Using the Locus receivers, I wonder if bearings are always fixed?

Suppose I do the following: Day one, set up a base, take the rover to 4 points and obtain static data, download and fix the base as a control, because I don't care about actual datum.

Day Two, I set up base receiver in the same place, take the rover and get 3 additional static locations, download and process.

I have wondered if I should re-shoot a point occupied the day before to assure that the BEARINGS are the same.

I mean, we know that the position of the base receiver will float, but can we assume that the meridian is accurate from one day to the next?

I can easily test this, but I wondered what experience others have had. The thing is, with conventional equipment, we are not locked onto our previous work unless we locate at least 2 points, which provides a bearing.

If the GPS cannot produce consistent positions from one day to the next, how does it produce consistent bearings from one day to the next?

Or does it?

ROTATING.

Your bearings (actually DX, DY, DZ's) will be the same.

Test me and see...

Lew,

I agree but......

The old school in me makes me shoot that extra tie back into the old control, "just for fun". Hasn't been wrong yet but.....

Jimbo

EDIT: Generally the static and kinematic procedures I use make two ties to each point anyway, using three receivers. It's just one extra setup for me to get a double check on the control.
Modified By James Webb on 5/5/2001 at 11:43 AM

I tested this shortly after getting my three units. We had a 300 ac job (small by GPS standards) and built a 14 points control network around/through it. I then ran four adjustments holding four different points (NAV position). Inversing between all points in all adjustments I found one bearing different by "one" second... must have been a roundoff :} I can live with that!!
Steve G

You wrote: "If the GPS cannot produce consistent positions from one day to the next, how does it produce consistent bearings from one day to the next?"
GPS "relative positioning" [survey] produces precise differences in position that are highly repeatable. If you fix the base position, then measure the same unknown point relative to that base, there will be only very small differences in position or bearing. If you do the same thing without fixing the base position, the differences in X,Y &Z don't change significantly, nor does the orientation.

---Brian

The answer to your question as to why positions can be different and bearings be the same is as follows. The origin of the coordinate axes used to compute positions is determined by the coordinates assigned to the base station used. But the orientation of the axes is determined by the coordinate system used by the GPS satellites. Thus the actual coordinates obtained for a station can change from day to day as different coordinates are used for the base receiver. But, since the satellite coordinate system does not change from day to day, the axes orientation does not change and this is what controls the bearing you get between two points.

Thanks, Geodesist...I had a notion this was the explanation.

With differential GPS we are using two frames of reference, whereas with conventional surveying we are using only one.

As long as the differential frame of reference is relatively small in its variations as compared with the scope of the GPS orientation, then direction can be considered "fixed."

Just summing up in my own words.

Yes the bottom line is every day you are using the same axes orientation and thus get the bearing. Also as a practicle matter the rotation between the various axes in current use, NAD 83, WGS 84, and the various ITRF axes are all so small that regardless of which one you used you would not see a measurable difference in the differential coordinates of the baselines of the length you are likely to be involved with (less than 100 km).