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
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 User Guidelines for Single Base Real Time GNSS Positioning .
Mark Mayer, post: 445739, member: 424 wrote: I recommend reading User Guidelines for Single Base Real Time GNSS Positioning .
Good advice!
