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    Re: How accurate is pinned lat & long on Google Earth?
    From: Bruce J. Pennino
    Date: 2017 Feb 27, 09:08 -0500
    Hope this is relevant. Some  general info which maybe will help you. All  U.S. states,  and I imagine all  districts & regions, have defined grid coordinate systems for surveyors. Our  U.S. system I believe is established and monitored by the U.S. Geological Survey. Here in Massachusetts we have a State Grid. You can go on line and find a “library” of Bench Marks . All BMs are defined via the  grid in a coordinate system and latitude and longitude ( I recollect).   BMs are round brass discs  checked regularly to confirm that they have not been moved or destroyed. Some areas have a large number of BMs, others there can be some distance between them. Sometimes there is a brass disc set in the front steps of the post office. Usually they are easy to spot. The accuracy of BMs vary....First order, second order, etc   Some are  only  x,y. Others are x,y,x.  Nav list has a number of surveyors who know far more about this.  Maybe you can you can use these to check your  IPhone.
    I recently had reason to question the distance  calibration of my total station. I laid out with a quality steel tape 1000 ft, 500 ft, 100 ft down to the width of a road. I then used my total station to measure these distances electronically. For curiosity, I checked everything against  Google distance calculator.  I was astonished how accurate Google was, all things considered.  My results were: my EDM was in error about 1 part per 4000 or very roughly one inch in 300 ft. Quite poor.  Routine boundary surveys are  supposedly accurate to 1/20000 maybe 1/10000.  My conclusion is that Google is quite accurate for routine non-survey use. My former brother-in-law said his ship’s navigation system (super tanker) said his location to 1 ft (plus or minus). but that was 10 tears ago. I say trust your I phone. Good enough.


    From: David Pike
    Sent: Sunday, February 26, 2017 2:33 PM
    Subject: [NavList] How accurate is pinned lat & long on Google Earth?

    I’m working on a learning game for students, but I need to know the lat & long of at least one point to decimetre accuracy.  To save trying to borrow a survey grade GNSS receiver and learning how to use it, could I just pick a point on max scale Google Earth, e.g. the corner of a patch of concrete, and pin that.  It appears to give a lat & long to 6 decimal places of degrees or about four inches, but how accurate is the lat & long really likely to be.  (Ignoring all that clever stuff about places having more than one lat & long)  Would it be more accurate than relying on using a $100 GPS/Glonass/EGNOS embeded hand-held receiver?  Thinking of the extra steps involved, I suspect not, but I would be pleased to be persuaded otherwise.  DaveP

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