Hi,

I am fairly new to surveying and have been thrown in the deep end on a job where I am having to combine GPS data taken with our companies Leica GS08 and a robotic total station (Leica TS15). The job is fairly small (less than 100 x 100 m site) so I dont think I need to apply any corrections to the GPS data.

On the site I have been placing nails into the ground and surveying these positions with our GPS Leica GS08. I then take out our Leica TS15 to survey these positions in and continue the rest of the survey. Back in the office I then start the alignment process.

To align in CAD I usually draw a line between the furthest GPS points and add the same line between my total station survey points. I can then bring the total station lines on to the GPS and rotate accordingly which theoretically aligns my total station data to GPS.

The issue I have is... yesterday I decided to put a third GPS point in and survey this to see how far out this point would be with the previous method. When I aligned my first 2 points in CAD this third point was 0.032 m out which is quite a lot considering the accuracy of the GPS at the time had a 3DCQ of 0.016 m.

My question is, what is the recommended way to align GPS and non GPS data so as the error between the stations is as minimal as possible?

I have a feeling the line method isn't quite correct! 

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Hi Nooby.. lol

First of all you are not too far off in your desk-top adjustment method.  When I began combining GPS and Ground survey data some 25 years ago, I did the same thing.

Second your perceived error of 0.032m total vs a point error 0.016m is not bad at all.  Check your GPS equipment specs and I think you could be reaching the tolerances of your equipment. 

Third, for the size of site you are describing I'd do this 1) GPS point 1, 2, and 3 as control.  Then GPS what ever else you want.  2) switch to total station and occupy any of your GPS points, then BS one and FS the other - I'd occupy and BS then longer of the two legs 3) make any adjustments or projections that you need do to the GPS data and drop it into your CAD program 4) import your total station data and rotate the long base line of this data set to the same GPS leg. 5) move the total station data by finding the mid-point of that long-leg baseline to the mid-point of the long-leg of the GPS data.  6) this should distribute the error between the total station data and the GPS data 7) check the residual error at the 3rd common point and it should be very close the the first two base line points.

This is NOT a way to balance a traverse or check closures, it it a quick and dirty way to merge to different data sets on a SMALL site such as you described.

Good luck,

Kevin 

Thanks for the answer Kevin, I really am a noob! I think I understand what you mean, although by the looks of it I am doing this when I get back to the office anyway. 

Longest GPS setting out points

Draw line between

Same TS points

Draw line between.

Align TS over GPS mid point line

Rotate TS?

You got it. If you want me to give it a look for you. Email me the data - akevinzz@aol.com
Adapt one gps control point cootdinate as correct calculate azimuth of longest leg and set to that bearing and re observe the triangle or traverse by TS and use as the local coordinate adapted for the site TS accuracy is the best for precise construction site (gps gig enough for dirt work)

Thanks Lalith, I haven't seen this method before however it seems to make sense although I think I may be doing this when I get back to the office rather than on site.

In the office:

1 - Take GPS data and draw a line between 2 longest setting out stations.
2 - Bring in total station data and align mid points over this line and rotate so angle is the same.

Not sure I'm any help with this gps and robots kinda new to me, Cad I know nothing. I just did 1.5 miles of topo with no control within over a mile. I made mistakes setting gps points that where to close to each other. Using my robot I found when traversing between them it got off almost half a foot. im only using a rover no base. Wound doing 2 rover gps points for occupation and back site with 2 set at the end. Traversed from those 2 setting control I needed over a mile. I found I was only .10 off. Maybe in my case to many rover gps, im not sure. I just rotated around 2 rover start points and 2 over 1.5 miles away. Not sure helps. Maybe to much is to many?   

Hi Keith,
Are you using a base and rover setup or just a dial in?
Have you applied scale factor to the instrument readings and are you using a transformation on the rover?
The distance quoted is at the upper end of the gps accuracy and I think that the height can be 3times the cq
I would be more inclined to make the gps data fit the instrument data because it is the more accurate of the two data sets.

Hi Paul,

Thanks for the response.


Im using a Leica GS08 GPS Pole with a bipod which is allowing me to setup over a nail, level the pole etc. Trying to fit the GPS to the total station? Wouldn't I need to do it the other way so as I can align everything to the correct co-ordinates?

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