TRAVERSE IN MOUNTAIN COUNTRY

Have you adjusted the horizontal collimation? I have had the same issue with short legs and steep verticals and the issue is generally resolved after adjusting the collimation. You need to do it often, say once a week. Changes in temp will change the collimation.

>> My question is specifically related to the steep vertical angles, and if it is "normal" to get lousy closures with steep angles, and what do you do to negate them or compensate for them?

I suspect that his instrument was a bit out of level when he turned the horizontal angles.

A simple test to show how an off level instrument can affect the horizontal angle is this:

Level the gun. Index (Zero Set) on a back sight and rotate the telescope up and down and observe the horizontal angle display. If the gun is leveled correctly the horizontal reading should not vary more than a few seconds from zero.

Rack the gun slightly out of level and repeat, you will see that as you rotate the telescope up and down that the horizontal angle changes considerably, and thats just from a static position. The horizontal error will magnify as the telescope is turned left/right along with up/down.

A loose tripod head will also impair the accuracy.

The colimnation was checked daily (this traverse took several days over a course of 2 weeks. The check each time says the unit IS in adjustment, and all of the number look good. The adjusment routine was carried out in low-land flat ground, if that makes any difference. The ppm's were reset taking into account temperature and pressure about every hour. Probably a 20 degree temperature diference between where the colimnation routine was done, vs. the location of traverse. Elevation difference is about 2500 feet.

thanx,
Geezer

That kind of “miss-closure” seems pretty excessive to me too.

When things get “steep,” and courses get “short,” then equipment, procedure, and computation parameters that might be of trivial concern down on the “flats,” become increasingly NON-trivial.

Things like forced centering, TILTING prisms, curvature/refraction, vertical convergence/divergence, environmental corrections, so forth and so on, start to make MORE difference than when you are down on the flats.

Will ALL of this explain ALL of the problem??? Probably NOT, but certainly SOME of it.

Of course there is the “GPS-ability” of the constraining stations in that kind of terrain to be accounted for too. The traverse may very well be BETTER than the GPS (in a relative sense)!

Loyal

I agree with Paul. Also if your setups are on steep terrain with a lot of duff on the ground it can be nearly impossible to maintain a level setup and walk around the gun to get fore and backsights. I've had to use two people on occasion, one looking each way with one sitting down and one barely able to look through the total station.
What did you use for targets? Tripods, rangepole with bipods?
Mountain work in heavily wooded terrain can be challenging, to say the least.
DJJ

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

> .... I've had to use two people on occasion, one looking each way with one sitting down and one barely able to look through the total station.

Been there, done that Doug.

Another overlooked item as far as equipment are the tripod shoes. They can and do become loose. A few turns with a 9/16" wrench will fix those.

I had a similar scenario posed to me recently by a customer of ours. He was not traversing with a total station of the type you specify but he took off from a pair of points with GPS derived coordinates and closed on another pair of points with GPS derived coordinates about 5,000 feet distant horizontally and of 600 - 700 feet elevation difference.

Interestingly enough, he was looking at about the same horizontal misclosure as you are.

After analyzing his traverse data and looking at his GPS data, I came to the conclusion that the scale factor accounted for virtually all his apparent misclosure.

So I would ask, based on the situation you describe, are the beginning and ending coordinates State Plane? Was the traverse data scaled? At the elevation you mention, that is a likely source of a significant portion of the apparent error.

Geezer,

I didn't catch where or if you said the misclosure was in bearing or distance. Is it apparent to be one or the other?

Pretty tough to look at a traverse closure when you do not have an angular closure.

Does your collumination routine include a tilt axis collumination routine? This is what would affect horizontal angles when sighting a steep angle. On the Trimble 5600 series instruments, there is a tilt axis error collumination routine. There is only one place in my town where I can run this routine properly. I say that because there is a recommended distance and angle. The only target I could find that met the requirements was a radio station tower. I think most people sight a power pole.

After it has determined the tilt axis error, it applies the error to the horizontal angle as you sight vertically. So if I were to zero the gun at a zenith angle of 90° and start turning the vertical motion, I should see the horizontal angle change as the tilt axis error is applied.

I would be curious if your gun holds zero as you run the vertical motion. JRL

Clearcut,

I really don't know where the error is as the traverse looks almost like a "9" in shape. The guys were trying to loop around the backside of a small peak rather than scale the cliffs.

thanx,
geezer

>
> My question is specifically related to the steep vertical angles, and if it is "normal" to get lousy closures with steep angles, and what do you do to negate them or compensate for them?
>
> Thanx in advance.
>
> Geezer

lousy, but not that lousy. we usually get at least 1:10,000. we quite frequently have very steep angles and some traverse legs as short as 50'. we do not use any special procedures, just double the angles and close the traverse.

You're swattin at flies. Traverse closure should have been around 0.15'. The GPS points are bad coordinates. Run the traverse back over the same points and you'll find the answer.

Pablo

>A fellow worker just ran a 1/2 mile traverse; 15 legs - half less than 200' with the shortest being 106'; and several vertical angles over 30 degrees.

>Probably a 20 degree temperature diference between where the colimnation routine was done, vs. the location of traverse. Elevation difference is about 2500 feet.

> I had a similar scenario posed to me recently by a customer of ours. He was not traversing with a total station of the type you specify but he took off from a pair of points with GPS derived coordinates and closed on another pair of points with GPS derived coordinates about 5,000 feet distant horizontally and of 600 - 700 feet elevation difference.
>
> Interestingly enough, he was looking at about the same horizontal misclosure as you are.
>
> After analyzing his traverse data and looking at his GPS data, I came to the conclusion that the scale factor accounted for virtually all his apparent misclosure.
>
> So I would ask, based on the situation you describe, are the beginning and ending coordinates State Plane? Was the traverse data scaled? At the elevation you mention, that is a likely source of a significant portion of the apparent error.

I'm putting my money on JerryS and the SPC scale factor. If I have read the posts correctly you have about 2600 feet of traverse and a 2500 feet elevation change which would put your termination point almost on top of your beginning point.

G-

I suggest that if you are not "Forced Centering", you would be introducing sighting error.

My $0.015.

Cheers

Derek

Could be a number of things.
I also agree about the gun being slightly out of level...
SPC could be another..

Yeah its not great closure...but its not the end of the world either..

At first we never doubled angles. Then we started doing that and was given a 15" allowance. Having been about the only person in the crews who had experience at this I laughed. The head hancho that I was being cocky and challenged me to turn a set in 10 or less. I told I'd turn them in 5 or less all day. On a rainy day we setup some points in the office and the challenge was on. Lets just say I got a free lunch that day. 🙂

For setting angles on the gun we always used the plumb bob string to set the gun angle's. Rod and prism were only for the distance shots. The down side to using that method however is if the gunner forgets to actually zero the gun on the backsight. I never did that thank goodness. One of the other guys did it several times at first and me and my partner got sent out to re-run those legs. I never saw any drawings but was told it stands fairly obviously when a point suddenly drops off the old logging road and shows up way down a number of contours down the side for no apparent reason.

Given all that care I remember one setup where I couldn't turn in less than a minute of error. AFter several attempts I called for one of the others to try. His were worse so we had the other guy try and his were worse yet. All 3 of us broke the setup down to be sure it wasn't just one guy's funky setup. We thought the gun was whacked or someone's plumb bob needed collumated. 😉 We packed it up and headed back to the office and checked the gun out there on some known points and it was good. We collumated anyway checked the rods for level as well. All was good and no one had any explanation. We went back the next day to the same points and I turned the first set in under 5". No one had any explanations so we just moved on and it never happened again. The frontsight had a fair amount of vertical but I had shot far worse verticals many times before and since and never had any problems. Go figure.

My bet it is GPS coordinates scale factor.

Traversing conventionally. Should have forced leapfrog centering. Mean of 4 horizontal rounds 2 FL and 2FR . Vertical angle mean of FL and FR if no compensation. Read forward and back distances. Ensure legs are in clear of spongy and intrusive material.

Error source if shooting LS close to rock face with large temperature gradient get refraction.

That said if it is a distance difference it will be scale factor as elevation and distance from central meridian are critical factors.

Tip, Use GPS between two inter visible points in area that you are traversing( does not have to be a part of traverse, just a long inter visible line) and obtain coordinates and then measure that distance with TS.

Compute join, apply scale factor and then compare with horizontal TS distance.

RADU

While 1.5' error is not an ideal traverse misclosure traversing from GPS pair to GPS pair (through the woods)...it is not that uncommon. We typically see better results than that (0.09' today in about 2100'), but occasionally I close over a foot like your crew chief. When the traverse is more or less straight...I just "twist" the whole thing graphically in CAD. Rotating about the first occupied GPS point, to the check in GPS point. This even slightly adjusts the initial back-sight...Distances usually check hot...
This method of adjustment is not always possible...but when traverse leg geometry allows, its my favorite adjustment procedure.

BTW...I thought a 1203 was a 3" second gun?