[time-nuts] nuts about position
kb8tq at n1k.org
Wed Apr 25 15:08:13 EDT 2018
One of the “interesting features” of the ongoing bridge rebuilding process around here is the
destruction of most of the benchmark locations. They were built into the old bridges and went
away when the new ones went up. Now there are cute little brass disks on the new bridges.
There is no information on the disk and no obvious plan to generate that data ….. When one
asks about it, the answer is: “nobody does it that way anymore”.
> On Apr 25, 2018, at 1:07 PM, Dan Kemppainen <dan at irtelemetrics.com> wrote:
> Anecdotal response.
> A few years back we played with using handheld garmin units (GPSMAP 62st) for locating property lines and corners. We averaged for 6 to 12 hours at known standards (section corners). This data was used to subdivide the section for property corner locations, and these corners were marked.
> The method for marking was to park the GPS close to the calculated corner location in Lat/Lon. Then we let the GPS survey for the same 6-12 hour period, corrected the GPS position and repeated until the desired corner was located.
> A year or two later due to a power line upgrade, many of the properties were surveyed professionally. We're in a small community and we got chatting with the surveyors. We helped them find roads/trails to get to section corners they needed to get to. They in turn helped us by marking the property corners in question with survey grade equipment (They said accurate to within an inch). The handheld GPS units were within about a foot or so of the professional units.
> Obvious concerns aside, what we were really doing was dividing relatively short distances (1 mile) between existing point into even shorter distances between the points. The limiting factor was really number of digits in the average reported Lat/Lon positions from the GPS.
> That said, with a few good reference points (section corners, or other standards) near the telescope and some time using a good handheld GPS and some careful math you should be able to drill down close to 0.3m level of accuracy.
> We did want to repeat the tests with timing mode receivers set to average for 48 hours, but haven't got around to it yet.
> On 4/25/2018 12:00 PM, time-nuts-request at febo.com wrote:
>> Message: 6
>> Date: Wed, 25 Apr 2018 07:56:14 -0700
>> From: "Tom Van Baak"<tvb at LeapSecond.com>
>> To: "Discussion of precise time and frequency measurement"
>> <time-nuts at febo.com>
>> Subject: [time-nuts] nuts about position
>> Message-ID: <2E0CF8A1D3C64D58BA771539C620ABE9 at pc52>
>> Content-Type: text/plain; charset="iso-8859-1"
>> List -- I had a recent query by a researcher who would like to pinpoint the location of his telescope(s) within 0.3 meters. Also (he must be a true scientist) he wants to do this on-the-cheap. He may have timing requirements as well, but that's another posting.
>> So I toss the GPS question to the group. Surely some of you have crossed the line from precise time to precise location?
>> How easy, how cheap, how possible is it to obtain 0.3 m accuracy in 3D position?
>> When we run our GPSDO in survey mode how accurate a position do we get after an hour, or even 24 or 48 hours? And here I mean accurate, not stable. Have any of you compared that self-reported, self-survey result against an independently measured professional result or known benchmark?
>> Do you know if cheap ublox 5/6/7/8 series receivers are capable of 1 foot accuracy given enough time?
>> If not, what improvement would -T models and RINEX-based web-service post-processing provide?
>> It that's still not close enough to 0.3 m, is one then forced to use more expensive multi-frequency (L1/L2) or multi-band (GPS, GLONASS, Galileo) to achieve this level of precision? If so, how cheaply can one do this? Or is the learning curve more expensive than just hiring an survey specialist to make a one-time cm-level measurement for you?
>> Something tells me 1 foot accuracy in position is possible and actually easier than 1 ns accuracy in time. I'm hoping some of you can help recommend solution(s) to the researcher's question or shed light on this interesting challenge.
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