ƒ??Real Elevationsƒ?

I'd say that depends on what is required on plot plans and the land development deliverables in your area.

Would I always carry elevations throughout all my observations and perform a network adjustment on them to get the most likely values? Absolutely.

Would I always tie to a physical benchmark? Not necessarily.

99.9% of our projects are tied to the NSRS through VRS vectors or post-processed static observations. Local BMs are few and far between, and often of mediocre precision. If we're not required to tie into one, we rarely do.

Every project elevation is local. "Real Elevations" are defined by the benchmarks.

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But to the point, I can think of many reasons why an assumed datum might be used on a project. As we move to easy access to quality, blue booked, (networked) GNSS corrections, the reasons become fewer and fewer.

Sure, I do it all the time. If it's a single-site non-permit, non-flood, non-MHW topo with a single instrument setup. Why charge the guy $250+ more just to be cool? My elevations ARE good.

@oldpacer?ÿ

You forgot the dripping sarcasm font.

Thanks for the input, all.

"...99.9% of our projects are tied to the NSRS through VRS vectors or post-processed static observations."

and

"As we move to easy access to quality, blue booked, (networked) GNSS corrections, the reasons become fewer and fewer."

Are what I expected.?ÿ Where I work, (in a bigger metro than my hypothetical) it takes less and less effort all the time to "approach" using NSRS grade elevations on almost every project, if not necessarily publishing that fact.?ÿ There are definitely exceptions, and I can absolutely see situations where, especially for something basic like a stand alone plot plan, that you might use assumed elevations.?ÿ As long as the sewer and the yard drain properly, most homeowners will feel like they got what they paid for in that regard.?ÿ

More and more of what I see actually being built though, at some point ends up on machine control, or some other GPS guided control.?ÿ Was just curious as to the thoughts and everyday practices of my esteemed colleagues here.

Thanks again for the feedback!

Still Learning!

@jitterboogie?ÿ I didn't forget, I did not know I was being sarcastic. Why are assumed elevations NOT good.

In NC we are required to perform a tie to NCSPCS if a NGS benchmark is within 2000' of any portion of the subject parcel.?ÿ If a PLS doesn't own a GNSS receiver, they can traverse out to or from the BM.?ÿ If you're into that making money thing, you simply slap your receiver on the first corner and getter done.?ÿ ?ÿ

There are plenty of properties that aren't within 2000' but I've found that getting all the projects on State Plane makes my life easier.?ÿ The free Orthos and LiDAR fit like a glove and destroyed control becomes a non issue.?ÿ

PLSs using local datums doesn't bother me in the slightest.?ÿ I'm more concerned about the guys who set foot long #4 rebar for lot corners.

Posted by: @learner

More and more of what I see actually being built though, at some point ends up on machine control, or some other GPS guided control.?ÿ Was just curious as to the thoughts and everyday practices of my esteemed colleagues here.

This MIGHT be an argument to have control offset by a 1000 feet or something from NSRS.

IF you publish NAVD 88 and the operator of that whiz bang machine control fails to calibrate to your benchmarks, then there could be a lot of money wasted.?ÿ

The fact that the machine control is GPS guided is not a reason to have the vertical datum be on the NSRS, IMHO.

@oldpacer?ÿ

I misread. My apologies. Assumed works, I was missing the 250 dollar reference. Thought that was for the whole cost of the survey.

Sorry.

Simple construction such as a house site gets at least two bench marks and the property corners are horizontal control with offsite control nearby. Vertical control will be leveled or total stationed.

More complicated surveys get more control points that don't include property corners. Those have as bare minimum three control monuments which are leveled. All jobs are tied to NAVD 88 bench marks in and around town. No job is 1000, 1000, 100. If no bench mark is nearby, control is set using the nearby CORS or HARN.

If a project does not have an absolute need for some bureaucratic paper shuffling tied to a "real" datum, then pick a benchmark, label it as being some number that will never be confused with a "real" datum value and roll on.?ÿ It's all relative.?ÿ K I S S

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As in everything, it depends.?ÿ Your scenario, a place with 25,000 people, still maybe.?ÿ If the job is strictly a horizontal affair, I don't really see the need to hit everything with elevation.

If I decide to GPS my control, yeah, I'll probably at least do measure-ups and carry elevations on my control.?ÿ But again, unless I foresee some reason, I might not.

I don't usually work in towns that size, and lots are big.?ÿ So I don't see much of a reason to worry about elevations on a 100 acre boundary survey.?ÿ Even if it becomes a topo for a new house and septic, I'll just go back and elevate a couple points near the proposed development, and it may or may not be real datum, depending on the circumstances and requirements.

Posted by: @dmyhill

IF you publish NAVD 88

Due to the large amount of confusion about what exactly constitutes NAVD88, my vertical datum notes will?ÿ usually read something like:

"Vertical datum is GNSS-derived NAVD88 utilizing GEOID18. Geoid separations were applied to solved NAD83(2011)[2010.00] ellipsoid heights at control points XXX and XXX to obtain orthometric elevations. Differential levelling was then employed to transfer orthometric elevations to project benchmarks."

I really, really hate it when I just see "Vertical Datum = NAVD88" on a planset or a survey. That tells me nothing.

The county flew the area and topo maps were created using lidar at 1 foot contour intervals. These maps have checked well to the bench mark system so its a valuable resource. For a strictly boundary job there is no reason to even consider elevations yet the process we use creates elevations and it's almost as simple to stay on the system as not.?ÿ

I can't see doing a topo or house layout and using 100', the tie ins to sewer and FEMA maps and our previous work in the area will all be using the same basis for elevations. If each job is 100' and on different systems we lose all the ability to map out individual projects and use that info for the next project. Why do 100' when we already tied each manhole and water tap, have all the inverts and invert elevations. It pays off over time to collect data that can be re-used.?ÿ

@rover83?ÿ

Datum AND how you determined it are both required on any topographic map signed by a land surveyor.

Not sure what is required by a drone operator and all the topographic maps derived by LiDAR from the counties that are not signed by anyone. Those have no rules, of course.

So, if you are in WA, and you do not see the following, you know you are not getting an actual survey:

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WAC 332-130-145(1)(a) Vertical datum used (such as "assumed," "NAVD 88," "NSRS," "unknown");
WAC 332-130-145(2) Statements of clarification for elements shown:
(a) Basis of elevations citing benchmark(s) used with elevation(s) (such as "city bench mark 20-01, elevation 456.32 feet, GPS observation including metadata");

@mightymoe?ÿ

A list of known benchmarks in a given city in my area are NGVD29 elevations because that's what the plan sheets are in.?ÿ If sewers are involved, you check the point where the new sewer will enter the system.?ÿ Hiding a note on Page 2 of a 50-sheet set of plans is meaningless.?ÿ People head for the sheet that is pertinent to their current project.?ÿ Even the city employees get a strange look in their eyes when you start talking about different datum numbers as those employees are not educated and experienced engineers or surveyors.

I do some work for a local C&D landfill.?ÿ Their 3-inch thick engineer's report for future use of the facility was prepared about 1985 and the elevations are NGVD29.?ÿ My report for permitting of each new opening in a cell references the 1985 report and uses those benchmarks as being the ones to which I make my check shots for elevations.?ÿ It's all relative.

@dmyhill?ÿ

For sure, I get that it is required, but I still see stuff like "Datum: NAVD88 using GPS" when it's obvious a survey of some sort was conducted and a licensee involved. It's also not rare to see project benchmarks that are obviously unable to be occupied by GNSS gear, with no explanation as to what was held or how elevations were transferred, despite that note about "GPS" being the "datum".

Some of the planset problems, I know, stem from engineers getting their hands on the control survey and inserting boilerplate language for their plans. Even our in-house engineers do this occasionally, despite multiple warnings to not touch our datum language.

But it's worth noting that I still get pushback from other/older licensees telling me datum notes like I posted above are "too in-depth" or "confusing" or "incorrect"...after I help them out by processing and adjusting data for their project because they don't understand processing, or datums.

@rover83?ÿ ?ÿIn my opinion, it's the younger licensees that don't get it.

My favorite bench mark is labeled "JOIN".