With 4 sets back and forth, I doubt that your traverse is out that much unless there is something wrong with your adjustments in your equipment.
You need two independent calculations made for your GPS locations. One for the beginning grades and another for the ending grades. Possibly in two different Projects that have nothing in common between the two resulting locations.
That should get past any differences in results in using any common geoid to obtain results.
That should give you a solution that should compare in your 3D inverse.
When I use the EC mode during data collection, I have better results in hilly areas. The difference is that I only encounter hundreds of feet at a time. The only occurrence near your elevation change was in Arkansas and had a change in elevation of over 1100 feet in one shot and that was in an 1800ft leg. There was a sheer wall of 700 feet.
I was so glad there was a road that went near there.
> Loyal,
>
> Yeah, we thought of the GPS problem, too. We ended up thinking that the GPS was good because the observations were taken in wide open area, no obstructions above 20 degrees or so.
>
> Sure hadn't thought about the tilting of the prism, but I can see where that would make a difference. Forced centering would be the "only" way to go in my book.
>
> Our bottom line take on the issue so far, was that it was just a combination of random areas from a wide variety of as many as 6 - 10 different factors.
>
> Thanx,
> Geezer
If you are not tilting the prism then that's the answer right there. There are error sources elsewhere, but this is a mistake that adds up in a hurry. Been there.
you say you ended on GPS points (not pairs)? RTK points or static? details please.
your traverse is probably fine. 4 sets per setup? how were the splits? how was the traverse by itself?
if all the sets were done from the same setup (same HI, same tribrach orientation & leveling) then they are not as redundant as they could be. Sure they are a nice average but if there is centering error it remains the same for all the sets. You can account for this by breaking the setup between sets (raise or lower the inst. tripod, recenter, relevel.) At the target end, tilting the prism does help quite a bit at short ranges (<200-300 feet).
A TPS 1203 is a nice heavy instrument with dual axis compensators. Some of the concerns raised above about leveling might be valid or might just be taken care of with the compensators. Our 1203 will simply refuse to take a shot if the level is too far out. Whether it gives bad results before that degree of tilt, I do not know. Never seen it happen with the 1203, but I have seen it with uncompensated instruments. Paul Plutae, I will have to go out in the yard and try your test on our 1203. Ran into similar questions doing solar observations with our 1203 vs. an old T2 and never completely resolved them. Always more to learn.
All this is just trivia. Important details, yes, but not important as your procedure for getting the GPS points and combining them with the total station work. I bet your traverse is fine and the problem is with your GPS points.
1) you don't have good enough GPS points (what was the sigma for each? do you believe the sigma that your GPS is giving you? How do you know it is believeable, i.e., what are you comparing it to?)
2) you don't have enough GPS points. One at each end is just not enough redundancy.
I'm not convinced that a pair at each end is enough redundancy either, but that's my personal belt-and-suspenders take on things. Three static GPS points at each end with a closed loop through them and cross ties back to as many of your traverse points as you can see. If you can only see that last traverse point, shoot it (and the other 2 GPS points) from each GPS point at that end.
3) how are you calculating all this? Any kind of least squares? I would be curious to run it thru star*net if you want to send the data.
PS. The only way you will EVER know your GPS points are "good" or even "how good?" is by cross ties and least squares. Whether that is with a total station and least squares, or by post-processing static with known good base stations depends on the work you are doing. I see a lot of guys who are quite good with the old methods (total station, compass rule, etc.) who miss this essential point. You can hang your traverse on some GPS points and sometimes it closes on paper, but without the least squares and the redundant procedures, the work is just not defensible.
dang. kilt another thread.
Hmmmm,
Did you adjust your TS for the barometric pressure differences that 2,500 feet of vertical difference will make in all your distances?
That could very easily result in 2 foot of difference.
First, any leg under 200' gets turned to plumb bob strings. The pole is just too wide in my opinion.
Second, I've had results like that when I took off of RTK pairs and not static pairs. I've never had problems hitting static pairs well. That coupled with the short legs could produce that error.
The pole is 0.1' wide and at 106' makes 00°03'14". Especially if a pointing error happens early in the traverse, it only gets WAY worse. We have a lot of hills in Cherokee County, Texas and while they're not mountains, it's not odd to see traverses with 106° and 79° for zenith angles.
Richard
As Jobo was fond of saying
"Forced centering is great if you never need to use that hub again."
Kris "Not a fan of forced centering" Morgan
> First, any leg under 200' gets turned to plumb bob strings. The pole is just too wide in my opinion.
Use the center of the pole then.
I will zero set on the left edge, turn to the right edge, then back to 1/2 the reading then zero set again, and then turn the angle
I don't use the pole, I use the target on top of the pole.
We traverse in rough country with 3 tripods, 2 tribrachs and targets.
> .. I use the target on top of the pole.
Targets are not always machined true to align with a plumbed rod, bottom to top.
> > .. I use the target on top of the pole.
>
>
> Targets are not always machined true to align with a plumbed rod, bottom to top.
Yep!
well yeah you have to check it first.
ALL traverses:
Three tripods (good ones, no aluminum)
Three tribrachs (I use Topcon 55501, because even with the optical plummet they are "see through" for the Instrument or rotating adapter optical plummet).
Two TILTING Prisms w/targets and rotating adapters (w/optical plummets and level vials) MATCHED to the instrument, so that the Target AND Prisms have the SAME axis (height) as the Instrument.
That last one is IMPORTANT if you want the best results, so I'll say it again:
MATCHED to the instrument, so that the Target AND Prisms have the SAME axis (height) as the Instrument.
If it wasn't SNOWING (again) I would go out to the truck and grab a tribrach/adapter/prism (traverse) set to take a picture of.
Loyal
Are you talking about the trunnions of both being the same distance above the tripod head? Just leveling the tribrach will introduce some difference there unless you are using Kern tripods.
jud
One last thing...or two
Vertical (zenith) angles BOTH ways are also important, but you really need the matching prisms/adapters for that to work properly!
The next (and maybe most important) thing to be careful of, is being SURE that the “horizontal distances” are being PROPERLY computed to a COMMON HEIGHT or surface. State Plane, UTM, or LDP methods SHOULD do this for you, but I would check your software to make sure.
There are several ways that I have seen software deal with reciprocal vertical angles, and NOT ALL of them do it RIGHT...ALL of the time. In fact, the same can be said for “one way” vertical angles and distances too.
Temperature, Barometric Pressure, EDMI Calibration, vertical collimation, deflection of the vertical, and of course prism offsets will all figure into the equation as well.
ALL of this stuff is EASY to account for (by a computer) IF your procedures, equipment, AND SOFTWARE are up to the task.
Loyal
Jud
Sorry, I missed your post last night in my haste to make Happy Hour!
The primary reason for using Topcon optical plummet tribrachs AND optical plummet rotating adapters (with level vials) is so that very little (IF ANY) 're-leveling' of the tribrach is necessary when the forward prism & adapter is replaced in the tribrach by the Instrument (ditto when the instrument is replaced by the backsight prism & adapter).
When traversing, I DON'T use the GTS-3b tribrach at all, BECAUSE it does NOT have an optical plummet (that's in the Instrument itself). By using a matched set of optical plummet tribrachs AND adapters with optical plummets w/level vials, I can simply leap-frog along, and maintain [very] consistent “HI(s).”
The other 'advantage' to this procedure, is that EACH station/setup is verified THREE times, by FOUR different optical plummets (the tribrach, the forward prism adapter, the Instrument, and the back prism adapter), and THREE different level vials. If something gets squirrely, it shows up while I'm still in the same Zip Code.
Two guys can move pretty fast this way (three guys can FLY), even being solo it still speeds things up for me (although I am getting slow).
Loyal
Jud
The key is matching brands.
I have a Leica instrument, some non-descript cheapo brand tribrachs (I didn't buy them). A couple of the cheopo tribrachs became non-serviceable but I found an old T-2 tribrach and a couple of 1980s vintage Sokkia tribrachs in the supply closet at a different location from our office. I adjusted those three using an adjusting cylinder and a target I pasted to the office ceiling and the bubbles using the total station dropped into the tribrach under adjustment. The T-2 tribrach is bullet proof (my former employer had dozens of them which I checked and never found a plummet out but the level vials did need occasional adjustment). The Sokkia tribrachs are good quality and are the easiest to adjust because the capstans screws are right out in plain view, no covers to remove.
So my tribrachs are in good adjustment but they don't match brands so the instrument will fit in them but not very happily. The Leica has a laser plummet so the only tribrach I have that will work is the one that came with the instrument. I realize you can get optical plummet tribrachs that also allow another plummet to see through but those cost dollars that I don't have.
Other than Loyal and RADU, most Land Surveyors whose ship came in would purchase their dream car a Lamborghini Countache but save a few bucks by using the tires and wheels off of their wrecked Ford Taurus.
Dave
I hear ya Dave, and I think that MOST (if not all) of us have had to 'make-do' with what we had, or had access to over the years.
I didn't start out 26 years ago (when I went solo) with ALL of the gear that I have now. I have however made it a point to SLOWLY acquire those things that make the most difference in “capability” every time I had a few extra bucks, OR a project that required additional or more sophisticated equipment. I have taken GOOD care of ALL of this equipment, and can honestly say that I still have every piece of gear that I ever bought (and it still WORKS). There have been a couple of bad choices (cheap stuff), but even that can be used for non-critical purposes. Heck...some of the “cheap stuff” has turned out to be pretty good, and has served me well for many years.
Personally I would take a WELL ADJUSTED mismatched set of odds and ends, over a shiny matched set of UNKNOWNS any day.
Loyal
Dave
Cheap equipment is rarely actually cheap in the long run is my experience. I bet those old T-2 tribrachs were expensive to acquire at the time but since they are still being used 40 years later then they are really very cheap.
I tell anyone who will listen that a 400 dollar tribrach you can use for 20 years plus is cheaper than a 100 dollar Chinese knock-off that you have to replace every 2 or 3 years because the cheap adjusting screws break off (and with very little pressure, they are almost loose).
How you treat equipment makes a big difference. I own a Topcon GPT-3005LW which I treat like a premature baby and get good results out of it. One trick I always do is cycle it off and then back on after fine leveling (I do that with the Leica at work too). That seems to tighten up the vertical results.
Hint-if you work for a large organization it helps to keep some old beater equipment around for loaners so I can remain assured that the TS issued to me is treated right. Only the real Surveyors get to use the good stuff.