I have an industrial site on the coast. I don't work on the coast much so help me get my head around this. The site butts up to a river mouth and is part of a large port area.
I ran static GPS on 2 points for 6 plus hours for 2 days. All of the OPUS results come back looking great and put my 2 control points within 0.1' vertically between the 2 days of data. The vertical difference between my 2 points check out with what I shot them with my total station.
When I hold those numbers it throws some of the drainage structures below 0. Not by much but say around -1.0'.
The engineer we prepared the survey for is freaking out saying there is no way anything on the site can be below 0 or it would not drain and would hold water all of the time because it is below MSL.
I don't know enough to explain what is going on.
Any ideas on how to explain that my elevations are right?
Actual "Sea level" runs with the tide, "sea level" as it relates to datum is fixed.
Mountains out of mole hills.
Switch it all over to NGVD29, that should give you about an extra foot!
:coffee:
Find a graph in your area that looks something like this and show them how to convert between all of the different datums.
Dugger
😀 :good:
When I hold those numbers it throws some of the drainage structures below 0. Not by much but say around -1.0'.
What do you mean it throws some of the drainage structures below 0? What is doing the throwing? software?
What is the elevation of the outlet portion of the project 1' below 0???
isnt Louisiana below 0:-( maybe not a good example.
Are you on the same datum as the engineer?
Water runs down hill for sure
Great example Dugar.
Snoop: Give us a location and the benchmarks you actually used, someone could create a graph like this rather rapidly... I used to include such a beast on the face of topos in the SF Bay tidal regions, but I still got questions like that anyway.
"isnt Louisiana below"
Not the majority of it, I'm getting nose bleeds at around 30'.
Depending on your location (Northern Pacific coast, particularly) there can be substantial difference between NAVD'88 zero and MSL zero.
Sea level (and Mean Tide Level) vary greatly due to location and geography.
But NAVD will drain while NGVD may not.
...but that's another issue.
Radar's sketch is great, but maybe there are too many elements in there. Just the fundamentals will usually make them see the light.
But I have found that no matter who I am dealing with, architect, engineer, land owner, they never get it until they see the sketch.
Snoop,
I live and work down here on the Georgia coast, and it's a regular thing to have elevations well below 0 NAVD88. If you are fairly close to the marsh, then you see negative elevations in the inverts of storm pipes, sanitary manholes, ditches, etc. all the time. It still runs downhill. Just for comparison, our highest natural elevations are around 47 feet NAVD88. I get light-headed because the air is so thin up there. Kind of like being in Denver.
Dale Yawn
Savannah, Ga.
It's very common in Florida to see negative inverts, even on drainage structures that drain into the inlets and bays. And they do hold water at high tide. It can be a problem because they are not "self cleaning", meaning the water backs up into them and doesn't get enough speed on the way out to wash out the junk.
And, as others have said, NAVD88 is not a mean sea level datum. It's about 0.9' higher than NGVD29 (which was more or less a MSL datum) around here.
> It's very common in Florida to see negative inverts, even on drainage structures that drain into the inlets and bays. And they do hold water at high tide. It can be a problem because they are not "self cleaning", meaning the water backs up into them and doesn't get enough speed on the way out to wash out the junk.
>
> And, as others have said, NAVD88 is not a mean sea level datum. It's about 0.9' higher than NGVD29 (which was more or less a MSL datum) around here.
Thanks guys - I think I have enough from your advice to explain it properly.
NAVD88 ZERO and MSL Zero are not necessarily the same.
Dale - it is the East Coast Terminals site off of Presidents Street.
That ought to do it!
> It's very common in Florida to see negative inverts, even on drainage structures that drain into the inlets and bays. And they do hold water at high tide. It can be a problem because they are not "self cleaning", meaning the water backs up into them and doesn't get enough speed on the way out to wash out the junk.
>
> And, as others have said, NAVD88 is not a mean sea level datum. It's about 0.9' higher than NGVD29 (which was more or less a MSL datum) around here.
Exactly the same here in Hawaii.
Years ago I did a survey that required some rough elevation numbers a couple thousand feet from the shoreline boundary. Not having a convenient benchmark, I figured that a sea level shot would do in a pinch. Adjusted for the tide, it actually matched up pretty well with a FEMA benchmark we ran in later.
I am with Buck on this
What datum was used for the engineers design?
Snoop
Snoop,
I have done a good bit of surveying there. Give me a call if I may be of assistance. I'm in the phone book, and will be pleased to help if I can.
Dale
