[TowerTalk] measuring elevation

[Bill Ralston]

"Eric Scace" <eric@k3na.org> wrote:

>    A stretch of garden hose (or several hoses connected together, or any
other length of tubing) makes an excellent elevation
> measuring device.

It does. Have done this before with some success when laying out
foundations. There is even a gadget you can buy for about $25 at Home Depot
that has a beeper on one end to tell you when the other end is level.  The
tubing is clear, so you can see the water level.  There are a few pitfalls:

- You can accidentally let all the water run out from the low end if you
aren't careful (my tower base is a 300' walk from the house, so doing this
once was enough for me to look for the next approach!)

- Strong variable winds (the norm at my QTH) can cause the water in the tube
to keep finding different ideas of level (had this problem when laying out
deck footings).

>    At the opposite end of the scale, my all-time favorite high-tech field
measurement device is a special surveyor's GPS receiver.

I think I've seen folks walking around with these things - backpack with a
funny looking dome antenna sticking up above their head.  Make hams at a
hamfest look normal in comparison.

I think they actually operate on a principal called differential GPS.  In
differential GPS, one unit is placed at a reference site, and the two units
each measure their position using GPS.  One unit communicates it's position
measurement to the other. Relative position is found by subtracting the two
position measurements (hence the differential).  Since the error in the GPS
measurement is highly correlated for both units, even though the absolute
accuracy of GPS is not that good, the relative error between the two units
is quite small.

Someday I'm going to rent one of those and find out where my tower really
is. :-/


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