Traverse

Please explain how you used "2 station gps."

Normally, people do not traverse with GPS (GNSS). They may use GNSS to measure end points and use a total station to connect all the points between. Or they may set up one receiver as a base and another receiver carried to all the points.

@bill93 So i just gps two points to orientate the obs. I shoot them with ts fixing just the bearing and moved on.

Gps means gnss 500 epoch for the two points 

Oh dear , I ve used Trimble Virtual reference Station

@enri-f

3 error in surveying

1 instrument error

Did your instrument @ total station and tribrach well calibrate. 

2 weather @ which we can't avoid normally under sun use umbrella

3 human error 

Your sight aim to prism

To make more efficient result repeat reading core left circle right

Your man power those setup back dan front sight station must be precise. 

Also need to put station at stable firm ground

3minute 56second out of tolerance

Our country tolerance misclose

1st class 1' 30" greater than 1:8000

Correction every stn less than 10"

2nd class 2'30" greater than 1:4000

Correction every station less than 30"

You don't have to loop back if you begin the traverse with two GPS stations and end with two GPS stations. It's still a closed traverse.

I'm not fully following either, but one question also might be how are you solving your GNSS coordinates.  Ellipsoid only (not likely)?  Using a geoid?  What datum is it exporting points to? Lat Long Geoid?  A local datum?  If you're exporting a map projection you will get closure errors (albeit small) until you convert to ground.

00°03'56" closure over 18 stations is ~13" of error per station.

At 630m total length, that's about 35m between stations, which combined with the 13" per station works out to about 2mm error per station. It's an approximation but a good exercise to get a reality check.

That's not bad for short, unbalanced legs and undulating terrain. Not all traverses are under ideal circumstances.

Pick the projection you want to use and set up the job in that projection. Set two points with RTN at the start of the loop and a point at the end to check into. If you want it to be even more precise then set your first point RTN at the beginning, then set a base up on that point and set the other two with RTK. Just try to have at least 180 epochs for each control point with GNSS. 

Are you using a 5” or better total station with tripods, tribrachs and round prisms on all setups? Are the fine level bubbles being used to level up? You traversed from one gps baseline pair to another gps baseline pair?

It more than likely either poor GPS solutions or equipment issues. Something like this in my experience, seems like maybe you used a prism pole for a backsight that had a 40’ vial. Instead of an 8’ or better vial on a tribrach. I did this exact thing once thinking I would do a traverse without having to pack as many tripod sets. Closed great linearly, but angular was the worst I have ever had.

@wal1170 Seems odd that the 40' bubble error would only go into angles and not into distance. If the problem were the lack of sensitivity in the bubble, then the error should have been random and not confined to being perpendicular to your sight line of the rod with the 40' vial.

Ok guys , I am back with some experience to share. I am not good in surveying for many reason , first of all the proper understanding of it.

 

Right, let me explain what I am talking about . 

 

Numbers > 630 long , 18 stations.

Enviroment > Road "downhill" with banks and trees , Field beside it .

Practice > 2 station gps and Loop back 

Description > Legs are not so well balanced 

 

Outcome > 0 misclosure on elevation (that I really don t belive ) and 3 minutes and 56 seconds misclosure. 

 

Yet , 1 in 8800 . 

 

As i said , I really find hard to understand what s going on . 

 

Also, should i got a better results if i gps ed more than 2 stn ?

 

Thanks 

Your traverse should be better than that. Your Start and End baselines should each be longer than 310' feet. Process your GNSS data and derive a scale factor from your BASELINE TO BASELINE data, NOT what your scale factor should be from where your are on your projection. Enter this Scale Factor into your Total Station OR Correct your GNSS data to ground based corrodinates and keep your Total Station Scale factor at 1.000. 

@lurker if you are shooting a tripod set for a foresight and a prism pole as a backsight. You’ll have solid distance checks, but angularly that is where the difference in sensitivity of the vial comes in.

@wal1170 how does the sensitivity of the vial cause the pole only to lean left or right and not forward and back?

The vial responds to all 360 degrees. It is not limited just to the left or right of the line of sight.

Can you post your raw data?

That might provide everyone a better understanding of the situation and allow for more useful comments.

Procedure questions that might also help us understand the issue better:

1) Are you using a plumbing pole held by hand, plumbing pole held by bi/tripod, plumb bob for line and then prism for distance, or tripod/tribrach setups for the backsight and foresight?

2) IF a plumbing pole (either held by hand or using a bi/tripod) or a plumb bob, are you sighting the prism center for line or are you sighting the point/string for line?

3) What are the angular specs on your total station?

4) What are your angle observation procedures - 1/2 a set, doubling or more, direct and reverse, etc...?