Incorrect Backsight

A list of coordinates is generally useless in figuring out what went wrong, and how to fix it. You should post the actual raw measurements - angles and distances. How did your software calculate coordinates for the bad backsight? Did you at any time take a check shot on your backsight? If not, then in the future I would advise you to always take check shots on your backsight periodically during a set up. It is good practice, to assure you that your set up hasn't drifted, settled, been bumped, whatever. And the check shots can be used to fix a boo-boo like this.

Also, how were the GPS coordinates calculated? Are they in a standardized coordinate system, or a custom one specific to your site?

Hi Peter,

I am very new at surveying and did not periodically check shots on the backsight. The surveying was done using the local South African LO-29 projection and WGS-84 datum. I have also attached the actual raw data in this reply.

If there are two?ÿ points?ÿin common then you can rotate to a line.?ÿ If there is only one point in common, then you're up the creek....w/o that paddle.

Don

You can't do a local site calibration in your processing software?

Hi, may you please give me examples of software that I can use to do a local site calibration?

I use Trimble Business Center v4.0 (TBC)

It looks like you are going to have to return to the site, orient on the backsight correctly, then re-measure to a few of the same points you measured before. Like foggyidea said, once you have a couple of common points, you can rotate the bad survey and align it to the good stuff, then you should have a few extra common points to check that your rotation was done correctly.

All you need is a COGO program.

First you need to determine what was the correct point that you backsited.

Then you can rotate and translate accordingly into new final locations.

I would reccomend that in the future that you stake to your first backsite and any problem will show itself at the beginning of your survey.

This is the one thing that I insist being show to?ÿ me when someone turns in their day of collected data.

good luck

Not be sound like captain obvious, but when you shot the backsight did you view it through the instrument and did you check back in when you were done with the setup??ÿ If you are sure you were shooting the correct prism then I would suspect the GNSS.?ÿ How was the GNSS data collected??ÿ Was it an RTK fix solution or a static post processing??ÿ A meter would not be unreasonable for an RTK float or an unprocessed autonomous position.

Yah that sounds like it's going to be the sensible thing to do

Mapuranga,

There are multiple things wrong with what you have done.?ÿ

Firstly, the horizontal difference of 1 meter that you have found would not affect the orientation of your total station shots.?ÿ

When backsighting, there are two things that are occurring, but are completely separate from each other. The first is a distance check. This gives you the errors (differences) in distance and elevation between the calculated point and the backsight point. The second is setting the horizontal angle to zero. This orients or aligns the total station with the control (base line). The distance check will not confirm orientation, and the orientation will not confirm the distance.?ÿ

I would suspect that it forgot to zero the backsight, which would explain the alignment issues.

The noted 1 meter of error may be due to the Trimble GPS operating autonomously, rather than fixed.?ÿ

If you can export an .fbk file from your data collection, I can assist in correcting the alignment, but the distance error will likely require revisiting the site anyways.

Surveyors can work magic with corrections and calibrations, but if you start out with bad control, the only way to fix it is to go back and establish (or tie into) good control.?ÿ

The ways of the blunder are many, and varied.

The most common way to rotate group of points - it pains me to recommend this - is to import your coordinates to a CAD program and rotate the lot of them. Then export the updated coordinates back out, if necessary.?ÿ?ÿ

There a lot of ways to recalculate coordinates from the raw data. By "raw data" we mean the angle and distance data read by the instrument. The coordinates are the product you get when the raw data is applied to the assumptions (ie/the control point coordinates).?ÿ ?ÿWhat data collection software are you using? Are you using the same data collector software with the Trimble that you are with the Topcon?

But I think you have something more than a simple rotational problem.

The distance between the stations BASE and BACKSIGHT in your RAW TOTAL STATION data is 18.071m. The distance between the same 2 points in the CORRECT BACKSIGHT file is 18.794m. A difference of that magnitude suggests to me that your Trimble GPS may not have been initialized when the points were recorded - that is, you have autonomous GPS positions that are only good to +/- a metre or so. That is one possibility.

Looking at the background picture in your original post - that's not a great place to be doing GPS. Bad (float / autonomous) positions from your Trimble is looking more and more likely.?ÿ?ÿ

Another possibility is that you have some problem with the grid systems you are using and are getting a huge scale factor applied. I don't know anything about SA projection systems, but the large negative "y" coordinates are a red flag to me.?ÿ Maybe negative numbers are commonly used for northings (southings?) in the southern hemisphere??ÿ ?ÿIDK. But my guess is not. The solution may be as simple as getting the right zone set in your data collector.?ÿ ?ÿ

Nevertheless I've rotated your total station points, holding the position of BASE and the direction to BACKSIGHT.?ÿ?ÿ

Summer Profet,

While technically correct, I would disagree with your statement that a 1 meter error has no effect on your orientation.?ÿ 1 meter would suggest an incorrect coordinate (backsight or instrument point) or a bad backsight shot.?ÿ Both would result in an incorrect orientation.

The essential missing element to your dilemma is the source(s) of the original?ÿerror(s) have not been positively identified in order to be confidently rectified and therefor the data is suspect and cannot be trusted. You literally have no other option but to resurvey this if the data is to be relied upon. The easiest solution is to re observe?ÿthe control points that you occupied and back sighted and confirm those measurements with the distance measured conventionally. You?ÿwill need to?ÿreach a satisfactory agreement between the two sets of conventional and GPS data.?ÿIf it were me I'd set a third point well in the open that can be observed from both stations so you will have something to check into with the total station?ÿto help you rest easy. I was always taught to avoid negative northing/eastings as they can cause issues with cogo computations. ?ÿ

Educations don't come cheap.?ÿ

I think Mr. OK might be on to something.?ÿ Using the LO29 project those coordinates are at the south pole.?ÿ Can you give us the lat and long?

The only thing that I am confident of is that the Trimble GPS were fixed and not measured autonomously. I checked on the survey report

While i think many good ideas are populating this thtead, here is another possible solution.

Go and re-observe gps, on a number of the specific points observed with total station.

This could supply you with some kind of redundancy, and checks on all of it.

And, for sure, always do something in the future for redundancy. Ie, several gps points, that are also shot with total station. And, getting a couple of shots that are over 200' is good too. 300 to 500 is way better. It supplies better bearings.

What's the weather is SA like right now??ÿ

N

?ÿ

While i think many good ideas are populating this thtead, here is another possible solution.

Go and re-observe gps, on a number of the specific points observed with total station.

This could supply you with some kind of redundancy, and checks on all of it.

And, for sure, always do something in the future for redundancy. Ie, several gps points, that are also shot with total station. And, getting a couple of shots that are over 200' is good too. 300 to 500 is way better. It supplies better bearings.

What's the weather is SA like right now??ÿ

N

?ÿ

After looking at your site photo, I would have little confidence in your GPS. First thing I would do is use a Georeferenced ortho-photo and my setup ties to get approximate coordinates and check them against my traverse. Not knowing your total station software, check the raw data and see if the backsight azimuth "zero angle" may have changes during the set up. This happened to me when using new software and?ÿa touch?ÿscreen.?ÿ

BTW, two GPS points are totally inadequate as there is no check measurement.

I SAY YOU RETURN TO THE PROJECT SITE OR RETURN THE CLIENTS MONEY.

Paul in PA