As a pure geometry exercise, I agree with your method if the crops are low lying crops and you can walk through them without tramping them down in places that you don't want the flattened. A modern transit would be much simpler to use in that case since you can locate points from the reference line by angle and range. In your method, if crop is tall then you'll have to lift your ropes above them with poles. GPS would be less awkward to use.

Another question is how you label the reference points. I know that once a surveyor gets ten or so stakes on the ground, things get confusing. You forget which stake is which. They have to be labeled carefully. In crop circle making, I suppose they must all be pulled up afterward - not a requirement in surveys. You can try only staking out a few things at one time, but that will also get confusing unless you have an observer who can tell you if you forgot to do something.

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