Open Traverse with GPS Points

You need to tell us what software you are using...
however, if you are collecting raw field measurements you should be able to process your ground work when you have good GPS data.

PS: I always collect all measurements, even seemingly redundant stuff. It helps for trouble shooting.

When I do such things 1 is 1 and its always 1 as long as I'm running control. Later, when I'm using the control as opposed to establishing it, I will include numbered shots on control in the data set (beginning and end) to prove that everything is set up right.

But everyone has their own work flow which is based on their experience and the resources at hand. I use StarNet. A lot of people have nothing more than the data collector and CAD to deal with data resolution.

Field Dog, post: 439286, member: 9186 wrote: Go to setup screen, enter occupied point as 2
and backsight point as 1000

Really all that does is put another https://en.wikipedia.org/wiki/Lite-Brite&apos ;">Lite-Brite peg on the software screen. You can collect the traverse measurements in the raw file, you can set up the dc or instrument to shoot the same distance any number of times, and average, if you're worried about a verification of the distance between those points.

On a route survey I will start on the first two and tie the two on the other end but I adjust using the very first point and the last all the way across the project. The initial setup backsite is very small compared to the long route you are surveying.

Peter Ehlert, post: 439291, member: 60 wrote: You need to tell us what software you are using...
however, if you are collecting raw field measurements you should be able to process your ground work when you have good GPS data.

Using Topcon MAGNET Field in the field. We're shooting property and section corners as we traverse. The office plans to adjust everything using our raw data before we start our topo.

Field Dog, post: 439329, member: 9186 wrote: Using Topcon MAGNET Field in the field....The office plans to adjust everything...

Adjust how? With what?

Mark Mayer, post: 439332, member: 424 wrote: Adjust how? With what?

I was told the office will adjust the GPS points, then run the traverse using the raw data. That's all I know. I just realized, that it doesn't matter if my backsight check is 0.07' off due to discrepancies in GPS, or if I use the distance I shot with the instrument. In case you're curious, each GPS point was shot on 3 different days for 10 minutes each session. We use a private network.

Adam, post: 439302, member: 8900 wrote: On a route survey I will start on the first two and tie the two on the other end but I adjust using the very first point and the last all the way across the project. The initial setup backsite is very small compared to the long route you are surveying.

When you say tie the two on the other end, you mean tie them to a section corner, NGS point, etc.?

I would tie the traverse down on two points that I had set and observed with GNSS.

Do this all the time. If I find on starting out the travers I have .07' in the back sight check I'll store a shot on the back sight check giving it a new point # and carry on the traverse. Depending on length of traverse I like having an intermediate GPS point to check in to and carry on the traverse to close into my final GPS point at the end. Typically what I see is the traverse just needs rotating as the angular error in the first setup gets multiplied through out the traverse so I may miss my closing GPS point by .5' but the overall distance between the first and last GPS control and traverse points only differ by .05'. I'm not going to waste my time adjusting out .05' in a dozen traverse points over a mile after rotating my traverse onto the GNSS points. All of this can be done on the DC in the field. If I can't achieve a respectable closure using this method then I have a bigger problem somewhere in the mix.

Field Dog, post: 439286, member: 9186 wrote: Are we talking grid distance vs. ground distance here?

Somewhere in the process GPS and total station measurements need to be put into the same system, whether that be SPC grid or something else. Your data collector might be set up to deal with it, or maybe the office software does it.

GPS measurements are initially in XYZ format and can be easily converted to lat-lon-ellip ht, State Plane grid, or a regional Low Distortion Progection. Total station data is at ground, which may be close enough to an LDP but will not match SPC until you apply the proper factors.

Somebody needs to understand the whole process in order to be sure it works right. If that person isn't you, it would be a great educational opportunity for you to learn from that person how it is dealt with.

A least-squares program is the best way to combine GPS and TS data after they are put into a common system.

Start at the farthest E-W or N-S point of your traverse and number consecutively. Number your GPS points 1001-1004, with 1 & 2 matching. Don't worry about the GPS adjustment at this time as you do not have a tight network anyway.

In order to get a good open traverse you just start have 2 geodetic quality points at each end. You best approximation of that would be to have a GPS triple at each end so you can actually adjust your start and end points versus traverse.

You have not explained anything of the quality of your 4 GPS points, as a minimum I suggest they be observed as long static network points versus nearby CORS. Not as individual observations or RTK.

For a good traverse you shoot every leg in both directions and carry elevations. Elevation agreement confirms your horizontal agreement with GPS positions

Paul in PA

It is helpful to set your instrument or data collector or when processing raw data to make adjustments for curvature of earth.

A Harris, post: 439419, member: 81 wrote: It is helpful to set your instrument or data collector or when processing raw data to make adjustments for curvature of earth.

And terrestrial refraction!

Field Dog, post: 439286, member: 9186 wrote: We're running an open traverse with two GPS points for
the first leg and two GPS points for the last leg.

Note: The GPS points have not been adjusted yet.

We were told to shoot the first backsight and use that
point # in our setup screen as the backsight because the
traverse will work out better. In other words, our traverse
is numbered 1, 2, ..., 14. Setup on 2, shoot 1, and store
1 as 1000. Go to setup screen, enter occupied point as 2
and backsight point as 1000.

Are we talking grid distance vs. ground distance here?
Why should the distance matter anyway? Aren't we looking
for the difference in bearings on the last leg? In other
words, my traverse closing bearing vs. the bearing between
the last two GPS points.

Note: The GPS points were set by us.

For the most part, yes. If you're running uncorrected to the grid, and the GPS points are grid points, then yes it should close better to a point.

The method, presumably your boss, has told you guys to use is to me, silly. We set up and run uncorrected to the grid daily between GPS points. All of the reduction of data, to the grid, is done at the office. The crew doesn't need to worry about different scale factors or projections. Just do what you know to do. That sounds like what they're having you do here. That being said, I don't know why when the software easily gets you there.

The ONE instance where I tend to do what they're telling you to do is when I am running elevations on topo. For that, I want to shoot the backsight and assign another number to it to have a relative (to the gun) elevation on that point, in case I need to do something with regard to the z component from that point. Makes the data reduction easier on me, but the software could handle it. I choose to do it this way.

Since we don't know EXACTLY what the scope of the job is, then it's hard to say why. I would suspect its a grid/ground issue though.

In the North end of my county, the grid is above the ellipsoid and makes about 0.02'/mile difference. In the South end of my county, it's well beneath ellipsoid, and makes about 0.18'/mile the other direction. Doesn't seem like much but it will play hell with a traverse and whether it's "me or is it memorex".

Field Dog, post: 439336, member: 9186 wrote: I was told the office will adjust the GPS points, then run the traverse using the raw data. That's all I know. I just realized, that it doesn't matter if my backsight check is 0.07' off due to discrepancies in GPS, or if I use the distance I shot with the instrument. In case you're curious, each GPS point was shot on 3 different days for 10 minutes each session. We use a private network.

0.07ft certain does matter, so does proper training and procedures.

Depending upon your GNSS Network type, VRS or RTN, length of baselines etc, not sure I'd use RTK at all for highway control. I suggest using Static on the coordinates at each end of traverse. Much more accurate than RTK.

leegreen, post: 439455, member: 2332 wrote: 0.07ft certain does matter, so does proper training and procedures.

Depending upon your GNSS Network type, VRS or RTN, length of baselines etc, not sure I'd use RTK at all for highway control. I suggest using Static on the coordinates at each end of traverse. Much more accurate than RTK.

assuming that the backsight point is less than 1000' away 0.07' is huge.
either the GPS coords are very crude, or there is a procedural error, or badly adjusted equipment... but your supervisor should be able to give you better direction on what to do and expect.
In this case the new work is based on kinda random data, if it was me, I would start with assumed coordinates and just collect the new measurements.... being "kinda close" to a "true datum" is dangerous. It could be accidentally be mixed with good quality stuff because "it looks right".

Just to be clear, don't use a single static receiver for point and send it to OPUS to process. You should run a static network using atleast two GNSS receivers, 4 would be best. Then post process yourself. You can OPUS as a check.

Thanks to all for replying! Hopefully we'll finish the traverse tomorrow, then I'll post some of the data for your scrutiny.