GPS to TS

Good question!

But how "far off" CAN a Total Station Shot be?

🙁
Loyal

If proper care is taken most RTK shots are within 0.03' Hz, and there are techniques you can use to both ensure and improve upon those values. Static points are much better. And Loyal is right... TS shots "can" easily be off by just as much.

Charlie c, post: 445582, member: 6798 wrote:
How can you use GPS for control?

Place multiple points with GPS

Resect onto three or more of these points with Total Station

Using a resection, rather than occupying a point directly, gives a measure of the uncertainties of the points, and also tends to average out the random inaccuracies

I set control points with GPS RTK, in open areas using a bipod for stability, and shooting each point five times, which the unit then averages for me. When I occupy those points with a total station the average horizontal discrepancy is 0.01 feet. I have done the same thing on a calibrated baseline and again matched the published data. I do not expect to have the same accuracy under canopy or near tall buildings, but then I don't need that kind of accuracy to locate curbs and parking stripes for a drawing.

Also don't forget to allow for scale factor differences between GPS and TS measurements.
Not sure if you're aware?

Richard, post: 445651, member: 833 wrote: Also don't forget to allow for scale factor differences between GPS and TS measurements.
Not sure if you're aware?

Question:

What scale factor would that be?

Assuming a point-point "measurement," both technologies SHOULD return the SAME "distance."

A GPS "measurement" transformed into some flavor of "GRID" (mapping projection), should be the SAME as a Total Station "measurement" transformed into the SAME "GRID."

Just say'n

Loyal

Loyal, of course they should return the same numbers after grid to plane adjustment. Richard was just making the point to ensure to do this adjustment. As Charlie C mentioned that he was new to GPS, Richard's comment was pretty valid.

The controller software I use does it automatically for me. I set up the job with a scale factor and it knows how much to adjust measurements depending on which instrument they came from.

What sort of gear are you using Charlie? Do you use one controller for both Total Station and GPS or are they totally independent?

Seb, post: 445662, member: 7509 wrote: Loyal, of course they should return the same numbers after grid to plane adjustment. Richard was just making the point to ensure to do this adjustment. As Charlie C mentioned that he was new to GPS, Richard's comment was pretty valid.

The controller software I use does it automatically for me. I set up the job with a scale factor and it knows how much to adjust measurements depending on which instrument they came from.

What sort of gear are you using Charlie? Do you use one controller for both Total Station and GPS or are they totally independent?

Good post.

I understand that, my "point" is simply that TOO MANY surveyors don't seem to understand just WHAT is being measured by GPS (or Total Stations for that matter), as opposed to WHAT the data collector (black box) burps out.

Take CONTROL of the data collector, don't let it control YOU (or your understanding of what is going on behind the curtain).

Loyal

The difference will vary depending upon you location and methods and the geographic area you are working.
My work is much of flat lander situations with elevations that vary from 200msl to 500msl at most and by using the "EC - curvature of earth" correction to TS acquired data my static GPS is always within 0.03' out in the open and not more than 0.10' under less desirable locations.

RTK in open areas is fairly accurate, you can take multiple readings with most data collector softwares on control points. The way I contain error is to set two control points on both sides of the location, I need to pick up, that way I am containing the error between these points instead of extending the error by going in a different direction. If I have .05 difference I can adjust .025 each direction.

[USER=6798]@Charlie c[/USER] The issue with GPS versus TS is that a TS will give you a raw, direct measurement out of the box, so to speak.
Depending on how your TS is set up, with or without scale factor applied inside the TS the difference to GPS will immediately show if scale factor is not on.
I run my TS with a sf of 1,and let the data recorder do its work.

Unfortunately a GPS (unless it's an all in one Javed) doesn't tell you anything and you're completely reliant on a DC to tell you 'something'.
It's knowing what that 'something' is that's vital.
In fact knowing all your settings are, so to elaborate on all that's been said before Charlie, make a note of your settings and don't just assume you can flip between TS and GPS be in the same measurement mode.

Charlie c, post: 445582, member: 6798 wrote: New to GPS.
Everything I have been told so far, is that a GPS point can be off by as much as .05 feet, or 0.1 feet from point to point..
How do you reconcile this difference when going from GPS to TS under tree canopy?
With this distance error, what do you use GPS for, just large acreage surveys?
How can you use GPS for control?
Thank you

Dealing with differences is the whole point of being a surveyor.

So with you, from whence do we start?

Paul in PA

I recommend reading https://www.ngs.noaa.gov/PUBS_LIB/NGSRealTimeUserGuidelines.v2.1.pdf&apos ;">User Guidelines for Single Base Real Time GNSS Positioning .

Mark Mayer, post: 445739, member: 424 wrote: I recommend reading https://www.ngs.noaa.gov/PUBS_LIB/NGSRealTimeUserGuidelines.v2.1.pdf&apos ;">User Guidelines for Single Base Real Time GNSS Positioning .

Good advice!

Loyal, post: 445667, member: 228 wrote: Good post.

I understand that, my "point" is simply that TOO MANY surveyors don't seem to understand just WHAT is being measured by GPS (or Total Stations for that matter), as opposed to WHAT the data collector (black box) burps out.

Take CONTROL of the data collector, don't let it control YOU (or your understanding of what is going on behind the curtain).

Loyal

Loyal is correct, GPS processing gives a mark to mark distance {sqrt(DX^2+DY^2+DZ^2)}. I reduce all total station distances (and zenith angles) to mark-to-mark for processing. A mark-to-mark distance is unambiguous (and the best way to archive, no HI/HT or zenith angle required). Makes comparing the two distances (GPS & TS) straightforward.

Part of the problem with these discussions is the confusion caused by equating differences in positions and a measurement. Those of us who have been using total stations (or their predecessors) on relatively flat surveys (less than a couple of hundred feet of vertical relief), with local coordinate systems (i.e. 5000,5000) assume that an inverse between two coordinates is the same as a measurement between two points. This is a useful fiction on small, flat surveys, but it is a fiction. The two are not the same.

Another problem is that many new users assume that GPS distances are different from total station distances. Not really. This confusion comes from most new users opting for State Plane or some other large area projection with large distortions when using GPS. Not wanting to wade too deep into the pool, new users start with predefined large area coordinate systems that are already loaded into the data collection software. This isn't a bad way to start using GPS. It took me a while to recognize that GPS and State Plane weren't synonymous when I first started using GPS.

Last, we've always been told that measurements are estimates. Sometimes very refined estimates, but estimates nonetheless. This becomes very obvious when you consider the transformation of a measurement to a grid coordinate or the inverse of two grid coordinates to a measurement. GPS provides ECEF coordinates which are of little use as measurements on the Earth's surface. The ECEF coordinates can be transformed into Latitude, Longitude and Ellipsoid height with very little loss (but because it is an iterative process, it is still "approximate" even if sub-millimeter). The Latitude and Longitude aren't easily inversed to derive a measurement, so we often project these coordinates onto a grid. The grid coordinates can be inversed using plane trigonometry to derive a value similar to a measurement made on the ground. The similarity between the grid inverse and a terrestrial measurement will be heavily determined by how close this grid is to the ground elevation where the measurement is being made. Because the surface of the Earth undulates so much, we must accept grids that are close to the ground but are seldom coincident with the ground. Large scale projections, like State Plane, compromise the similarity for large scale applicability with grids that can be vertically separated from the ground by a considerable amount. You can have grids that are less useful over large areas that are more similar to the ground for a particular area (such as a job site or a county).

Loyal, post: 445595, member: 228 wrote: Good question!

But how "far off" CAN a Total Station Shot be?

🙁
Loyal

Person's gotta know their equipment, I agree.

So, how "far off" can your total station shots be?

Under ideal conditions my 5" TS has a hard limit on the angular spread of about 20". EDM distance spread of 2mm. So at 100m, for instance, a radiated point will be within a (near) rectangle 10mm x 2mm. Add environmental influences after that.

There is very little issue merging GPS and total stations under good GPS conditions. Under less than favorable conditions GPS should always be looked at with suspicion.

As long as GPS is in the open, both the GPS and TS are working with the same projection parameters, then errors between RTK GPS and the TS should be very small.

Generally we will see .02' or less HZ.

Conrad, post: 447072, member: 6642 wrote: Person's gotta know their equipment, I agree.

So, how "far off" can your total station shots be?

Under ideal conditions my 5" TS has a hard limit on the angular spread of about 20". EDM distance spread of 2mm. So at 100m, for instance, a radiated point will be within a (near) rectangle 10mm x 2mm. Add environmental influences after that.

Fair question.

I wasn't speaking in terms of "angular/edm SPECS" as much as I was about the Computed/Returned "coordinates" (positions) generated via Total Station Observations. It has been my experience (ever since "total stations" & EDMI became commonplace), that a significant number of surveyors don't really understand WHAT the TS/EDMI/Data Collector is burping out, and therefore have problems correctly generating reasonably "accurate" Coordinates/Positions from said data.

I will stipulate that MODERN Data Collector Software is fully capable of "doing" all of this stuff correctly, but to do so requires the surveyor (the weak link in many cases) to properly setup said Software in the first place. Things have got a lot better over the years, but I still see quite a bit of FLAT EARTH BS, and run into folks @ conventions and meetings that just DON'T GET IT.

Now the vast majority of my work over the last 49 years has been in the Mountains (and bumpy deserts) of the American West, and I realize that there are MANY folks who have never needed to deal with steep vertical angles, significant topographic relief, or "large" projects (10s-100s of sq. miles). That's okay, BUT when folks take that postage stamp/pancake land mind set out into BIG World, things far apart REALLY fast.

Loyal