GPS mistakes.

"The top mistake I am seeing, is the combining of geodetic and plane surveying. Think rotational error.
One Point Setup. Geodetic north, at the base. (This is a RADIAL line to the north pole) All goes well, until the user MOVES the base.
Now, we do ONE POINT SETUP, and once again, we are on a radial line to the north pole, once again. Also, the scale factor has changed a little.
Since both setups are set up using the same protocol, the user ASSUMES that they are the same. They are not."

I'm not sure what you're saying so; bear with me. Since my GPS is giving me State Plane Coordinates; I'm getting Grid North from the GPS, right? You're saying that surveyor's are applying the convergence to get True (Geodetic) North, and then rotating to this in the data collector?

I usually post process my base through OPUS. Then I translate my file to these post processed coordinates. When I'm done with the fieldwork I apply the scale factor to my coordinates to get them to ground coordinates. Then I subtract a constant amount to get my base coordinates to N10,000 E10,000. But, I keep my grid bearings and show that as a basis of bearings on the plat.

If I wanted True North for my basis of bearings; I would rotate the file in the computer. After the job was complete. I would think this needs to be the last step.

Am I missing something.

It's all in the setup or configuration.
It sounds like your sequence above is correct.
When I first started, I had a GPS network, to TEST my activities on. It was tied into the Hot Springs Airport Station.
This kind of testing, allowed me to check my methods.
I really blew a job, once via one point setup... because GPS (TDS, Carlson variety) when used for GPS, uses Lat Lon AND rectangular coords, at the same time, and the relationship is called a "Projection".
When you use "One Point Setup", it can use GEODETIC at the base, (Theta is zero), or it can use GRID at the base, (Theta varies).
If you don't occupy at least 2 points on a localize, it can give you as a default Geodetic from BASE to Rover, and USE that bastard brg, in the localize. (one point localize). I though it was giving me GRID. It was not.
I recommend a "Proving Ground" as you meddle with it, to be SURE you are getting it right.
If you rotate it in the "Computer at home" to "True North" (Geodetic) virtually the same. Then, there also should be a LOCATION on that survey where it is TRUE. Thus, you'd have to know theta at a given point on that job.

State Plane Grid North is BASED on Geodetic, at the Arkansas Central Meridian. As you move E or W, then the Theta grows. Theta is zero, at the CM.

Thus, a survey based on SPC, is based on Geodetic... at the C/M. (Central Meridian).
And, the angular difference between C/M and your survey, is theta.

One thing to remember is:
"Everything is always remembered".
If you shortcut a step, on a survey, (such as fake ties to a corner) then it will always bite you later.
Same is true, with being sure of what your software is doing.
N

Nate The Surveyor, post: 402024, member: 291 wrote: The top mistake I am seeing, is the combining of geodetic and plane surveying. Think rotational error.
One Point Setup. Geodetic north, at the base. (This is a RADIAL line to the north pole) All goes well, until the user MOVES the base.
Now, we do ONE POINT SETUP, and once again, we are on a radial line to the north pole, once again. Also, the scale factor has changed a little.
Since both setups are set up using the same protocol, the user ASSUMES that they are the same. They are not.

Are you claiming one point localisations are useless away from the CM (or whichever spot they were aligned to) if the survey is aligned to SPC? Or are you saying the people who you've seen doing them don't know how to do a proper localisation?

I'm saying that IF you do a "one point", based on autonomous, (geodetic at the base) and then MOVE your base, and BOTH bases are set up with one point, (geodetic at the base again) that the bearings are not the same, due to the theta change.
ALSO, the autonomous solution twice, (no opus) error will be there.
Not that it would help alot, if brgs are based on geodetic.
N

I'm a younger guy, my GPS experience is pretty much all network, it sure sounds like what Nate is getting at is that "traversing" a base to an unknown position is not such a good idea.

On the flipside, I recently cleaned up some crap a surveyor 25 years my senior created by using a zone one north of the one he was actually working within in a Lambert projection, talk about rotation and scaling.

I'm with you now, Nate.

I didn't have anyone knowledgeable to mentor me with GPS. I'm easily confused by some of the terminology.

When I first got my Rover; I checked my (new to me) RTK system with a pair of OPUS points to verify that I had things set-up correctly.

BK9196, post: 402211, member: 12217 wrote: I'm a younger guy, my GPS experience is pretty much all network, it sure sounds like what Nate is getting at is that "traversing" a base to an unknown position is not such a good idea.

On the flipside, I recently cleaned up some crap a surveyor 25 years my senior created by using a zone one north of the one he was actually working within in a Lambert projection, talk about rotation and scaling.

If you have raw data there is no need to rotate and scale to change zones. I just go back to raw GPS data and apply different zone, Geoid or projection.S o of my projects span across SPC zones. I often select the zone, then change it later.

Nate The Surveyor, post: 402210, member: 291 wrote: I'm saying that IF you do a "one point", based on autonomous, (geodetic at the base) and then MOVE your base, and BOTH bases are set up with one point, (geodetic at the base again) that the bearings are not the same, due to the theta change.
ALSO, the autonomous solution twice, (no opus) error will be there.
Not that it would help alot, if brgs are based on geodetic.
N

Perhaps you are mixing up terminology. What you describe is not what I'd call a 'localisation.' A one point localisation would be a deliberate matching of unknown quality autonomous reading to a known Cartesian point in order that the GPS unit will adjust for local ground scale, shift, and usually local rotation, wherever you move. It is done deliberately to make sure the numbers coming from the unit are consistent with your local coordinates.

What you describe is not what a well-read GPS user, and the manufacturers, understands by the term 'localisation'. I hope those less experienced users reading this thread will not be dissuaded from learning how to do one point localisations, because they are very useful.

One point setup CAN be quite useful.
Even on geodetic bearings, and ground scale.
You just have to KNOW what it is... And if you do it again, a mile or two away, that those 2 jobs are NOT on the same projection.
Lee green above is quite correct, that if you have the raw data (read latitude and longitude), it can be re projected, onto another projection. Ie, Arkansas north, or Oklahoma, or north Texas....
I feel that each mfr TENDS to address these items in varying ways.
Learn how your particular software does it, and what it does. Some are more straightforeward than others.
I learned to do one point projections, then apply theta, so that I was on ground scaled, local coords, and grid, (AR South) bearings. Saving many steps. But, now getting onto genuine spc required more work, if a job needed it.
No matter. At least, I had a useful system, that kept my bearings on grid. Usually, the csf varied less than 10 ppm, between jobs. (100 ppm = 0.10' per 1000 feet)
A projection introduces error, but not much. Managing the flatening is important.
Local systems work... If you stay local!
N

Nate,
Lats and longs are not 'raw data'. They are positions. RTK raw data is the delta x, y and z relative to the equatorial plane. It generally includes antenna heights and quality information as well. There are numerous ways to use it to relate data sets and even combine several autonomous jobs with certainty.
I agree we need to educate ourselves about the tools we use. Most of the time it just takes a little nudge to realize it's not magic...

Very true

leegreen, post: 402218, member: 2332 wrote: If you have raw data there is no need to rotate and scale to change zones. I just go back to raw GPS data and apply different zone, Geoid or projection.S o of my projects span across SPC zones. I often select the zone, then change it later.

Not understanding how to use your GPS software is not a mistake, it's a problem.

More concisely:
2 jobs, 5 miles apart, separate files, separate base stations, one point setup, geodetic north at both bases, are not on the same basis of bearings, even though they are both based on geodetic, at their respective bases.
I have been seeing surveyors treat them like they are identical, when they are not.

Sounds like Nate's "one point" GPS setup is an autonomous position. This appears to be source his concerns. One should always collect static data at the base. Yes even during RTK, collect static data. So when you have these projects five miles apart, you can tie them together using CORS. Also suggest running at static session connecting each project. I believe GPS for surveyors must include static data and post processing software. GPS should be treated much the as just another tool, it does not change the our standard operating procedure (SOP). Always tie the project down. Always run checks, close your traverse, cross ties to other control points. Leave monumentation. Seems this concept of checks and balances in surveying is what the next generation is lacking. A coordinate with 4 decimals means nothing at all. Leave footsteps for others to follow. Show your work, make note's.

I've always said that GPS files should be fixed in the office before leaving.
There is no reason GPS data should control a projection, it's the other way around.
Projections existed long before GPS and should be determined and decided on irrespective of GPS equipment.
GPS is the tool that gets you into a position on a projection, there is no need to gather information for a project then decide on the projection later; unless you are tying into existing control and it doesn't work in a GPS environment.
If you want a true north bearing around some point, there is no reason to travel to the site first, rather pick a point off a quad or Google and imput the parameters into the job, then go to the site and begin, collect data, shift your base after you determine the precise location and you have a LDP.

As far as state plane, why would anyone go to the area and survey, then go back to the office and change everything, that makes no sense to me, walk out of the office with state plane in the data collector. Shift over to a more precise location after sending data collected at the base point using CORS/OPUS/RTX-PP.

Pick your control system, print out all the parameters and put the hard copy in the job file, do it at the time you set up a job, write it on the folder and never change it through the entire process. It will save time and make the entire process more effective.

If you survey in an area, say a city or county, why not chose a projection for that area, if it's a LDP fine, state plane fine, modified state plane fine, just use it and stick to it, load it in each job before going to that area it should be a very simple process of copying one projection from a base file to the new job file. It takes me about 10 seconds to print out a report and place it in the job file, even less to copy the projection over.

How do you guys handle different zones?

Nate The Surveyor, post: 402247, member: 291 wrote: More concisely:
2 jobs, 5 miles apart, separate files, separate base stations, one point setup, geodetic north at both bases, are not on the same basis of bearings, even though they are both based on geodetic, at their respective bases.
I have been seeing surveyors treat them like they are identical, when they are not.

You lost me at separate base stations and one point setups, guess Im a fan of checks and balances

BK9196, post: 402365, member: 12217 wrote: You lost me at separate base stations and one point setups, guess Im a fan of checks and balances

Insert sarcasm here____________

Well, there are folks around here, doing just that, and have always done it like that, and to them this is NORMAL.
Call it what you like... not gonna change them!