Benchmarks

Thanks, Mark. I'm on it. Good link.

Dave

Tom,

"I download the data from any of their close base stations and process the data."

So that's kinda like a mini OPUS? I'm going to file that one under "investigate further". I noticed the water district had a page with "...static data files available at a 5-second epoch rate and stored as 1-hour files in a zipped Rinex data format". I've not used rapid static before. That would be great where the radio reception suffers.

I'd gladly visit with you as you're passing through. Hit me with an e-mail when you're ready to travel.

Dave

Bill,

"...whether the tilt of the geoid is significant in 3 miles..."

It's bad enough that I've got to worry about RTK inaccuracies. Now I'll also lose sleep over tilt from the Geoid! Thanks. :-O The subdivision doesn't back up to a mountain range, but it's hard to tell what densities of rock lie below the surface.

Dave

Unless the scope of the work calls for the elevations to be in a particular datum, why not just use an assume elevation and note that's what you did on your finished drawing? Do establish a couple BMs on the site, preferably something that will survive construction and it can always be translated vertically at some later time. I do this fairly regularly, but never by way of RTK. If I only had one receiver, like your saying, I'd do two long static sessions on the benchmark, one in the morning and one in the evening and process them through OPUS. What I'd be shooting for is to pin down the geoid separation from the ellipsoid in your particular area. If I had two receivers, besides doing static on various benchmarks, I'd static on two inter-visible points on site and then check the elevation difference conventionally and compare that delta to my GPS derived delta. You could do it with one receiver, but two cooking simultaneously is mucho better. Build in some redundancy into your measurements so you can go to bed with a warm fuzzy feeling that's not alcohol induced.

I guess the only part that's bugging me a little is the question in the back of my mind of who is overseeing your work and providing you with some direction? I get the sense that you're being left on your own to figure this stuff out. That isn't the way it should be in my book but I commend you for your seeking mind.

Best of luck to you Dave. Willy out.

Dave,
Sometimes these younger fellers are a bit hard on us old timers. When I was in school, and you too probably, there wasn't any GPS to speak of yet, and I had to learn it on my own in the school of hard knocks. Heck, everybody has to learn somewhere. I've still got a steel tape, though, and a chain, too.

> No sir. I'm a carpenter. I work for a living. 🙂
>

Yeah, us surveyors jump out of our trucks for two minutes, look around, get off early, and get paid a lot of money!

> Point in question: If the water district base is using NAD 83 (2011) epoch 2010.00 as a horizontal datum, how would I go about discovering what vertical datum they're using?
> Dave

Where the elevation is published,it should state the datum. If you want a real elevation, you would probably be better off with an OPUS solution. If you are required to have tie to a municipal bench, I can't conceive not needing a surveyor.

Was not trying to flame you. But without a GEOID or a localization I dont think you can transfer elevations effectively with RTK.

RTK Is OK For Local Elevation Work

When I transfer an elevation by GPS I want a minimum of 2 OPUS-RS points on the site. With no shortage of CORS I expect under a 0.10' to a local benchmark check.

Much of the US requires longer OPUS observations to get equivalent precision.

Paul in PA

JT,

"When I was in school...there wasn't any GPS"

Hell, when I was in school, I don't think that electricity had been invented yet.

And not only am I trying to figure out GPS, I got my first Smart Phone yesterday. I'm drowning in technology.

Dave

Willy,

I always try to build a little redundancy into anything I do--overlap a couple of points, take a shot on yesterday's work, etc.

Thanks for the GPS workflow suggestions. I'm really glad to hear that GPS is capable of accurate elevations. I wish I had a second receiver.

As for supervision, the PE is calling the shots. No PLS in the office right now. (A temporary condition) So yes, I'm on my own.

Dave

RTK Is OK For Local Elevation Work

Thanks, Paul. I just looked at the map of CORS stations. You're right--there's no shortage of 'em.

Dave

dmyhill,

"Yeah, us surveyors jump out of our trucks for two minutes, look around, get off early, and get paid a lot of money!"

That's why I'm trying to break into the Surveying Racket. Money and hot chicks. B-)

It looks like the consensus here is that RTK is insufficient for tight elevation work, but that Static or OPUS will do. That's the information I was looking for.

Dave

RTK Is OK For Local Elevation Work

around here we have no shortage of various governmental and quasi-governmental control monumentation to reference. really, it's remarkable. so i always do, if for no other reason than to see that i'm in the ballpark. start and end the day checking into the closest 2 or 3 monuments, each day i'm on site. but i'm doing (almost exclusively) site specific topo and design work with little to no floodzone implications. on those jobs that do, i'm running hard levels as, again, there's gonna be a monument or three close enough to trump the time i'd spend with OPUS (i only have one antenna right now myself).

but datum is the snag, ain't it? hundreds of shots on local control (which is gonna be 93 or 96 almost all the time), i'm gonna come up roughly .20 higher than published elevations. i expect it now to the point that if i don't then i'm looking at that day's work a little closer.

but my standard MO is to set the site benchmarks, check them repeatedly, and run a closed loop through them (doubles on the robot, which has shown to close on itself more than sufficiently for the work i'm doing).

the interesting datum issue that i keep getting dragged into around here is with the capitol view corridors, a series of descriptions with controlling X,Y, and Z elements, against which the developers of our last 2 decades of urban infill seem to really like to toy with. oh, and the corridors were established in 1978 on, of course, NAD27/NVGD29. it's amazing how many of these cats that are throwing 35 story buildings with appurtenant 12 story parking structures continue to get burned by designers and civil firms who jam the entirety of all that to within a blown angle of million dollar+ penalties (or the tear-down of the entire top floor of a 400 car garage).

Pay careful attention to your antenna heights. One of the most common and most avoidable sources for error. Fixed height rods are the way to go if you have them.

Willy,

I'm absolutely anal about rod heights and instrument heights. And yes, the 5700 is lashed onto a fixed-height rod. The Active Prism I use for the semi-robotic total station isn't, but I always check it everyday and set it for 6.5 feet.

Dave

RTK Is OK For Local Elevation Work

Flyin Solo,

I'm always amazed at the vertical accuracy of modern robots. I've seen S6's that could radiate elevations on a large construction site as tight as I could get them with a level loop.

Dave

RTK Is OK For Local Elevation Work

Dave-
Just to add some further food for thought,when using real time GNSS positioning:
1. Know what horizontal and vertical datums your legacy plans, plats, drawings, etc. are based upon and what horizontal and vertical datums you will need to work with or in which you will deliver the data.
2. Know what accuracy and precision (not the same thing) you need for your work or deliverables in order to know what tool you will use.
3. ALWAYS have redundant locations for important points
4. ALWAYS get a check shot before capturing new data (and after reinitialization, if possible)
5. ALWAYS have solid, continuous communication to the network or base while locating a point
6. Be aware of and avoid multipath conditions if possible for important points.
7. Your data quality is dependent on three things: the accuracy of the physical or virtual control, the precision of your GNSS derived ellipsoid heights, and the quality of the hybrid geoid model (for NAVD 88). If one of these is too inaccurate for your needs, your elevations (orthos) are no good.
All that being said, did you know that with good conditions, using the SAME initialization and a small project site, you could actually determine relative orthometric heights using real time procedures to sub-centimeter precision? Things can be complicated with real time work and "it depends" is really the answer to most questions.
Kudos to you for your bravery in stepping up to use real time, your quest for knowledge, and for keeping your cool in these discussions.

RTK Is OK For Local Elevation Work

> 3. ALWAYS have redundant locations for important points
I would say ALWAYS have redundant measurements to important points. One resolved location is enough.

RTK Is OK For Local Elevation Work

"You're right--there's no shortage of 'em."

There sure is a shortage of them in NE Arizona!!

RTK Is OK For Local Elevation Work

"...did you know that with good conditions, using the SAME initialization and a small project site, you could actually determine relative orthometric heights using real time procedures to sub-centimeter precision?"

I have a project where I find vertical repeatabilty is ±0.02' where horizontal repeatability is ±0.05'...