California Surveyors Forums

The City of Los Angeles is listing on the Navigate LA site benchmarks with a NAVD 1988 Adjusted value.

I think this is a dangerous thing to be doing.

The attached PDF shows two examples.

Example 1:

The new 2000 elevation of this bench mark is +/- 2.5 feet Higher than the previously recorded NGVD 1929 elevations.

Example 2:

This bench mark does not have/show any previous elevations, and I believe this type of listing is the most dangerous type that is listed.

Probably 99.9% of the sub surface utilities in Los Angeles were laid in place using the NGVD 1929 bench mark elevations. If one (surveyor) uses a 2000 elevation then problems, or even a tragedy, could exist if excavation takes place.

USA is fine for horizontal locations, but this is all about vertical positions.

http://www3.gendisasters.com/california/16894/los-angeles-ca-gas-pipeline-explosion-june-1976

I remember that 1976 incident. I was told that the first ones to see the puncture happen was a survey crew..maybe. I was also told that the backhoe
operator who punctured the line just bailed out of the cab and ran as far as he could..most likely.

The error for this one particular incident was a high pressure gas line closer to the surface than was believed.

I wont use the 2000 values for an Architectural - Design Survey. I am doing an As Built for a High School that is next to the listed BM in Example 1. The sub surface records show that there is a 6'' So Cal Gas line in the area of planned construction and a 30 inch, yes thirty inch, So Cal Gas Line in Riverside Drive.

I have been wanting to post about this for a while.

The City of Los Angeles is listing on the Navigate LA site benchmarks with a NAVD 1988 Adjusted value. I think this is a dangerous thing to be doing.

NGVD29 has been unsupported by NGS for almost 25 years. All new floodplain mapping of which I'm aware is being referenced to NAVD88. When would you have L.A. enter the 21st century?

Datum changes always present opportunities for confusion, but -- fortunately for the public -- most elevation data is handled by people trained and licensed to manage it properly: land surveyors.

.

Jim..

The 2000 elevation simply does not relate to what is below the surface by 30 inches. It also is far removed from all surface improvements by the same amount.

but -- fortunately for the public -- most elevation data is handled by people trained and licensed to manage it properly: land surveyors.

Oh snap!

Kinda like doing boundary surveys...right?

Am I missing something? Doesn't the fact that the City references the datum remove all confusion?

DanFrink wrote:Am I missing something? Doesn't the fact that the City references the datum remove all confusion?

lol, Bronx Cheer to your questions.

BTW, I REALLY hope you are being facetious.

Both benchmarks are a bolt set in the top of a curb, not exactly a stable vertical mark.

I think every Surveyor could benefit from working as an Inspector for a few years. You would be surprised how much concrete and asphalt we insist on setting benchmarks in move around. The monument structure isn't just the bolt; it is the unstable concrete it is set in too.

The difference between NGVD29 and NAVD88 is around 3' in most of California; 2.5' is not unexpected.

Both benchmarks are a bolt set in the top of a curb, not exactly a stable vertical mark.

So where would you put them. OC sets theirs in Catch Basins and OLD LB ones are in wells but all other agencies seem to use TCs. Some old Federal BMs were br caps in the side of a building, real PITA to use trying to hold a elev rod vertical while on your knees trying to keep the bottom of the rod at the stamped X on the cap.

My issue with most datum info is that it's in the front of the book and not mentioned on any of the data sheets. I think any BM data sheet should show the last 29 datum elev along with its first 88 datum elev so reference can be made to old plans.

Let me see, what datum is San Diego on, City of Fullerton, City of Long Beach. I will state I have not used any of these in over 5 years so they might have updated - LOL.

I go with Land Butcher and see no real cause for concern in regards to the datum shift. When you do a topo you need to cite what you used for datum and if the underground improvements are vital, then I would suggest using a backhoe to pothole them.

Berk Blake, PLS CA wrote:I go with Land Butcher and see no real cause for concern in regards to the datum shift.

I'm not concerned either, on the job I am currently working on.

The PDF shows a portion of the area that new construction will take place in.

When you do a topo you need to cite what you used for datum

Nothing personal Berk, but IMHO any surveyor who does not use a plan BM for a design survey that relates to above and sub surface improvements is akin to a surveyor using prorate to pincushion an original tagged iron pipe that is visible to anyone in the area, and to relate the 1988 Datum vertical difference to the 1929 Datum, the prorated position would land 2.5' away from the original pipe.....

1988 Datum is dandy for an elevation certificate, but impractical and very risky to use for tying into 1929 Datum based improvements.

and if the underground improvements are vital, then I would suggest using a backhoe to pothole them

Contractors pothole all the time, but if one (backhoe operator - grade checker) thinks that there is an extra 30 inches to go, they wont slow down digging until a pipe is punctured.


I can hear it now "Boss! The grade stake says 646.1 and we checked a few and the gas pipe is at 638, here, look for yourself on the profile of where the gas line is. We only went down 3 feet!!"

I REALLY hope you are being facetious.

Just kidding. This was my initial though, makes sense to me.

Berk Blake, PLS CA wrote:I go with Land Butcher and see no real cause for concern in regards to the datum shift. When you do a topo you need to cite what you used for datum and if the underground improvements are vital, then I would suggest using a backhoe to pothole them.

1988 Datum is dandy for an elevation certificate, but impractical and very risky to use for tying into 1929 Datum based improvements.

Where I work we've been using NAVD88 for years, successfully integrating utility design and as-built plans that are referenced to NGVD29. It's not a big deal.

.

A good backhoe operator will find the pipe before hitting it. I had one find a 12" hp gas line that PG&E didn't even have on their maps. Relying on record elevations of facilities like that is a recipe for disaster.

In my opinion, setting bolts in the T/C for benchmarks is not good practice but that isn't to say I don't know is widespread due to the cheapskate nature of our profession.

You don't see the water utility setting ad-hoc cheopo boxes made out of plywood for their valves but I swear that's what it would be if land surveyors ran the water utility.

A benchmark should be a rod monument driven to refusal as deep as it takes to get a stable monument. "But that costs too much!" We can spend $20million in staff time to determine the elevation but 25 cents is too much for the monument. That attitude seems to be widespread in our profession.

Other professions/industries get outside help to build sufficient facilities but land surveyors seem to have this do-it-yourself mentality that it costs too much.

Imagine if all monuments were, large, substantial and prominent. I bet we would get a lot more respect from the public.

Paraphrasing 4722 - "LA now publishing NAVD 88 datum on BMs is dangerous!" Somehow I don't see this as dangerous.

I find myself agreeing with Frame, Frink, Karoly & Blake. Frame indicated "NGVD is no longer supported." and "he's been using '88 for years."
Frink get's it to a point: "doesn't referencing the datum remove all confusion?" - Yes Dan, but only for those who understand the differences. That takes me back to Frame: "most elevation data is handled by people trained and licensed to manage it properly: Land Surveyors." If you are confused or frightened by the change from '29 to '88, then you are probably not trained to handle the datum changes properly, you are likely working beyond your level of competence, and thus outside of your license. Remember, we are licensed to protect the public.

When 4722 replies to Frame that "the 2000 elevation simply does not relate to what is below the surface by 30 inches. It also is far removed from all surface improvements by the same amount." I think what he is trying to say is that the NAVD88 value published in 2000, does not correctly relate (30" difference) to the "As-Built" plan information based on NGVD29. If the "As-Builts" are correct, what was supposed to be below the surface by 30" should still be 30" below the surface, no matter if the BM elevation is '29, '88, or assumed to be 100.00'. And yes, Dave, our monuments should be stable, large, substantial and prominent.

What IS dangerous, is the PLS who thinks his topo indicating the H & V location of underground improvements based on "As-Built" plans is perfect, because he is working in the datum of the original construction. This is far more deadly than a dreaded boundary pin-cushioner, and every bit as unprofessional.

If the utilities are high-risk facilities, they should be marked out, and positively located (Potholed per Blake) as a part of the original topo, as well as construction. Potholing does not always mean backhoe either – Vactor trucks are also used for positive utility location. Find the actual utilities, and indicate their location through whatever datum you are using.

Unlock the mysteries of different datums: Sign up for the CLSA - CSRC geodetic workshops coming soon to both Southern and Northern California.

http://www.californiasurveyors.org/PDFs ... rkshop.pdf

Rob_LS wrote:Paraphrasing 4722 - "LA now publishing NAVD 88 datum on BMs is dangerous!" Somehow I don't see this as dangerous.

Since just about everyone missed the point of my post, I'll elaborate.

I was talking about the contractor, grade checker, layout person etc etc..not LS's.

I have been on a number of pothole surveys and have NEVER given a cut to the top of pipe. I don't even remember doing line layout before the dig, only being called after the pipe is found to do XYZ on it.

In my opinion, setting bolts in the T/C for benchmarks is not good practice but that isn't to say I don't know is widespread due to the cheapskate nature of our profession

I don't know what county Kilroy is in but; San Diego "BP in TC" Brass plug. LA city/county bolt, spk, lead and BM tag all in TC. OC alum cap in CB. Riverside/San Berdo - do they even set anything or just rely on the old BMs set along the RRs 70 years ago.
Occasionally, or seldom I run across a concrete post with a Br cap.
ALL SET BY GOVERNMENT AGENCIES.
So who is the cheapo?

Nice summary Rob, agreed. LS 4722, I did miss the intent of your original post. I do see the danger for other professions, but not for Surveyors (unless they are very poorly educated in surveying). To me this is a very basic principal that should be understood even at a technician level.

Dan

DanFrink wrote:Nice summary Rob, agreed. LS 4722, I did miss the intent of your original post. I do see the danger for other professions, but not for Surveyors (unless they are very poorly educated in surveying). To me this is a very basic principal that should be understood even at a technician level.
Dan

It's good to see one person acknowledge the fact that a tragedy could take place if the 2000 elevation was used by the wrong people. I do not know how it is in your area, but in LA County there is a lot more activity taking place in construction and unlicensed surveying by individuals that can barely speak English.

For people like you and of course Jim Frame, anything surveying related could be handled with ease.

Now..if surveyors would quit fibbing on record maps..