The Tale of Two Datums

[ClemsonVet]

One surveyor finds control and brings it to a site, sets benchmarks for that site. civil site plans are designed and construction of a building starts.

a second surveyor finds control from a different source and brings it to a site, sets bench marks. civil road plans are designed immediately adjacent to the site above and construction starts.

for this question it doesn't really matter where the control came from. There is a vertical datum difference of 4" between the two.


Question,,,,

what happens when these two projects meet? are they going to lock in or will they be off 4".

thanks

please bill your time to marketing.

 

[phamENG]

Well that depends, doesn't it? If the road elevation for Datum 1 was supposed to be +92'8" and for Datum 2 it would be +93', then it'll line up. If they're both intended to be +92'8", you might have a slight problem.

Now please provide your marketing department's address so I can send them my bill.

 

[bridgebuster]

Each set of plans should have stated what vertical datum is being used. In NYC there are probably two dozen datums. However, it’s easy enough to convert from one to another.

 

[LittleInch]

Not sure why the second surveyor didn't use the first benchmarks but of course there will be 4" difference.

In reality they might just make up the difference on site to avoid a step where no step should be.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[SwinnyGG]

Not sure why the second surveyor didn't use the first benchmarks but of course there will be 4" difference.

If I'm contractor A, I am never, ever, ever, ever, ever, ever, ever going to allow my site control to be set off of contractor B's benchmarks which don't match the ones on my drawings.

I'm setting benchmarks per my plans, and I'm quintuple checking to make sure they're right before a shovel touches dirt.

 

[MotorCity]

It sounds like the building and the road are two independent projects. If thats the case, there is no reason the benchmarks need to match, its all relative. If they each had used the same source for setting the benchmark and are 4" off, thats a problem. It means one (or both) of the project elevations is off. For example, say an elevation = 550.25. Surveyor "A" for the building might set that as his benchmark at 100'-0". Surveyor "B" for the road may call that same elevation 600'-0". Does that mean that the two surveys are 500' apart? No, its two different names for the same elevation.

 

[LittleInch]

I've assumed that the same datum point is used relative to some local or national grid or using the local grid datum.

The question isn't clear.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[jgailla]

I had a site contractor friend who didn't know about the NAVD88 vs. NGVD29 conversion. She built a site and then tried to tie into a road, coming in about 1.5 feet off. There was a lot to tear out and regrade to get everything to work. The problem is tying in outside the project.

 

[LittleInch]

I had a site where when they built the access road, the initial contractor either made a huge mess or, I think, the datum post got knocked over and someone just replanted it about 5m out in x, 2m in Y and about 0.4m in Z.

They then built the road according to the plans.

Problem only surfaced when I was on site trying to work out how my GPS was >5m out as my pipeline was supposed to be 5m off the road when I was standing in the middle of the road which had already been built.... That caused a whole heap of issues.

The disparity between offshore datums and onshore ones also causes a lot of fun....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[coloeng]

If the civil site contractor uses the benchmark set by surveyor 1 and the civil road contractor uses the benchmark set by surveyor 2, there should be no issues. If either of them use the incorrect benchmark, then you will have issues.

 

[ShafeW]

Not always a surveyor or contractor problem.
e.g. Engineer 1 provides roadway plans to engineer 2 for reference. Engineer 2 reads roadway elevations from those plans, not paying attention to the datum used, and designs his pavement and sewer tie-ins 4" in error. He always needs to check what datum outside plans are using, and if he doesn't understand how to convert, consult his surveyor for a conversion.

 

[FacEngrPE]

Due to datum confusion, now I require all surveys and site plans to include a datum note, which is most often written by the project surveyor.
NGS on datums and reference frames

https://www.ngs.noaa.gov/datums/index.shtml