Thread: squaring up large areas

Good question I was wondering that my self.
I am stuck with the 3,4,5, formula too.

Measure the diagonals.

If you have an odd shape, run string lines to create a rectangle and measure the diagonals of the rectangle, then take and measuements for the odd shape from that.

Al

I use a surveyor.

If you need accuracy over that sort of area, and can't handle a theodolite, that's your only solution.

If it's farm work or whatever, you could use a GPS.

Cheers,

P

Sorry to disagree bitingmidge, but I wouldn't use a GPS unless you can afford to be up to 20m out (or more).

Thinking out aloud ...

1. Mark out one corner using the 3,4,5 rule (or 6,8,10 or 9,12,15 or ...) and then use a string line from the corner and through a vertices's (sp?) to the other corner. Once you have two sides, then next two shouldn't be difficult.

2. Measure out 30m, 40m and 50m of string/rope.

5, 12, 13 also works. 5 & 12 are the sides, 13 the hypotenuse if it is sqaure

Originally Posted by Vernonv

Sorry to disagree bitingmidge, but I wouldn't use a GPS unless you can afford to be up to 20m out (or more).

Ahh yes, sorry. You'd have to know a surveyor with a fair dinkum one!

Cheers,

P

Originally Posted by bitingmidge

You'd have to know a surveyor with a fair dinkum one!

Yep, one of those would work a treat. Very accurate bit of gear, but also very expensive.

Yes, Differential GPS. Funny stuff. The US sets up the GPS system and realises that maybe an adversary may also able to use it for military purposes. Solution - introduce a secret "random" error into the system. The US navy then sets up correction signals that transmit the error corrections so you can remove the errors (later) - this is differential GPS. From memory differential GPS is accurate to about 10-20cm.

Originally Posted by Vernonv

Sorry to disagree bitingmidge, but I wouldn't use a GPS unless you can afford to be up to 20m out (or more).

Of course farmers which use gps with an accuracy of 2 cm wouldn't count I guess.

Originally Posted by outback

Of course farmers which use gps with an accuracy of 2 cm wouldn't count I guess.

Same system as the surveyors, not your $100 Tandy variety!

Mind you, I've used "farmer's levels" a bit and they have a "surprising" level of accuracy!

Cheers,

P

Other than for cropping (mainly), there aren't too many farmers around that have/use high accuracy positioning systems.

I'm always amazed at the number of people (not just farmers) that expect their $200 GPS received to be accurate to the metre (or sub-metre).

Even the 5 metre accuracy most receivers quote in nearly impossible to achieve under normal usage conditions.

Over that large an area, a string or rope will stretch too much to provide reliable measurements. Best bet is to hire a surveyor. Alternatively, use a steel tape to measure each leg and hypotenuse. If your tape is too short, lay out a smaller rectangle at one corner, and extend its legs in the required directions. In either case, use a spring scale at the pulling end of the tape to assure the same tension in the tape for each measurement. For the shorter tape, re-check the overall diagonals by using taut string for alignment, and measure segments with the tape and spring scale.

Joe
retired civil engineer, but not a surveyor

Originally Posted by joe greiner

Over that large an area, a string or rope will stretch too much to provide reliable measurements. Best bet is to hire a surveyor. Alternatively, use a steel tape to measure each leg and hypotenuse. If your tape is too short, lay out a smaller rectangle at one corner, and extend its legs in the required directions. In either case, use a spring scale at the pulling end of the tape to assure the same tension in the tape for each measurement. For the shorter tape, re-check the overall diagonals by using taut string for alignment, and measure segments with the tape and spring scale.

Joe
retired civil engineer, but not a surveyor

Yep and don't forget to take into account the thermal effects upon the tape as it is calibrated to give accurate results at given tension and given temperature.

It depends upon what degree of accuracy you are after - most formwork is + - 10mm so reasonable tolerances are probably all you are after and a tape with consistent tension is most likely adequate.

I'm with Joe