Benchmark list for Cities in San Mateo County

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sako
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Benchmark list for Cities in San Mateo County

Postby sako » Tue Sep 03, 2019 8:22 pm

City of Menlo Park has 12 Benchmarks throughout the whole City. I have seen improvement plans where it shows BM's from San Francisco Water District (SFWD), does anybody have access to the BM list of SFWD?

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Re: Benchmark list for Cities in San Mateo County

Postby PLS7393 » Tue Sep 03, 2019 8:56 pm

If using a SFWD benchmark, be sure you understand what datum your using?
I know SF use to have several different datums (I want to say seven), and very confusing.
Get GPS, check a benchmark and move forward on NAVD 1988. Using old NGVD 1929 does us no good, and can easily be converted to NAVD 1988 using the VERTCON Conversion Program from NOAA

https://www.ngs.noaa.gov/cgi-bin/VERTCON/vert_con.prl
Keith Nofield, Professional Land Surveying
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Re: Benchmark list for Cities in San Mateo County

Postby SPMPLS » Wed Sep 04, 2019 6:04 am

Yes, VERTCON or VDATUM will provide a conversion from NGVD29 to NAVD88. However, I recently contacted Dan Roman, NGS Chief Geodesist, regarding the accepted accuracy of the conversion. Dan said that it is in the +/- 15 cm range at 1 sigma. If that is acceptable for your purposes, then go for it. If you use VDATUM to perform this conversion anywhere in California, the stated vertical uncertainty of the results will be +/- 18.78829 cm.

Vdatum accuracies 29 to 88.jpg


Calaveras County.jpg
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Re: Benchmark list for Cities in San Mateo County

Postby PLS7393 » Wed Sep 04, 2019 6:22 am

In your case SPMPLS, the geoid models are not as strong in the mountains where you are, verses the flat lands, so you need to understand your working conditions.

I've been informed that the bay area (San Mateo Co as identified on the post) is pretty safe to use the VERTCON conversion and when I use my GPS to test and compare elevation data, I am typically within a satisfactory tolerance. The key is to understand and delivery a good satisfactory product to your client.
Keith Nofield, Professional Land Surveying
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Re: Benchmark list for Cities in San Mateo County

Postby SPMPLS » Wed Sep 04, 2019 6:34 am

Look at my second example. It is in Monterey. Here is another from Santa Barbara. It has nothing to do with the quality of the geoid models or topography. There is no such model for NGVD29. NGVD29 was established based on a series of 26 tidal bench marks, assuming mean sea level was the same at each, which was obviously a flawed assumption. NAVD88 was established from a single bench mark at Father Point/Rimouski, Quebec, Canada (PID TY5255). The relationship between the two datums is very location dependent. When the NGS Chief Geodesist tells me that the resulting VERTCON/VDATUM conversion is reliable in the +/- 15 cm range at 1 sigma, I put some weight in it. They built the model used for the conversion.

Again, it has nothing to do with the quality, or lack thereof, of any geoid model.

Santa Barbara County.jpg


NGVD 29 blurb.jpg
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Re: Benchmark list for Cities in San Mateo County

Postby btaylor » Wed Sep 04, 2019 8:07 am

PLS7393 wrote:In your case SPMPLS, the geoid models are not as strong in the mountains where you are, verses the flat lands, so you need to understand your working conditions.

I've been informed that the bay area (San Mateo Co as identified on the post) is pretty safe to use the VERTCON conversion and when I use my GPS to test and compare elevation data, I am typically within a satisfactory tolerance. The key is to understand and delivery a good satisfactory product to your client.


Whenever I've had the opportunity to compare things with GPS, Vertcon comes out quite good in the Bay Area.

On a side note, if the job in in Menlo Park, Santa Clara County is close by and Santa Clara Valley Water has done a great job with their BMs (providing you are able to use GPS to tie into them).

https://www.valleywater.org/flooding-sa ... ol-network

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sako
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Re: Benchmark list for Cities in San Mateo County

Postby sako » Wed Sep 04, 2019 8:59 am

Guys, I have been using Vercon for long time when needed. What I'm asking for is if anyone has a list of BM's from SFWD.
SWVD benchmarks are great when nearby.

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Re: Benchmark list for Cities in San Mateo County

Postby SPMPLS » Wed Sep 04, 2019 9:01 am

Certainly making a direct tie from an NAVD88 bench mark(s) via leveling or GNSS to a NGVD29 bench mark is the proper way to determine the relationship between the two datums at that particular bench mark. Checking that relationship against a VDATUM/VERTCON modeled value is a worthy exercise, but even if it checks within reason, you most certainly are going to use the results of your observations/leveling. NGVD29 leveling had some major flaws in some of the level lines. Just because you checked VDATUM against survey results for one location doesn't mean that it is valid for all locations, maybe not even reasonably close locations.

My point is, using VDATUM/VERTCON alone to convert a NGVD29 elevation to a NAVD88 elevation doesn't ensure that you are on the NAVD88 datum. Possibly not by a significant amount. Depending on what you are using that unchecked, modeled NAVD88 elevation for, it could end up being a very costly shortcut.

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Re: Benchmark list for Cities in San Mateo County

Postby sako » Wed Sep 04, 2019 10:49 am

SPMPLS wrote:Certainly making a direct tie from an NAVD88 bench mark(s) via leveling or GNSS to a NGVD29 bench mark is the proper way to determine the relationship between the two datums at that particular bench mark. Checking that relationship against a VDATUM/VERTCON modeled value is a worthy exercise, but even if it checks within reason, you most certainly are going to use the results of your observations/leveling. NGVD29 leveling had some major flaws in some of the level lines. Just because you checked VDATUM against survey results for one location doesn't mean that it is valid for all locations, maybe not even reasonably close locations.

My point is, using VDATUM/VERTCON alone to convert a NGVD29 elevation to a NAVD88 elevation doesn't ensure that you are on the NAVD88 datum. Possibly not by a significant amount. Depending on what you are using that unchecked, modeled NAVD88 elevation for, it could end up being a very costly shortcut.

If I have a NAVD88 BM, I won't bother with NGVD29 BM's. These days most of surveys are required to be based on NAAVD88 for FEMA purposes. Some Cities also require it. There are some cities who's BM's are still on NGVD29 ( like San Jose). In San Jose converting with Vertcon is my last resort if I don't have SCVWD BM's nearby. I think cities with NGVD29 BM's should run level and update their list to NAVD88.

P.S.I still don't have SFWD BM list.

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Re: Benchmark list for Cities in San Mateo County

Postby MikeT » Mon Sep 09, 2019 9:31 am

I would call SFWD directly.

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Re: Benchmark list for Cities in San Mateo County

Postby Brad L » Tue Sep 10, 2019 7:22 am

I believe the SF PUC would be the agency to contact. The SF PUC surveyors in the SF bay area are out of their Burlingame office. However, I don't know if they have a LS in the Burlingame office.

FYI:
FEMA put out a Flood Insurance Study from 2015 that states the conversion from NGVD29 to NAVD88 is +2.75.

"The conversion factor from NGVD29 to NAVD88 was +2.75 feet for all streams and Stillwater elevations in San Mateo County."

https://www.menlopark.org/DocumentCente ... l-1?bidId=
Brad Luken


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