Week 4

Activity 1- Two Peg Test

The two peg test is a simple method for calculating the accuracy of an automatic of digital level, by calculating its collimation error. When the circular bubble is placed in the center, the line of collimation should be on a horizontal plane, the amount of deviation from this plane is the collimation error within the instrument. All instruments have some level of collimation error, but due to knocks on the instrument, the error may become larger and the two peg test is used to determine whether an instrument is within an acceptable tolerance.

Equipment Required

Process

For the process below a peg distance of 30m will be used, but this distance can be altered depending on the space allowed. Position D should be roughly 10% of the peg distance.

1) On reasonably level ground, hammer in pegs (or mark out with paint) two points A and B, at 30m apart. This will form the baseline for the test.

2) Set up the level along the baseline, mid-way between the points, at point C.

3) Sight to a stave on point A and take a reading, followed by a further reading at point B.

4) Along the same baseline, setup the level approximately 3m (10% of the peg distance) from point A, as shown in figure 1. This will become point D.

5) Sight to a stave on point A and take a reading, followed by a further reading at point B.

6) Calculate the collimation error using the equation below.

Calculating Collimation Error

Using the process above, four readings have been acquired C>B, C>A, D>B, and D>A. To calculate the collimation error, the following equation should be used:

Collimation Error (e) per L m = (C>B - C>A) - (D>B - D>A)

When the level is in perfect calibration (C>B - C>A) = (D>B - D>A), but any difference within these numbers is considered the collimation error per distance calculated, for example 30m for the above procedure. When the error is above ±1mm per 20m (or other approved value), the level is out of tolerance and must be calibrated.

Example Calculation

A two peg test was recorded with pegs A and B being 50m apart and peg D being 5m from peg A.

Field Data

With the level at position C the following data was recorded:

Staff at position A-1.283m Staff at position B-0.860m

With the level positioned 5m from peg A.

Staff at position A-1.612m Staff at position B-1.219m

Calculating Collimation Error

Collimation Error (e) = (0.860-1.283) - (1.219-1.612)

Collimation Error (e) = (-0.423) - (-0.393)

Collimation Error (e) = 0.030m per 50m

Practical Video Guide

Activity 2- Markeaton Street Fly Levelling

Fly levelling is a way of obtaining the level difference between two point. Within this exercise there are 6 points at each of the 4 locations around the building. Fly level round the building and obtain the height of each of the 24 points around the building, relative to the benchmark, which has a height of 25.678m AOD.

Equipment Required

Process

1) Place the stave on the Temporary Bench Mark (TBM) and the level in an appropriate place moving towards A. Record this as a backsight and in the remarks section call it “TBM1 25.678m AOD” .

2) Move the stave past the level towards A, obtain the next reading and record this as a foresight.

Note: remember because the stave has moved, record this on a new line.

3) Move the level past the stave towards point A and record this as a backsight.

Note: As the stave has not moved, record this backsight on the same line as the previous foresight.

4) Repeat steps 2 and 3 until the level is next to the marks for the point A location.

5) Once at Point A, obtain a series of intermediate levels for each of the 6 points labelling them appropriately in the commends section as A₁,A_2…..A₆, until all points have been recorded.

Note: For the intermediate sights, as you move the stave each time, remember to do a new line.

6) Move the stave towards point B and record this as a foresight on a new line.

7) Keep leapfrogging with the level and stave until you reach point B.

8) Repeat these steps until you have obtained all readings on points B, C, and D.

9) Continue fly levelling round to the original bench mark you started. The last reading should be a foresight and on a new line.

10) Count the number of foresights and backsights and ensure that you have the same number of each.

11) Complete the quick check list below, to ensure your survey is accurate to 1mm per change point.

Key Points and Tips

1) Try and pace out when moving the stave or level and keep the number of paces between even. This will help minimise any errors from incorrectly levelling the instrument.

2) Plan set up locations to maximise visibility and simplicity, such as by setting up on the corner of a building to get visibility on both sides.

3) NEVER move both the instrument and the stave at the same time.

Booking Results

Bookkeeping is very important, if a book is correctly filled out errors can often be spotted. There are some rules to ensure that this can be easier:

· Each line in the book relates to a position on the ground for the stave, so if the stave moves, use a new line.

· If the stave stays in the same position, book the next result on the same line.

· The first reading obtained is always a backsight and this should be to a known point, e.g. a Temporary Bench Mark (TBM).

· The last reading should always be a foresight back on a known location. The known location can either be the same point used to start the survey or a different point on the ground with a known height.

· If multiple readings are taken from one level position, the middle results are intermediate sights.

· There should be an equal number of foresights and backsights at the end of the survey.

Example layout for fly levelling

Using the rules above, the below table can be used, with the first reading being a backsight and continually leapfrogging the level and stave (creating foresights and backsights) until ending on the final foresight. At the end of this we can see that there is an even number of each.

Quick Check

To check the error in a survey, obtain the sum of the backsights and the sum of the foresights. When you minus the foresight from the backsight, the difference will give the overall error within the survey.