[time-nuts] nuts about position
jimlux at earthlink.net
Wed Apr 25 21:21:10 EDT 2018
On 4/25/18 11:18 AM, Brooke Clarke wrote:
> Hi Tom:
> As part of a FireWise community mapping process I'd like to get GPS
> coordinates of the fire hydrants (Lat, Lon, Ele). Is there a civilian
> GPS receiver that makes use of WAAS and/or DGPS corrections?
I think almost all handheld receivers these days use WAAS for improved
performance. WAAS should give you 1 meter kind of accuracy,
particularly if you compare a known location in the area.
DGPS was a thing back in the 90s (I fooled with a Trimble Scout with a
pod that received corrections over FM broadcast SCA) - I'm not sure it's
widely used today.
The USCG DGPS transmits corrections on MF beacons, but is being
decommissioned. Most of the inland stations have been shutdown.
If you're a surveyor, you get corrections from a network (CORS or
something similar), or you're primarily interested in "relative"
position - you set up your base station and your RTK rover tells you
where it is within 1 mm + 1ppm of distance from base.
http://www.xyht.com/ has regular features on the latest GPS survey gear.
You might be able to convince the local survey equipment rental house to
come out and demo the gear (or give you a good price on a rental)
Or, why not just do the survey optically (!) - none of this new fangled
GPS stuff. Rod, level, theodolite. If Everest could do it in the 19th
century in India, you can do it too.
If you can find a couple benchmarks to work from, you can get accuracy
of 1 part in 1000 with a decent 200 foot tape measure and something to
sight with (a cheap laser level at night works pretty good to keep your
line straight). You're doing a series of triangles - SSS completely
defines it, so no angles need be measured.
With decent survey gear 1 part in 10,000 or so is straightforward.
1 ppm is hot stuff with conventional optical gear - you're going to be
making multiple measurements, compensating for refraction, etc. It's
like GPS at 10cm accuracy - lots of things cause errors of that magnitude.
A nice theodolite (like a Wild T2) is readable to 1 second of arc.
That's about 5 microradian. At 100 meters, the horizontal uncertainty
would be 0.5mm. Yeah, not quite 1ppm, although you could probably do
multiple setups and average in on 1ppm.
Of course, you'll then need to go out and get a decent tripod, a rod and
target, and a rod person to wave the rod, etc.
But another poster did comment on "why not use the telescope" you could
precision point to a series of stars and calculate using celestial nav
where you are. Although, that might be painful to the 1 meter sort of
accuracy - the "tables" probably don't really account for deviations
from ellipsoid and so forth.
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