Forced Centering aka Leap Frogging Traverses

> 1. I am not that comfortable about measuring up from the mark on the ground to the prism on the tripod. Any suggestions for tricks and techniques? What are the downfalls to be aware of?
If you are uncomfortable have your assistant also measure it, and check each other. Don't have an assistant? Measure before and after you turn your angles. or measure it once in feet, then again in meters, and make the conversion. There will be a constant difference between the height of instrument and the height of a target on the same tripod. KNow it and use it to check your measure ups.

Be careful that you don't bend the tape while doing the measurement, especially if there is a wind. I like to use a folding ruler for this because it doesn't bend as easily. You will find that the limit of measuring error for measure ups is about +/- 0.01'.

> 2. My data collector software will convert slant height to correct HI if I measure to the notch below the TS. It will add to the additional vertical distance from the notch to the lens. So I should be okay with this…right?
You should be okay.

> 3. Can I use the 360º prism with auto lock for traverse? I can religiously make sure the TS is pointed to one mini prism on the 360º and use the same one for all the shots.
As long as the range isn't too great. I often used the 360prism with my Trimble 5603 with no noticeable degradation.

> 4. With target on prism pole when certain obstacles are in line of sight I could simply raised the pole to have clear sight. With tripods I am limited. Do I need to create another point in middle in what already happened to be a short traverse leg? I don’t see any other way within my limited knowledge.
If you have no other alternative you might be best to abandon the forced centering for that one leg.

> 5. Any hint on how to be consistent in carrying elevation with forced centering method?
The trick is to get your measure ups entered right. See step 1.

> 6. I have three tripods and two extra tribrachs with optical plummet. One challenge is the tribrach on the gun do not have optical plummet. So with the third tribrach which belongs to gun I have to setup each time with the gun on it. Any thoughts how anyone of you manage this kind of situation?
Either get another tribrach with an optic plumb or you can get a carrier/adaptor with a plumb. In your case, I suppose you will have to use the plumb on the gun to set up your foresight. Not convenient, I know!

> 7. After reading several posts on the forum there seems to be no consensus if one should slide the instrument around on the plate to make it sit over the point again? One school argues it should be fixed so that accuracy can be maintained. Another school counters that with an argument—instead of keeping the error locally to one point we will be propagating it to remainder of the traverse, hence increasing error ellipses! Any thoughts for a novice here?
If you move the tribrach on the tripod head you aren't forced centering. If feel the need to move the tribrach after having once set it for the foresight it is a sign that your tribrach isn't in good adjustment. In forced centering you set the tribrach up as the foresight and it stays in exactly the same place while you foresight it, then occupy it, and finally backsight it.

As they say in Canada, keep your head up and your stick on the ice.

1. Nothing to it. The tape should be straight and not bent, if you have a hard time with that use a folding rule. Blundering the measure up is a common mistake, say it out loud while looking at the tape as a mental check and to help you remember until you record it in the dc and or the book.

2. Do one or the other but not both, pick a method and stick with it.

3. Up to you, there is documentation somewhere that will tell you what the spec is on the prism verses others, if it fits in what you are looking for then you should be fine. Trust the technology to do its thing but find out what that thing is so you understand your hardware.

4. If you are traversing through points that are so blocked you are jacking the rod up above obstacles and then traversing through em you are kind of swimming in different waters than forced centering. The assumption, in general is that when you say "traversing" you are talking about along a line of sight that is tripod to tripod visible.

5. I like to run gear that has the same Height from tribrach to center of gun as to center of glass. Leica gear is built that way to start, some other gear is not and some gear is adjustable so you can dial in the prism HI with a threaded element.

6. Kind of a awkward arrangement, you could buy another tribrach with plummet, you could remove the plummets from the two "extra" ones and use the gun to set up every tripod, this would be the tightest in some respects but time consuming.

7. Keeping the tribrach in exactly the same place is a old school way to maintain tight positional tolerance, if I am following your saga you are already swimming in LSA soft, and are not building a watch or anything anyway, I would encourage you to adjust up over the point as you go, if the difference is more that a hundredth, you are doing something wrong. Be advised though that once you do you are no longer "forced centering" at least as I understand it, but just traversing with legs.

Aloha, Mark:

Pleasantly surprised to see a response so quickly. I didn't aspect until tomorrow since I posted it pretty late. Thank you!

Couple more questions...

1. What would you consider to be far of a range for 360º prism?

2. I did a quick search on Amazon.com for folding rulers. There are a lot of them! Any suggestion for brand that is reliable and you would trust?

Thank you again!

Aloha, PC:

Thank you so much for your valuable suggestions and input. I know I can count on you all! I am swimming in LSA soft but with some large flotation device attached to me! 🙂

Thank you.

> 1. What would you consider to be far of a range for 360º prism?
That depends on the instrument. But without fear of being too far wrong I'd say 500 feet or so. If your instrument gets the distance quickly, then it isn't too long.

> 2. I did a quick search on Amazon.com for folding rulers. There are a lot of them! Any suggestion for brand that is reliable and you would trust?
I get mine at Home Depot. Be sure to get the type that is graduated in feet, tenths, and hundreths Engineers).

> 3. Can I use the 360º prism with auto lock for traverse? I can religiously make sure the TS is pointed to one mini prism on the 360º and use the same one for all the shots.

I'd suggest that the first thing to do is to test your 360º prism by setting it up successively on the same point with a new orientation of the mini prism toward the instrument each time. On the face of it, it seems possible that using the robot to a target like that may introduce more error than the centering system does. I am, however, unrobotic and may be needlessly concerned on that point.

> I'd suggest that the first thing to do is to test your 360º prism...
That's a good idea and simple to do. I did it with the old 5603 and found that the orientation of the 360° prism made no difference.

I use a steel tape that has a magnet on its end.
That is brilliant if your mark is magnetic.
One thing to watch if you're chasing mm's is those 3, 5 metre, 10, 15' tapes have an end to compensate for interior and exterior measurements.
I also find my work boots (foot) are good to hold the tape to the mark. Takes a bit of care but comes second nature.

I also carry those short 1' extension poles for pesky areas where sight can be obstructed.
Typically your instrument maybe just thst that much higher than target above tribrach and so sightlines vary from one end to another. Also useful over fences etc.

Re instrument tribrach.
I have set up my non plummet tribrach with a target set up then plumb a point after bringing instrument forward.
That has obvious problems but with care can work. Need to ensure your bubble on tribrach is working.

If you are in control, trust your offsider then you should be okay.
I always stress to my offsider to say if he has any doubt about anything. He's extremely trustworthy which makes for a good team.

>
> 2. My data collector software will convert slant height to correct HI if I measure to the notch below the TS. It will add to the additional vertical distance from the notch to the lens. So I should be okay with this…right?

Isn't the instrument measuring to the center of rotation in both axes, above the point on the ground? Not to the center of the objective lens (which may be higher or lower, depending on the VA?

Shouldn't the HI measurement ALWAYS be to the hash mark on the instrument?

As for the HI's/HT's: I use a Trimble S6. It has a notch near the bottom of the instrument to measure the slant HI to. I have found this to be an accurate way to measure HI's. The DC does the conversion and stores it as a true vertical measurement to the instrument horizontal axis. I feel I can get ±1 mm without too much trouble. I use a metric folding rule that also has feet and hundredths. If you have one of those, you can record both in the FB as a check. I also measure the slope HT to seco mini prisms, and enter that in the DC. But, that is not as accurate as measuring the the instrument. If I want good verticals (trigs), I use the following info: the S6 is 0.196 m above the tribrach plate. The prisms are 0.083 m above the tribrach plate. So, I can easily compute what the prism SHOULD be true vertical, and can change it if needed, usually in the office.

I do not use the 360° prisms for traversing. They introduce small errors if not pointed directly at the instrument.

None of our tribrachs have optical plummets (by choice). We use small rotatable optical plummets that sit in the tribrach, and of course the instrument has a built in rotatable plummet.

Many times I don't even set points on the ground (if not needed). In those cases, my HI is 0.196 m and my HT's are 0.083 m. When I do have a point on the ground, I would never move either the prism or instrument over the point AFTER an observation has been made to that setup. I would rather have an isolated small error at a single point than an accumulating error that just keeps getting larger as I go along because I moved something.

Another trick I do sometimes is to set YPM (yellow paint marks) as references around the point and shoot reflectorless to them. I can then use those to resect if I need to come back to that point. This is because I do a lot of setups in water where I cannot set a ground point. That can also help in locations where it is difficult to set a nail or rebar, or where it will be knocked out.

A lot of good info already mentioned. 1 thing we do that hasn't been mentioned-
when measuring up to the prism on the backsight, we turn the prism parallel to the ground and measure to the side of the target. I think this helps to better identify a more precise point to measure to without as much eyeballing/rubbernecking.
This does give a slant height and you can read/estimate it to 0.005'. Our targets are 0.59' wide, so the our "radius" is 0.295' - - your HS for your target will be
.
.
HS= SQRT [slantheight^2-radius^2]

(for most applications 4-6' the HS will be about 0.005' less than your slant height.)

Don't forget to turn your target back to it's correct position oriented toward the instrument.... otherwise when you try to orient the instrument on the backsight- you'll do a facepalm and "commence the walk of shame" back to the backsight. Remember- It doesn't take any longer to complete an accurate/precise measure-up than it does to do a crappy/sloppy one- you just see the results in your vertical....

> Don't forget to turn your target back to it's correct position oriented toward the instrument.... otherwise when you try to orient the instrument on the backsight- you'll do a facepalm and "commence the walk of shame" back to the backsight.

I wish I had nickel for every time...

If you're really trying to be that accurate with the elevations I would carry HI on the traverse, but nothing beats a good old fashioned level loop. Run a closed level loop turning on each traverse point.

Also, 3 optical or laser plummet tribrachs are a requirement for this procedure in my opinion. That's the way I've always leap frogged.

"4. With target on prism pole when certain obstacles are in line of sight I could simply raised the pole to have clear sight. With tripods I am limited. Do I need to create another point in middle in what already happened to be a short traverse leg? I don’t see any other way within my limited knowledge"

I have a mini-prism that I can hang on a plumb-bob from the tri-pod. It gives some adjustability below the tri-pod.

Aloha, Mark: Thanks!

> > 1. What would you consider to be far of a range for 360º prism?
> That depends on the instrument. But without fear of being too far wrong I'd say 500 feet or so. If your instrument gets the distance quickly, then it isn't too long.
>
> > 2. I did a quick search on Amazon.com for folding rulers. There are a lot of them! Any suggestion for brand that is reliable and you would trust?
> I get mine at Home Depot. Be sure to get the type that is graduated in feet, tenths, and hundreths Engineers).

Aloha, Kent:
Thank you for the suggestion! Will do the test this week.

> > 3. Can I use the 360º prism with auto lock for traverse? I can religiously make sure the TS is pointed to one mini prism on the 360º and use the same one for all the shots.
>
> I'd suggest that the first thing to do is to test your 360º prism by setting it up successively on the same point with a new orientation of the mini prism toward the instrument each time. On the face of it, it seems possible that using the robot to a target like that may introduce more error than the centering system does. I am, however, unrobotic and may be needlessly concerned on that point.

Aloha, Richard:
Thank you for the additional thoughts!

That is a very good point on the tape end that is designed for interior and exterior!

Extension pole idea is very useful...but I think I need about 2' extension to have clear line of sight.

Richard...I don't understand clearly what you are suggesting on the tribrach without plummet setup. Sorry, hoping you could elaborated a bit more for me.

I had to look up offsider mate! 😀

> I use a steel tape that has a magnet on its end.
> That is brilliant if your mark is magnetic.
> One thing to watch if you're chasing mm's is those 3, 5 metre, 10, 15' tapes have an end to compensate for interior and exterior measurements.
> I also find my work boots (foot) are good to hold the tape to the mark. Takes a bit of care but comes second nature.
>
> I also carry those short 1' extension poles for pesky areas where sight can be obstructed.
> Typically your instrument maybe just thst that much higher than target above tribrach and so sightlines vary from one end to another. Also useful over fences etc.
>
> Re instrument tribrach.
> I have set up my non plummet tribrach with a target set up then plumb a point after bringing instrument forward.
> That has obvious problems but with care can work. Need to ensure your bubble on tribrach is working.
>
> If you are in control, trust your offsider then you should be okay.
> I always stress to my offsider to say if he has any doubt about anything. He's extremely trustworthy which makes for a good team.

Aloha, Bob:
This is instrument specific. I was referring to the Trimble TS I am using. See attached images. Assuming...I understand you questions here...
>
> Isn't the instrument measuring to the center of rotation in both axes, above the point on the ground? Not to the center of the objective lens (which may be higher or lower, depending on the VA?
>
> Shouldn't the HI measurement ALWAYS be to the hash mark on the instrument?

Aloha, John:
Thanks for chiming in!

I understand most of it; however, few questions below...

I do remember seeing the images posted in the past with S6 setup on the river bed and you in your canoe next to it!

> I also measure the slope HT to seco mini prisms, and enter that in the DC. But, that is not as accurate as measuring the the instrument.
This is not clear to me. Could you please clarify how this measurements are done?

> If I want good verticals (trigs), I use the following info: the S6 is 0.196 m above the tribrach plate. The prisms are 0.083 m above the tribrach plate. So, I can easily compute what the prism SHOULD be true vertical, and can change it if needed, usually in the office.

I don't understand this one John... How are you getting these values? We have the height value from ground mark to tripod plate, bottom of the tribrach to its top plate and the bottom of the instrument to it designated mark(s). Where does this values fit in this variables? Sorry if this is a silly question!

Thank you so much!