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    Re: Estimating height of eye
    From: Bruce J. Pennino
    Date: 2013 Apr 11, 10:28 -0400
    However, if the towers are surveyed  accurately inplace, and their heights are precisely known, and they have electronic distance prisms on the top, then the tractor simply becomes an EDM just like my EDM on the tripod.  No connection to satelites.  A continuous signal goes from tractor to the towers, and the towers locate the tractor.  Now the vertical could easily be accurate to an inch  and horizontal accuracy is less than 0.5 inch ....just as any surveyor can do with an EDM.
    A surveyor friend of mine is a "one person shop".  His tripod has a prism on it, and the prism sits on a servo motor. As my friend walks around with a mini EDM of some sort, the prism continously adjusts and follows him. When he wants to survey a point. He "clicks" on a hand held device and records the point with a coded description. He goes from point to point as quickly as he can walk. I believe he routinely provides surveys accurate to 1: 20,000 or maybe 1:30,000....I forget.

    ----- Original Message -----
    Sent: Thursday, April 11, 2013 9:43 AM
    Subject: [NavList] Re: Estimating height of eye

    Others know far more than I do, see Richard's post, but I don't believe 1 inch x/y.Even with the towers, which help a lot, I'm thinking maybe the real/true accuracy for x/y  is +/- 0.5 ft at best. Accuracy of  Z with the towers... I just don't know.   I know high end construction Caterpillar grading equipment is computer controlled via satellites connected to the home office where the site is coupled to Auto Cad drawings in a job file.
    The operator , more or less, sits there and enjoys the view while the computers and robots do the work.  I could check, but I suspect this is good enough for "rough " grade of a few inches at best. Who knows about Caterpillar equipment? 
     But the simple truth is, who can really check the farmer's information?  You will suddenly have several "experts" disagreeing on the coefficients in their geoids.....if you know what I mean.....

    ----- Original Message -----
    Sent: Thursday, April 11, 2013 1:26 AM
    Subject: [NavList] Re: Estimating height of eye

    Recently talking to a Canadian farmer. He has GPS on his combine. Motor attached to steering wheel. More expensive setup is a servo that drives the steering hydraulics directly- no backlash. But what really interested me is that there are stationary earth-bound tower mounted correctors spread around the farmland which can bring the accuracy to an inch.

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