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Bruce Small
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I routinely set control points (in relatively clear places) with my Leica 1200, then later occupy those control points with a total station. The typical difference is 0.01 feet, and if I ever saw more than 0.025 feet I would start over. I've also taken the 1200 to a measured baseline and consistently saw the same 0.01 feet. All shots taken five times, one after another and averaged, using a bipod. 

A horizontal difference of 0.10 feet means something is wrong with either the equipment or the procedure, or both.

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MightyMoe
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@bruce-small 

You are seeing similar results to our R10's.

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Norman Oklahoma
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 Here is a residual report from StarNet. These are points tied by GPS-RTK using a Topcon GR-3 and subsequently traversed. Typical of what I do. There are about 50 points traversed and side tied in this network and these dozen tied by GPS.      

image
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MightyMoe
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@norman-oklahoma 

Interesting, not what Bruce and I are discussing but OK.

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lukenz
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@bruce-small 

If I convert that to millimetres that we use here that is 3mm which is static gnss level accuracy in my experience, very difficult to even get a pole adjusted to that precision (pole run out etc.) especially if you change the height/rotate pole between shots to have independence between shots.

 

You stated five repeat shot averaged but how long were the shots and was there any significant time spread between them.

 

I'm also using Lecia 1200 base/rover which is GPS/GLO. Base on legs, rover on pole with bipod and using 30 epoch obs at 1 hz, <1km baselines. Only expect to get around 10mm hz 25mm v repeatiblity at 95% CI with multiple 20min+ time separated shots with perfect open sky conditions. Usually have 10-15 sats in solution.

 

Find it hard to believe results can be that much more accurate in other places in world?? I get the same amazing results sometimes also but not everytime such that I'd expect it. We are closer to the bottom of the world but still have good satellite numbers and geometry

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Bruce Small
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@lukenz  My procedure is thus: On the rover rod, always the same rod height of 5.6 feet, with a bipod, always facing the same direction on each control shot (usually north, but it could be a distant radio tower), take the five shots in about one minute total, which means the Leica has automatically re-initialized between each shot. That procedure pretty much means the little bit of bubble error that might be there is the same for each point, so everything is relatively accurate. With the reflectorless there is no rod error because I shoot a target on the ground 0.10 feet above the mag nail (I use a painted and scribed concrete brick for the sight). Ta da!

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Stephen Ward
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@bruce-small Or it's a really long line.  In East TN typical grid to ground difference is .01 per 100' so a 1000' line on grid would tend to produce an answer 0.10' different it shot with GPS on Grid and a TS on ground coordinates.

 

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oldpacer
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If I used NRTK to set control points, I would hold a "midpoint" between the two network pairs in order to better isolate each ones corresponding error.  Both Network Pairs have error (way more error than your total station, but I am neither going to ague for that nor say NRTK does not make good control). I would leave you receiver on the backsights while you are working and post postprocess when back at the office. 

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JERRY ATTRICK
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Posted by: @jakehart

so if I set 2 points with my gps rover

Come on, Mr. Jakehart,

As Hi-Staker said, "it depends". You have to be a little more forthcoming with your data. Your statement leaves out a lot of information. 

I know nothing about you and how you go about doing what you do, so no disrespect is intended or implied. I do not know anything about the project that you reference nor what specifications the project demands. Maybe a tenth of a foot is more than good enough. I don't know. There is a guy on U-tube doing traverses with sticks in the ground, fer krissakes.

I know the limits of my equipment and my processes and do exactly what you describe on many of the projects I do. I don't have the time, expertise and budget on any of my projects to seek one hundredth of a foot results with my RTK systems. Except maybe that "GPS on BM" point that I did a year or so ago. I expect better than a tenth of a foot, however.

I don't post-process any of my data. I will send data to OPUS and DPOS if I need ties to a particular coordinate system.

So, if are you just looking for some spirited banter on the subject, job well done. Lots of great ideas out there, as usual.

I love this place.

JA, PLS, SoCal

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Field Dog
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Posted by: @hi-staker

Are you using your GPS in a ground system or a grid system?

Is geodetic distance synonymous with grid distance?

Posted by: @hi-staker

Have you baselined your total station?

Baselined as in checked on a calibration range? In 1994 we used a Topcon GTS-4A for an FDOT project, and we had to check the instrument on a calibration range before starting the project. We removed a side panel on the instrument and set a series of DIP switches to their proper positions.

 

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lukenz
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@field-dog 

Typically I would understand a geodetic distance to be an ellipsoidal distance which is different to a grid or ground distance.

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Jitterboogie
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@lukenz 

heres a great summary of the lot of them:

https://amerisurv.com/2013/08/23/ground-versus-grid-low-distortion-projections-part-1/

 

god Im going to miss working in Just one LDP when I start my new job next week......

 

😥 😣 😫 

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Field Dog
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@lukenz 

I'm curious about the wisdom of Topcon for including the geodetic distance in their point-to-point inverse output. How is that data useful to me?

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lukenz
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@field-dog 

From what I gather on here you guys in the USA are only interested for grid/ground distances for boundary/topo work. Here in New Zealand (and in Australia I believe) we show distances as ellipsoidal on our survey plans;

Area = ellipsoidal

Bearings = grid

Coordinates = grid

Distances = ellipsoidal

This approach has advantage of grid bearings making all nearby surveys consistently orientated to official projections and grid coordinates make calculations easier than lats/longs. Having ellipsoidal distances gives pretty consistent distances/areas everywhere in country (elevated areas are scaled down to ellipsoid/"sea level").

 

Long winded way of saying that geodetic/ellipsoidal distances for cogo may not be needed in your locality if your not doing geodetic control work but in other parts of the world it's a very useful/necessary feature.

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Field Dog
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@lukenz 

Thanks for the excellent explanation!

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MightyMoe
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@field-dog 

Here is a quick Florida calculation:

Two points on the Florida East Zone

On the Central Meridian for each point of long=W81d, at lat N27d and N27d02'

both are at sea level to make the picture easier to understand.

inverse between them:

Grid and Geo azimuth 0d00'00"

Grid distance=12,116.912

Ellipsoid distance=12,117.625

Ground distance=12,117.576

elevation for both points=0.00

ellipsoid height for the north point=-83.85  south point=-83.38

numbers are USFT. 

 

You can tell from the numbers where the ellipsoid is. Is it above or below you?

The radius of the earth is about 20,000,000' which means 1ppm corresponds with 20' of elevation change. As you can see there is 80' in the geoid height which should create 4ppm for the elevation scale. 12,117.576x.000004=0.048',,,,,12,117.576+.048=12,117.624 or very close to the ellipsoid length using rough calcs. 

Now you know if the ellipsoid is above you or below since the ellipsoid length is longer. 

And you can back in all your scale factors. 

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Nate The Surveyor
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Mr field hound,

Your questions lead to many other questions. 

N

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Field Dog
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@nate-the-surveyor 

Hopefully the many answers won't be too far behind them!

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