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From: VanguardLH <V@nguard.LH>
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Subject: Re: precision coordinates
Date: Sun, 23 Jun 2024 19:18:25 -0500
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Andy Burns <usenet@andyburns.uk> wrote:

> bad sector wrote:
> 
>> How would I map with precision about 900 planted trees on 
>> google-earth-pro? As a manual method I would think of taking a hundred 
>> readings on each with my phone and averaging them out, but that is a LOT 
>> of work (did it once with a bubble-sextant to win a bet). Is there an 
>> fdroid app to do this sort of thing (not interested in signupware)? Any 
>> other ideas?
> 
> Surveying like that probably requires either differential GPS or 
> real-time kinematics GPS.

DGPS stations don't provide 100% coverage of the USA.  See:

https://gssc.esa.int/navipedia/index.php/Differential_GPS

The light blue is single station coverage.  Dark blue is overlapping
coverage.  There are still some uncovered areas.  Coverage also requires
line-of-site, so if you're in a tree dense valley you might not be able
to get the DPGS signal.  DPGS stations are land based.

GPS III satellites eliminate the need for DPGS.  They are 3 times more
accurate.  Instead of an average accuracy of 5 to 10 meters, they have a
range of 1 to 3 meters.  I don't know if smart phone makers can access
the GPS III satellites, though.  Might be accessible only by gov't or
military agencies, so you'd have to resort to the land-based DGPS
stations if reachable.

https://www.lockheedmartin.com/en-us/products/gps.html

As for the OP's request to add coordinates to [online] maps, well, how
old are the maps?  A pic of an area could be 3 years old, so what others
would see with the given coordinates could be before the trees were
planted, or when they were still seedlings and not visible on the map.
I've seen aerial maps that were lacking the buildings that I knew were
there.  With Google Earth, you can zoom in to see the capture date in
the status bar, or use the "Historical Imagery" feature to see the date
stamp.  

The GE pic of my house is dated 5/23/2023, or just over a year ago.
Still shows the massive oak in the backyard that was cut down a month
ago, and the zoomed-in image looks like a blocky cartoon.  No way it'll
show seedlings or saplings.  Insufficient detail.  I can see the
handicap ramp at the front of the house, but no one else would know what
that elongated blob represented.

I haven't played with Google Maps/Earth to know if you can request
access to higher detailed satellite images to see something like
saplings, or make out the brand and model of car on the street.  It's
not like what you see in the movies where a spy satellite focuses down
to a pack of cigarettes.