Hello:
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.
Bruce
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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