How to measure height above sea level

[TSN79]

When drawing sewer pipes, I often include the height above sea level within one centimeter to get the correct fall, simply cause I've been told to do so. I'm just curious as to how this height is actually measured by the people who actually lay these pipes? Is there some kind of instrument that tells you this height directly and with such good precision?

 

[star.torturer]

GPS style Satalites do it. I think

they time how long it takes to get from the satalites position to the ground and back to the satalite, feed oit into a program and, whala its there. that's what i suspect.

 

[TSN79]

And it's good down to one centimeter?! Don't "normal" GPS handy-things have a precision of like 10-20 meters or something...? Maybe these instruments are perfectly tuned for this...thx!

 

[star.torturer]

the Military keep all the best ones so they might have one up to 1mm , maybe.

you could always use a topography program for it

 

[FredGarvin]

I have had surveys done for my installation. They used GPS. The accuracy I was given was, if all goes well, down to a couple of inches. That being said, that was also doing, what they referred to, as a chain traverse (or something like that) in which they started at their control point and traversed about 20 points in succession back to the original point. In that circumstance I could see the couple of inches being an accumulated error. You do have to ask yourself if the accuracy you are looking for is necessary.

 

[civil_dude]

Elevations are typically transferred from a known point, called a bench mark, to a project point with 'Level Loops' using equipment that is just for leveling. You run a loop to determine closure, i.e. subtract the change in elevations going one way from the change coming back and divide it by the length of the run.

There are different levels of rigor for different types of work. The closure depends on both the error and the length of the loop. A typical closure of 1:10,000 is achieved with good equipment and methods.

GPS is not typically used to transfer elevations if a high degree of accuracy is required.

 

[russ_watters]

When taking many stationary fixes and averaging them, you can get much more accurate than the typical individual fix accuracy of 3m (with Dgps).

 

[Mech_Engineer]

When taking many stationary fixes and averaging them, you can get much more accurate than the typical individual fix accuracy of 3m (with Dgps).

IIRC, Differential GPS can be accurate down to 1cm (theoretical). They do use DGPS units with enough accuracy on an airplane to accurately measure pitch, yaw, and roll, so I know they are very accurate in the correct applications.