

 

As I understand it, the most comprehensive map of the Earth ever made was made using ASTER data from the Japanese satellite Terra. Here's a quote from an article that mentions it's elevation accuracy:

 

"In addition to the resolution issues, the ASTER folks have expended a lot of effort to determine the vertical accuracy of the dataset. The stated accuracy is 7-14m stated in terms of standard deviation. This should mean that 99% of GDEM data will fall within +/- 42m maximum of the actual elevation. Vertical accuracy was determined by subtracting GDEM elevation values from those of other datasets, including the NED and SRTM plus comparison with control points."

 

If you really don't trust that surveyor, then see if you can get a second opinion from another one. But online sources like Google Maps or consulting a smartphone just won't cut it.

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Some good points being brought up on the limitations of a 'do it yourself' survey using some of the cute gadgets and information now available. Elevations particularly are best left to the professionals. Lets hope that this thread is a one time fishing expedition that may snare a land shark. MEL

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I appreciate everyones expert analysis and I am accepting of the facts. I misunderstood the findings in the elev. cert. that the home was below sea level....when in actuality the home is just under the 100 year BFE....thanks for educating me.

 

One final question (I think). The surveyor told me that because the home sits at 3'9" below the BFE benchmark, but the first floor (which sits above the crawl space) is at 9" below the BFE....that I could raise the home by 4-5" and they would then "round up" to 0"....or at BFE for the first floor. My question is....what do they use to define the first floor elevation....do they shoot the bottom of the front door? I will ask them tomorrow, but was hoping for feedback from you'all.....thanks!

 

 

The online sources you used would use data sources that would not have the kind of elevation accuracy you would need. As for your iPhone, I've seen a post by someone asking why their iPhones (including a 4S) were 100 feet off. GPS based elevation (assuming that's what your iPhone is using) has a rather limited accuracy that may be worse then it's positional accuracy. Even the altimeters used by aircraft aren't perfect since they have to be adjusted to the current barometric pressure.

 

As I understand it, the most comprehensive map of the Earth ever made was made using ASTER data from the Japanese satellite Terra. Here's a quote from an article that mentions it's elevation accuracy:

 

"In addition to the resolution issues, the ASTER folks have expended a lot of effort to determine the vertical accuracy of the dataset. The stated accuracy is 7-14m stated in terms of standard deviation. This should mean that 99% of GDEM data will fall within +/- 42m maximum of the actual elevation. Vertical accuracy was determined by subtracting GDEM elevation values from those of other datasets, including the NED and SRTM plus comparison with control points."

 

If you really don't trust that surveyor, then see if you can get a second opinion from another one. But online sources like Google Maps or consulting a smartphone just won't cut it.

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The elevation of the lowest floor will depend on your house. You mentioned a crawl space. If one is present, then the floor of the crawl space may be the lowest floor. The instructions for the Elevation Certificate include several diagrams that depict different configurations of buildings, and how to determine the elevations listed in Section C of the certificate. A copy of the instructions for the elevation certificate can be obtained online from FEMA.

 

The lowest floor is used for insurance rating. I believe for rating the difference between the lowest floor elevation and the Base Flood Elevation is rounded to the nearest foot. Other factors used to determine the rate is the flood zone - A, AE, V or VE are the more common zones. The Flood Insurance Rate Map (FIRM) is used to determine the flood zone.

 

If your house was constructed in 1975 then it is considered to be "Post-FIRM" since the construction occurred after the first FIRM for Mt Pleasant. The initial FIRM for Mt Pleasant was 04/02/1971. The current FIRM is dated 11/17/2004. This information is also used to determine the insurance rate for the house.

 

Before considering any modifications to the structure, a Floodplain Development permit will be required from the authority having jurisdiction. For purposes of the floodplain permit, the elevations should be determined to the nearest tenth foot. As mentioned by others, a survey is required to gain the necessary accuracy and precision. The floodplain permit is in addition to any building permits you may be required to obtain.

 

If you have questions about the FIRM or elevation certificates you might try FEMA's Map Assistance Center. The center can be reached by calling 877-FEMA MAP. Also, the surveyor that completed the elevation certificate should be able to explain how the elevations for your house were determined.

Edited by wister6813
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