Yesterday I was setting out a site for the position of a new house.
Working off an existing mark, I checked enough of the control and boundary positions to be happy. Set out the grid lines - all good.
Then the builder asked us to set a datum nail he could use for height, which was easily done.
He then asks for some quick checkings on some of the rough earthworks. We find that they have not gone down far enough - about another meter of excavation needed.
I'm suspicious so check another couple of points - by the numbers it looks like the architect wants the house dug down into the hill quite a bit - at least a half meter or more.
But wait - that's not what the plans show. It is drawn to be almost level on the driveway.
I checked the ground levels shown on the plan at three points I could clearly identify, and they all show about a one meter difference between our numbers and the heights shown on the architect's plan.
So what would you do now?
(to be continued)
I've seen those kinds of differences in elevation around here, and it happens to be the difference between NGVD'29 and NAVD'88. Of course I don't know anything about where you are, but it sounds like the architect's design is in a different datum or assumed datum if it was done from a real-life topography at all.
What would I do? I would go to the architect or someone and tell them that the vertical data doesn't jive and that you need to get on a common datum.
There is a note on the plan saying all levels in terms of the city drainage datum. There is no datum point shown or referenced on the plans.
My heights come from two 2nd order benchmarks near the site which are in terms of that datum.
I believe my numbers to be good.
What to do?
What we did was to phone the architects, who put us on to the surveyors that did the underlying topo.
Apparently they took their heights from a manhole lid at the bottom of the hill.
Hoping to be able to use it, we find the manhole - but our height for it busts their original number by 200mm. And if we use their numbers we are still left with a 500mm bust.
By now I'm pretty sure the underlying survey is punk.
So what now?
Double check my assumptions and look for a given elevation on something existing I can use to tune up to whatever he's using for his datum.
I would like to do that, its the quick way to get the builders going.
But there are some liability issues here.
The house is shown as being tight (600mm) on some recession planes and at the end we have to certify that the building was built in terms of the plans.
Jim stop and get architect on site. Perhaps have ready get critical levels in your datum ie for storm water drainage outlet and sewer outlet.
Architect knows all critical design function parameters....
RADU
Because the site is elevated and steep I'm much more concerned with busting recession planes than with drainage connections.
But stopped is where we are at.
I've sent a hardcopy of the details and numbers to the builders (our client) for them to take back to the architects.
Well Done
I agree with RADU
