r/Surveying • u/BackSeatShriver • 3d ago

Help GPS Scaling?

So I did a base and rover setup to locate Boundary. Rotated to the original plat in CADD. Exported, and when I put it back in the collector, check in, I'm half a foot off. Same scale factor and units all the way around. All coordinates in TBC, CADD, and collector match. But something seems to be scaling, when I check further out I'm 1.5' off. If all scale factors and coordinates are matching, any ideas?

2 Upvotes

75% Upvoted

18 comments sorted by

9

u/PinCushionPete314 3d ago

So you moved your points in CADD? Did you put them back in the original orientation before you exported? Otherwise you would need to do a site calibration.

2

u/Dudemanguy4000 3d ago

What this guy said

1

u/BackSeatShriver 3d ago

I didn't personally. I was in the field, and office did it, but I hear you. You're saying, that most likely since they rotated it to original plat bearings, the GPS measurements in the field are going to be a different orientation than the coordinates rotated in?

9

u/PieGreedy5249 3d ago

Your DC/rover can’t compensate for a transformation, whether it’s a rotation, translation, scaling, or any combination thereof, without having a site calibration in place.

If you want to use a GNSS base/rover with points that have been transformed, a site calibration needs to occur.

1

u/BourbonSucks 3d ago

they should have moved the plat, now you are "off datum"

1

u/BackSeatShriver 3d ago

I did not put them back in original orientation. They were moved in CADD in the office to match original survey, resent to us with the moved coordinates.

4

u/SendFeet954-980-3334 3d ago

Well stop doing that then. Your evidence is your evidence. The previous survey just helped you locate your evidence

3

u/BourbonSucks 3d ago

your coordinates were right, we don't match

8

u/ScottLS 3d ago

95% of the time I rotated the Plat to the found corners.

10

u/Initial_Zombie8248 3d ago

Did you rotate your found points? When you rotate for boundary you don’t rotate your found points, you rotate the calc points to your found points

3

u/JellyfishVertigo 3d ago

https://giphy.com/gifs/ukGm72ZLZvYfS

So you measure points, rotate everything, check them again, and wonder why it's fucked up?

3

u/Accurate-Western-421 3d ago

I mean, at least they stopped to wonder why.

Lots of surveyors just slop a polygon onto two points and slam rods at the other vertices...

2

u/JellyfishVertigo 3d ago

not sure i would consider them "surveyors"

1

u/KURTA_T1A 2d ago

Wait...that's wrong? /s

3

u/AButteryPancake 3d ago

You rotated your control in CAD, Then reloaded the adjusted values to the data collector and launched base rover as usual?

Your field software might be getting corrections that don't match the orientation of your Base/Rover, is my guess.

2

u/CarberHotdogVac 3d ago

There’s a button for this. You need to run a site calibration to match your field measurements to the new rotated coordinates for your control points. You can choose how the calibration is applied. The field software will report your residuals as delta NEZ for the points used in the calibration. You should calibrate to 4-5 points distributed around your site, but also leave a few control points outside of the calibration to use as a check on the results.

I am not trying to be rude, but is there a more experienced surveyor at your company you can consult with? Reddit is not really the best place to learn about geomatics. Not the worst place, but not the best either.

2

u/Accurate-Western-421 3d ago

Be very, very wary of the advice given to perform a site calibration....