Data Acquisition

Unfortunately every measurement ever made, interferes with the item being measured, even though we commonly accept such a "measurement" as being accurate.

Albert Einstein and others realized this, after they understood there is no absolute fixed frame of reference, but each observer is entitled to assume their reference point is fixed (although to others, that is not so).

Further stated by Heisenberg, in his famous Uncertainty Principle.

So we can never truly know anything, really,just "educated guesses".

For instance: the number pi is used in so many equations,but cannot ever be accurate - it was designed that way, likewise with the irrational number rho.....

Unfortunately, there are numerous places on the planet where GPS technology for civilians is woefully inadequate, particularly when measuring elevation. I have been trying to obtain elevation readings for several locations along a track where we are installing a water system in a rural village in Tanzania. We have had numerous readings taken at the exact same location with several new GPS units. None of the readings agree remotely with each other, and none of those readings agrees with the elevation readings from Google Earth.

Hello aqua doc

Dependent on the variations, it may be because of the tidal force exerted by the Moon, on the Earth's crust, which varies over several tidal hours.

I have never trusted GPS units, normally folks expect these are accurate, and trust the one they use, never realising it is inaccurate until they check its readings against other units.

May I suggest you go back to an older technology, more laborious though it be, of theodolite and chainman/staff operator.

I do note that some local Authorities have abandoned the GPS system, and gone back to the theodolite surveying method, for the same reasons you are finding.

With your water system, establish a fixed location as a datum point, and work from that point. (No doubt you are doing that, anyway)

In the past I have had no trouble with properly surveying with theodolite method, for transmission lines and water projects etc....

I agree with aqua doc's observations regarding elevation, however, in my experience, I have found that the lat. & lon. readings are pretty close to being "dead-on".

The elevation problem can be readily addressed - Get a good altimeter.

Hello TheJMAN,

"The elevation problem can be readily addressed - Get a good altimeter"

The altimeters in common use, rely on barometric pressure for the altitude sensing.

This means they are weather dependent......

15 meters? My 5 year old Magellan is spec'ed at 7 meters, and when in WAAS mode is 3 meters. I agree that the altitude is a bit flaky, but if you give it enough time it usually settles out to a reasonable accuracy.

As for uses:

1. Using a 802.1b pcmcia card that logged signal strength... every second, the GPS unit logging (through serial) the lat/long every second; got a pretty good idea where some new access points needed to be for better coverage.

2. Using the GPS and some 3d terrain mapping software, did a good initial job on a radio telemetry system. Not good enough to rely on alone, but good enough to weed out the places where it wouldn't work.

The problem with "High Level Technology" is that it requires electrical power to run it.

The singular advantage of the Theodolite and chainman/staffman, was that providing the other end could be clearly seen, readings could easily be taken,and calculations or profiles for gradients & levels were easily done.

Now, please don't read that the wrong way, but most of the World's greatest engineering works of Power Dams, Canals, Pyramids, China's Great Wall, Railways, Roads, Bridges etc. were done very successfully, without the use of "High Technology GPS Systems".

Often all that was used, was a basic "Spirit Level" and line-of-sight readings taken, then marks made on "profile boards" to enable workers to excavate or fill, or build to the line.....