Hi.
I'm Rami Tamimi.
If you've ever done any kind of survey leveling, then you know you're gonna need to do some
calculations.
These include the height of the instrument setups, as well as the elevation of the points
you are reading.
seen my last video about differential leveling in the field, I want you to go and check that
out.
It shows you the full field procedure of how we collect backsight and foresight readings.
The link to the video will be below in the description.
Or you can click on the corner right now and it'll take you to it.
Once you've watched that video, come back here and I'll explain to you the calculations
needed.
Let's go ahead and take a look at some of the data we've collected.
Now this right here is the project that we did last time.
This is the manhole, which was the benchmark that we shot.
The gate shot, which was the first turn point.
Then there's the three points that we shot: orange, blue and green.
We shot the gate again.
And then we shot the manhole again.
I want to just emphasize that this manhole reading and this manhole reading are the same
point, so don't be confused by the fact that they're on different lines.
Same thing with this TP1 gate and this TP1 gate shot as well.
Those are the same points.
The reason we put these points on different lines is because they were recorded through
different setups.
Therefore, they have a different instrument height.
However, the elevations in theory should be the same.
And then the other thing I want to point out is, these three points right here are considered
side shots.
Side shots are points that do not influence the instrument setup.
What I mean by this -- these points will not change the instrument height because we never
took a backsight on any of these points.
They were simply side shots, so we just took foresights.
Side shots are like special points that we just need a specific elevation on.
They could also be control points that we have record elevation on and we may be in
the field checking into one of them to make sure that our work is good.
That's the use of a side shot.
The reason I bring this up is, I want you to make a slight alteration to your field
notes.
Since we never took any backsight readings on these points, what we're going to do now
is take a look at the foresight reading, and I want you to copy that number into the backsight
column.
So the reason we're doing this is going to help our calculations and just make things
a little bit smoother for us.
To begin, let's calculate our error.
And the way we do that is by taking the sum of the backsights, minus the sum of the foresights.
What we're going to do is we're gonna calculate what the sum is.
So that's 3.83 plus 3.73 plus 5.97 plus 3.19 plus 5.32 plus 4.35.
Our total is 26.39.
All right, now let's take the sum of the foresights.
4.50 plus 5.97 plus 3.19 plus 5.32 plus 3.71 plus 3.67.
26.36.
So we have an error of 0.03.
0.03.
That's three-hundredths of a foot.
That's not bad.
We're going to use this error to calculate what our adjusted elevations are going to
be.
But first, let's just figure out what the height of the instrument setups are and what
the current elevations are.
To do so, what we're going to do is start by taking the 795 benchmark elevation.
This is given to us and this is what we're going to base all of our work off of.
To get the height of the instrument, for the first setup, you're going to add the backsight
to your elevation.
So that's 795 plus 3.83 -- gives us 798.83.
This is the height of the instrument at the time that we took this reading.
Now to calculate the elevation of TP1, we're going to subtract the foresight.
So subtract 4.50 and we get 794.33.
Okay now we have our first elevation.
To continue this process we're going to do exactly the same thing.
We're going to take the elevation of TP1 and we're going to add the backsight of TP1 and
subtract the foresight of the next point.
So 794.33, we're going to add 3.73 and now we have an HI of 798.06.
Now we're going to subtract the foresight of orange minus 5.97.
The elevation is 792.09.
Now we're going to add the backsight of orange, which is 5.97 -- 798.06.
Hey, look.
These two numbers are the same.
Remember when I said side shots don't influence the instrument setup -- this is what I'm talking
about.
Because the backsight and the foresight are exactly the same, whatever you subtract from
the foresight, you're gonna add back into the backsight and you're get the same height
of instrument.
This is because we never actually moved the instrument.
When we shot all these points, the instrument height was the same.
I'm gonna go through the math just to prove to you that it's like this, but in reality
you wouldn't even have to do this because the instrument height would be exactly the
same for all of those shots.
The only time it's gonna change is when we picked up the instrument and went back to
check into the benchmark.
Okay, let's keep going.
So now we're going to subtract 3.19 to get the elevation of blue minus 3.19, 794.87.
Now we will add back that 3.19, give us the same HI, 798.06 and we're going to subtract
5.32 and we have a new elevation of 792.74.
Now we're going to add 5.32.
Same 798.06 minus 3.71 -- 794.35.
And we're going to add 4.35 and now we have a new HI which is 798.70 minus 3.67 gives
us 795.03.
Okay, now you might be noticing a few things.
First of all, TP1 which was the gate shot, and benchmark one, the manhole shot, have
different elevations on the bottom than they did on top.
This is because of the error.
Another way to check our error is to take the final elevation of the benchmark and subtract
it from the internal elevation of the benchmark.
When we do that we get 795.03 minus 795.00 is equal to 0.03 and look, that's exactly
the same.
So we know that we've done our error calculations correctly.
What do we do about this?
How do we adjust the elevation?
The formula to figure out the adjustment factor is actually quite simple.
All it is, is the error divided by the number of stations.
So what we have here is 1-2-3-4-5-6 different stations.
You don't count the initial benchmark reading.
So we have six stations and so what we're going to be doing is dividing our error by
six.
So the formula is error divided by number of stations and that looks like 0.03 divided
by 6 which gives us 0.005.
That is going to be our adjustment value -- adjustment is equal to 0.005.
So now because our final elevation of benchmark one is higher than the initial elevation,
we are going to subtract this from our readings.
In order to do this you want to look at the first station.
The first station, you're going to subtract just the 0.005.
The second one you're going to subtract it twice.
0.01.
The third station 0.015.
Fourth station 0.02.
Fifth station 0.025 and the sixth station 0.03.
Now I understand that I'm kinda out of the box here so forgive me.
This looks a little sloppy so what we should be getting here is 794.325.
Here we're gonna get 792.08.
Here we'll get 794.855.
I put a zero up here just to be consistent.
This one is going to be 792.72, 794.325, and this last one is going to be 795.00.
Okay we have calculated our adjusted values.
How do we know that this is now good?
We can take a look at any numbers that are consistent.
For example, the benchmark reading which is up here is now the same down here so we know
we're good there and TP1, which is right here is also down here and these values are the
I hope you guys enjoyed this video and learned something new.
see guys next time.