Back2Basics: Long BS & Short FS

In traditional surveying ( once with theodolite and Chain) , but now with a total station it is a good idea to keep your fore sights long as practical. However when you traverse it is impossible to keep traverse lengths roughly the same, because of obstructions and terrain. What I do when I know I am going to be forced into a short leg is to if possible set or observe a distant feature with a recognizable point and then set my traverse point X on that line. So that when setting up on X I observe that distant feature or the set picket (stake) so that I can turn the angle from a long back sight.

You may also be interested that translating angular measurement to linear measurement that the rule of thumb is that an angular error, misclosure, of one minute of arc equates to one inch in three hundred feet ( 1’ = 1”/300’).

In the “old chaining days” angular closures were far superior to distance measuring with a chain, so it was critical to get the angular closure to seconds of arc hence BS and FS lengths were critical. However with the advent of the total station you can traverse a closed figure with very small misclosure because distance and angular measurement precision are both very high.

Misclosure errors and blunders caused by the instrument operator….. .

Cheers

RADU

When running a traverse, I try to make the least amount of set-ups as possible, while tying to keep the legs as balanced as possible. This is difficult to do so I will usually set temporary points first, at all the stations, then adjust as needed.

When doing a topo or stake-out, I will try to keep my foresights shorter than the backsight; this will keep the error less than or equal to the error at the backsight. I will always take a check shot on another known point,this helps me sleep at night.B-)

> When running a traverse, I try to make the least amount of set-ups as possible, while tying to keep the legs as balanced as possible.

Why is balancing the legs important?

As someone pointed out in the previous thread: It's not as important today, as it was back when we used a transit and chain. But if you went to the extreme, you wouldn't take a 10' backsight and turn to a 1,000' foresight.

Also, try to stay away from angles close to 0 or 180 degrees. It's not so important today, with the accuracy of our digital instruments but it was weak angles, turned with a 20" gun, that diluted a good traverse.

Edit: I took a second look at my post and see that I didn't really answer your question.:-$

If you take the error in a 10' backsight and force it into a 1000' foresight, you create 10 times the error. This is also the reason to keep the number of points in your traverse to a minimum. If you have a .04' error in each point, the less points the less error.B-)

@yswami: To intuitively grasp the implications of short and unbalanced sights in meatspace, you can make three points on paper along a good straightedge (like a scale or drafting triangle) to represent unbalanced instrument stations,

___V________V________________________________V____

then use any two points to reestablish the third point precisely and accurately with the straightedge after lifting it from the paper.

😀

Here is a trick question

If you traverse around a mile (20,000') with 40 points and miss the closing distance by 1', is this good enough?

1:20,000 closure.

:snarky:
Radar

> Aloha,
> Okay, here is another basic subject…
> I learned that I should always have long backsight and short foresight--to reduce errors. Let's say I have 300 ft from station to BS point. Does that mean my foresight should stay within 300ft? What are the good justifications to shot long FS that exceeds the distance of BS? What are the precautions that need to be taken if I do? It doesn't make any sense to only shot 290 ft, then breakdown and setup instrument all over again. I probably will introduce more error in the process… If I have clear line of sight for another 700 ft to traverse shouldn't I take it? (I don't have GPS so all the work need to performed with optical instrument.)
> Your thoughts?
> Thank you!

Yes and no. It's always a GREAT idea to have your backsight's as long as possible. Sometimes though, it can't happen. If GREAT precaution is taken, then the risks can be mitigated. Your example is fine. I'd crank the hub 700' with a 300' backsight.

In our hills, we try to keep our hubs at a minimum of 200'. Sometimes it doesn't work. If it's close to 200 or less, the rule is to turn the angles to a plumb bob string. I know, it's archaic, but it works because it's WAY skinnier than the 0.1' wide range pole. Robotics really doesn't care as long as your instrument and pole are in good adjustment.

However, sometimes, you just gotta break all the rules. We were running a 9 mile loop one time for O&G. At the end of the second mile, I had a crazy turn with a 300' backsight to a 127' foresight. Shouldn't be an issue right? Well, the next foresight was 2800' down a pipeline from a 127' backsight. So, we turned the 127' foresight down with 8 sets, so we really could see what the angle was. Then, we moved ahead, and I sent the rodman to the next foresight 2800' down the pipeline. I stacked the plumb bob string in a lath and stayed there to watch it while the instrument man then turned 8 more sets from the short backsight to the long foresight. From there, every sight was 2500 feet or so and fairly balanced, except for a 5900' shot down the state line from a 3200' backsight. 🙂

So a nine mile loop, with a real possibility for a problem only 2 miles in, turned out well and we closed the loop within a half a foot, which for what we were doing, and where we had come from, was WAY more than acceptable.

My point is, if you're gonna break the rules, make up new rules to compensate for the rules you just broke. 🙂 Check check and recheck. When you think you've got it, check it one more time. 🙂

Here is a trick question

> If you traverse around a mile (20,000') with 40 points and miss the closing distance by 1', is this good enough?
>
> 1:20,000 closure.
>
> :snarky:
> Radar

Not just yes but hell yes. 🙂

Here is a trick question

for what? a particle accelerator? probably not, for a gas line, probably so...

Make traverse lines as long as possible for the job you're doing. It doesn't matter whether a backsight is shorter than a foresight or vice verse. That is to say, if you have a setup where one of the targets is longer than the other, it doesn't matter which one you sight first. it will be the same angular configuration either way.

When setting out a point at a particular angle and distance, it is good to have a backsight that is longer than the point you are setting.

Here is a trick question

> for what?

A private property boundary, adjoining government land....;-)

Ideally you will want balanced sight distances. In real world application, that is not always possible, so you have to address those times. Kris described a great procedure in his post.

Even with modern instruments it is useful to balance angles. Even though you may not notice the reason as much as with older equipment.

When you are setting the instrument on the back sight, you have some amount of angular error in sighting the target. If the sight distances are balanced (barring other sources of error), you carry that same amount of error through to the next station. If you sight farther away to the fore sight, the error increases in a proportional triangle.

With careful procedures, in the real world, you are likely talking hundredths of a foot. However, you can see that the error increases as you move farther away from the source of the bad angle.

One reason I prefer a longer back sight distance is that there is a sweet spot where aligning the scope cross hairs with your target (pole or string) involves less guesstimation than if the target is very close. Of course if the site line is too long, you can go past that sweet spot.

In my opinion, balancing the sight distances perfectly is much less important than choosing the proper target for sighting. Again, as Kris Morgan points out it is often wise to break out the old plumb bob as needed.

Back2Basics: Long BS & Short FS Leveling

Now, if you want to put the robot up for a minute and break out a level rod and auto level, now balancing stuff really begins to be more important. Why? Well, if you can keep your backsights and foresights balanced, then you do not need to worry about curvature of the earth.

For those ULTRA-RARE moments where we need a level loop now, and I tend to step them off ahead of time and look at the relief. So, if I'm gonna get short on a turn, I'll probably begin slowing transitioning the turns shorter and shorter so that it's not a big deal and then as I can, slowly make the turns longer and longer, out to about 250'. That's about as far as I wanna turn with a level.

Some people don't make the distinction between the two. I suspect that they were blanket rules for crews even though the surveyor knew when to cheat and how to cheat, the crew didn't and some of the archaic rules have carried on to this day. At least that's my take on some old procedures with new equipment.

Back2Basics: Long BS & Short FS Leveling

I suspect that they were blanket rules for crews even though the surveyor knew when to cheat and how to cheat, the crew didn't and some of the archaic rules have carried on to this day. At least that's my take on some old procedures with new equipment.

Few today even consider the effects of small angles or unbalanced distances, primarily because they have been taught by those who themselves had no clue. The few who do consider what small angles or unbalanced distances can to to a traverse or when doing the computing, know the whats and the whys. Don't consider those basic rules archaic.
jud

Here is the real trick question which might help you visualize the mathematics of the problem. If you are measuring angles between existing points, does it matter if you shoot the long line first then the short line, or is just as accurate to shoot the short line first then the long line.

> What I do when I know I am going to be forced into a short leg is to if possible set or observe a distant feature with a recognizable point and then set my traverse point X on that line. So that when setting up on X I observe that distant feature or the set picket (stake) so that I can turn the angle from a long back sight.
>
> You may also be interested that translating angular measurement to linear measurement that the rule of thumb is that an angular error, misclosure, of one minute of arc equates to one inch in three hundred feet ( 1’ = 1”/300’).
>

Aloha, Radu:
These two suggestions are very practical and I believe these will improve my field procedure a lot. Thank you:-)

>...while tying to keep the legs as balanced as possible. This is difficult to do so I will usually set temporary points first, at all the stations, then adjust as needed.
>

Aloha, Radar: I don't understand the "temporary points first" idea. When I am stuck with short FS where do I setup the temp. point? Could you please explain a bit more?

> Here is the real trick question which might help you visualize the mathematics of the problem. If you are measuring angles between existing points, does it matter if you shoot the long line first then the short line, or is just as accurate to shoot the short line first then the long line.

Yes;-)

> @yswami: To intuitively grasp the implications of short and unbalanced sights in meatspace, you can make three points on paper along a good straightedge (like a scale or drafting triangle) to represent unbalanced instrument stations,
>
>
> ___V________V________________________________V____
>
>
>
> then use any two points to reestablish the third point precisely and accurately with the straightedge after lifting it from the paper.
>
>
> 😀

:'(