Coming back to the reference I mentioned much earlier in the string, one of the
graphs is shown here:
https://besjournals.onlinelibrary.wiley.com/action/downloadSupplement?doi=10.1111%2Fj.2041-210X.2011.00118.x&file=MEE3_118_sm_FigS2.doc
It shows the typical variation in location indicated by consumer gps units over
time and how you may be able to get a better location by averaging over a
longer period BUT that this location is still likely not to be the 'correct'
location.
The particular graph shown here it is part of a 5 hour run with several
consumer grade gps units and set of smartphones (only a small set of the data
shown in that graph), the smartphones tested had some kind of averaging built
in as they slowly got to within 1-2metres of the 'correct' location after about
2 hours and stayed approximately there for the remains of the 5 hour run.
What I did not try is what would happen if you did the 2hours of averaging on
several separate days, presumably on the archaeological fieldwork there might
be the chance to do several days of 2 hour runs and with these averaged as well
then half metre or even better might be possible but is not guaranteed.
One separate bit of information that could be of use for archaeological
fieldwork is that besides the survey grade Leica dgps we also used 0.5m
accuracy kit (the same leica kit but just using Egnos instead of Smartnet) to
find buried markers for total station surveys. You would think that 0.5m
accuracy plus a metal detector should be able to find the metal survey pin but
it often took a very long time in rough grassland habitats so definitely having
a way to get more accurate than 0.5m is better even if it is only to find the
buried origin marks for total station. By comparision when full differential
gps is switched on you walk exactly to the buried pin with no deviation and hit
immediately.
________________________________
From: Qgis-user <qgis-user-boun...@lists.osgeo.org> on behalf of QGIS.USER
<qgis.u...@raycar.plus.com>
Sent: 27 May 2020 16:19
To: 'Garth Fletcher' <ga...@jacqcad.com>; qgis-user@lists.osgeo.org
<qgis-user@lists.osgeo.org>
Subject: Re: [Qgis-user] wishing for accurate lattitude/longitude from, a cell
phone
CAUTION: This mail comes from outside the University. Please consider this
before opening attachments, clicking links, or acting on the content.
Hi Garth,
Thank you for the correction and the additional information. Much
appreciated.
My current thinking is that in the archaeology we do, the intra-site (relative)
measurements are quite good but what is inaccurate is the absolute
measurements. We can set out our grids with cm accuracy but can only locate
them on the ground with 10s of metre accuracy. It would be good to have a low
cost way of establishing the absolute position even if that took time and/or
was off-line.
Ray Carpenter,
Chapel Archaeology
-----Original Message-----
From: Garth Fletcher [mailto:ga...@jacqcad.com]
Sent: 27 May 2020 15:25
To: QGIS.USER; qgis-user@lists.osgeo.org
Subject: Re: [Qgis-user] wishing for accurate lattitude/longitude from, a cell
phone
Hi Ray,
Apologies for the typo - I had typed iGS3, but iG3s is the right number.
iGage <www.igage.com<http://www.igage.com>> iG3s, now replaced by the iG4 which
adds Galileo
tracking but otherwise seems very similar to the iG3s. $2400 US.
These track satellites from the US GPS, Russian GLONASS, Chinese BeiDou
and, with the iG4, European Galileo constellations.
Their sole function is to record from all the satellites they can track.
They produce a RINEX format file which can be sent to a post processing
service such as Canada's Geodetic Surveys' CSRS-PPP:
<https://webapp.geod.nrcan.gc.ca/geod/tools-outils/ppp.php?locale=en>
The longer the observation (recording) duration, the better CSRS-PPP can
converge to an accurate location. In my experience in New Hampshire's
heavily wooded environment, a 30 to 45 minute observation time generally
gets me to better than ± 1 meter accuracy. Yesterday a 6 hour long
observation in a small field surrounded by forest converged to within 1
inch. Dense forest canopy reduces the number of satellites that can be
tracked. Also, some times of day are better than others in terms of the
number of satellites and their geometry, see:
<https://www.gnssplanning.com/#/charts>
The iG3s was perfect for my specific conditions, but I think it is not
optimal where many locations within a site must be accurately measured
** relative ** to each other.
Such sites would be better served by differential measuring where two
receivers are used. One is the "Base" which is kept in a fixed location
while the other is the "Rover" which is moved from place to place.
The Base constantly transmits its data to the Rover over a RF link. As
long as both are fairly close (within a few kilometers of each other),
both see essentially identical satellite errors which can be cancelled
out to produce a very precise ** difference ** in location between Base
and Rover.
Note that the absolute accuracy of their positions may not be as high,
but the relative accuracy can be at centimeter level. In other words,
the absolute accuracy might be ± several meters, but the relative
accuracy will be ± centimeters.
Relative location is all that is needed for site mapping. Accurate
absolute location can then be determined by carefully measuring the
location of the Base, which only needs to be done once.
The major advantage is speed as there is no need for long observation
times at each location. There is also no need for internet or cell
phone connectivity - the only requirement is that the Base be able to
transmit its data to the Rover at all locations of interest.
Earlier in this thread <https://emlid.com/reachrs/> was mentioned as one
source of such Base/Rover systems. At $ 1600 per pair they woud be
less expensive than a single iG3s or iG4 and could be much more
effective. However, I do not have any personal experience with such
instruments.
On 5/27/20 9:11 AM, QGIS.USER wrote:
> Hi Garth,
> Thank you for some very useful data. Mine own experience in archaeology
> fully supports your findings. Like most things in archaeology, money is
> scare, time is plentiful:-) So when you talk about getting sub-metre
> accuracy using something called "iGS3", my interest peaked:-) However,
> despite a reasonably thorough Google, nothing relevant turned up. What is a
> "iGS3" please?
>
> Thanks in advance.
>
> Ray Carpenter
> Chapel Archaeology.
Cordially
--
Garth Fletcher
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