Yes, rate of convergence.
The rate of convergence will change as the working latitude value changes.
The amount of convergence in a line running east and west of two miles would be consistent with the difference of a closure of between 1:16,000 and 1:50,000.
Without jumping thru a lot of hoops, I usually force the closure of the angles between beginning sets and closing sets and weigh them according to leg length.
:gammon:
are you reducing your total station data to grid? You should likely be entering an avg combined factor on your data collector to reduce your data to grid.
I usually sight a screw above the prism that holds the target onto the prism holder or the candy cane between the tribrac and prism holder.
Other times while I am sighting and see only the prism, I will focus beyond the target and actually take a reading while focused on the reflection of the objective lens of my instrument.
It usually depends upon which one I can see best depending upon what the background is behind the prism.
For me the use of a red and white stripe candy cane eliminates most any distortion.
:totalstation:
convergence has nothing to do with it.
State Plane is a conformal projection. Over short distances angles are preserved. One to two miles would fall into this category.
But scale factor would give him misclosure to the general direction of the traverse, East and West. He says his misclosure is North and South.
Just curious why he is not seeing the same problem running north south then.
1
Agreed that conformal projections preserve angles over short distances. Poster is near Atlanta, which is Georgia West zone. Convergence at Atlanta's longitude is about 59 minutes 26 seconds.
Wouldn't failing to account for that make a difference?
My old professor wrote this article!
> Agreed that conformal projections preserve angles over short distances. Poster is near Atlanta, which is Georgia West zone. Convergence at Atlanta's longitude is about 59 minutes 26 seconds.
>
> Wouldn't failing to account for that make a difference?
Only when Geodetic bearings are considered. Convergence is the difference in Geodetic Az and Grid Az. T-t correction relates to turning plane angles on the curved surface of the Earth, but isn't particularly noticeable over short distances (from what I've read).
going out on a limb here, but this is my theory: because of the nature of the GPS constellation and the coordinates gleaned from GPS data, coordinate uncertainties are typically lower/smaller/better in the east-west axis than the north-south axis. if you are traversing from control of this type, and using a weighted coordinate strategy, an east-west traverse would expose more north-south uncertainty than a north-south traverse would expose east-west uncertainty.
i hope i have described this well enough. anyone agree with me?
Good point, bill. Then I am probably wrong about the scale factor issue.
Are you using a projection?
> My company does a lot of road work. As a result we run a lot of long linear traverses(1 to 2 miles) between GPS control pairs. We have noticed that when we run a traverse in the East to West direction our closures are not what we would like them to be. Maybe just a coincidence but the mis-closure always seems to be to the north when running to the east and to the south when running to the west. We have had our instruments calibrated as well as the tri-bracks. All GPS pairs were part of a static network with 1 hour observations with multiple redundancies. Our raw field closures are around 1:16,000 unadjusted and 1:50,000 adjusted by compass rule. We are expecting better closures based on the fact that we are running 1" Total Stations and turning 5 sets of angles.
>
> Strangely enough the few traverses we have run with control in the north to south direction have closed very well(1:50,000+ Raw).
The key piece of information that I haven't found in this thread is what MAP PROJECTION (if any) the traverses are computed on. Are you in fact using a standard map projection or are you using the GEODETIC bearing between a pair of GPS-derived positions to use as the starting azimuth for your traverse?
Least Squares Adjustment. I would run the GPS and the conventional in min constrained adjustments just to identify blunders.
Another thing to verify is that you are using the correct prism constants. The conventional traverse would still look great on its own if you use totally bogus prism constants (results in a blunder in the scale of your traverse).
I could be wrong, but I wonder if you are placing too much confidence in the azimuth between your GPS pairs? This is what leads many folks to rotate their traverse in CAD because they have a great distance fit but the weak azimuth they begin their traverse from impacts the closure negatively.
What was held fixed in the GPS control?
I would bet some of the LSA gurus around here would be willing to offer advice if you posted some your data in Star*Net DAT format.
Are you using a projection?
:good:
Are you using a projection?
Kent
We are using state plane coordinates derived from geodetic positions obtained by Static GPS and reduced in Leica Geo Office with a least squares reduction.
Kevin
Unfortunately Georgia DOT requires we use a compass rule adjustment on our "traditional" traverses. We can and do use LSA on the GPS control.
Yes. We figure a SF for each traverse between two pairs of control points.
Frustrating, but such is government.
I would run a LSA on the traverse anyways just to look for blunders.
Then address any blunders, adjust by DOT standards.
I am in the process of doing that right now.
