Lecture 1 -Leveling

In this lecture, we will provide a brief introduction to basic principles of surveying and outline one of the most fundamental forms of surveying - leveling

Concepts Covered

1) LevelingDefinitions (benchmark, HI, backsight, foresight)Basic theory of levelingLeveling the instrumentReading the survey rodBooking your workIn-class survey

2) Discuss field assignment #13) Assign everyone into survey groups4) Identify the location where we will all meet in the field

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Surveying – An Introduction

Definition:

The discipline encompassing all methods of gathering and processing information about our physical earth and its environment.

Types of Surveys:

Cadastral Surveys (boundary surveying)

Hydrographic Surveys (involving water bodies)

Construction Surveys (engineering surveys)

Geodetic Surveying (locating points in space)

Topographic Surveys (ground-based mapping)

Photogrammetric Surveying (aerial methods)

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Field Work In This Course

Week 1 Leveling

Definition:The process of finding the elevation at a specified location relative to another known elevation.

This week, each group will perform level surveying at a specified site on the university campus.

We will break up into groups later in this lab.

Week 2 TapingWeek 3 Transit and AnglesWeek 4 Field Survey for Course Project (Phase 1)Week 5 Field Survey for Course Project (Phase 2)

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Leveling Equipment

Level

Levels are instruments used to establish a horizontal line of sight.

Tripod

Leveling rod

Field Book, pencils, ruler or straight edge

Stakes, hammer, paint, ribbon

Hand level

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Definitions

Bench Mark (BM) A permanent object that has a known elevation.

Temporary BenchMark (TBM) A moveable object that has a known elevation.

Turning Point (TP) A fixed object used when determining the elevation of other points.

Think of turning points as “stepping stones” in your level survey.

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Height of Instrument (HI) The elevation of the line of sight established by the instrument.

Backsight (BS) The reading on the rod when held on a known or assumed elevation.

Backsights are used to establish the height of instrument.

Foresight (FS) The reading on the rod when held at a location where the elevation is to be determined.

Foresights are used to establish the elevation at another location, often a turning point.

Intermediate Foresight (IFS) The reading on the rod when held at a location where the elevation is to be determined but not used as a turning point.

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Basic Theory

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Calculations

For our leveling, we need to apply two very simple equations:

Height of Instrument = Known Elevation + Backsight

and

TP Elevation = Height of Instrument – Foresight

For the previous example:

Height of instrument = Known Elevation + Backsight = 100.000 + 0.973 = 100.973

and

TP Elevation = Height of Instrument – Foresight= 100.973 – 4.987 = 95.986

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Booking Your Work

Every surveyor in North America records his/her level surveys in the same manner using the six columns displayed on the left-hand pages of your survey field book. The columns are typically labeled as: station, backsight (BS), height of instrument (HI), intermediate foresight (IFS), foresight (FS), and the last column contains the elevation values. We will use the column labeled IFS later. Here is how we would record our values for the previous example.

Station BS(+)

HI IFS(-)

FS(-)

Elevation

BM 100.000.973 100.973

TP #1 4.987 95.986

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Arrangement of Cross Hairs

When you sight through the telescope, you will see a vertical and a horizontal cross hair and two horizontal stadia hairs.

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Reading the Rod

Rod readings are taken using the centre cross hairs.

For now, ignore the presence of the stadia hairs.

Rod readings are taken to three decimal places (or the nearest millimetre).

Rod readings can be read to two decimal places with certainty.

Estimate the third decimal place

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The rod is delineated to the nearest hundredth of a metre (centimetre)

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What is the reading for this rod sighting?

Answer: 2.363

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What is the reading for this rod sighting?

Answer: 1.308

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Duties of the Rod Person

The rod must be plumb to give a correct reading.

No matter how much care is taken by the instrument person when reading the rod, if the rod is not perfectly vertical when read, errors will result.

Waving

Waving is the procedure used to ensure that the rod is plumb when a reading is taken.

The method consists of slowly rocking the top of the rod, back and forth.

The instrument person continuously reads the rod and selects the lowest value.

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Closure

For all differential leveling, it is good practice to close the leveling loop.

Closing the loop is accomplished by returning to the original starting point.

If we were to complete our level loop with complete accuracy, our computed final elevation would be exactly the same as the benchmark elevation used to initiate the survey.

This comparison of the starting elevation and the computed ending elevation is termed closure.

The accuracy of the survey can be easily determined by comparing the sum of the backsights with the sum of the foresights. They should be equal.

Depending on the precision required, permissible values for the closure of a level loop can be specified.

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Setting Up the Level

1) The legs of the tripod must be tightened securely.

2) The legs of the tripod should be firmly pressed into the ground with the tripod base plate roughly horizontal.

3) When leveling a four-screw level, the telescope is rotated until it is over two opposite screws as shown below.

The telescope is leveled by using the thumb and first finger of both hands to adjust the leveling screws until the bubble is approximately centred.

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Rule #1

The leveling screws are ALWAYS turned in opposite directions by equal amounts simultaneously. If one screw is rotated faster than the other, the screws will either bind or the telescope will loosen.

Rule #2

The left thumb rule ….

The leveling bubble will always move in the direction of your left thumb.

4) Rotate the telescope 90 degrees until it is located over the other two leveling screws as shown.

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Again, level the telescope using the leveling screws.

5) When the scope is level, rotate the telescope another 90 degrees and make any minor adjustments to level the instrument.

6) Rotate the scope another 90 degrees and again, make any minor corrections as required.

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7) Continue rotating and leveling the scope until the instrument is fully level along both axes.

8) As a final check, gently spin your telescope and allow it to come to rest, no matter what direction it faces.

Examine your leveling bubble.

It should be exactly centred.

If it is not, repeat the entire leveling procedure.

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Differential Leveling

In the previous example, we established a new elevation relative to a nearby benchmark. In actual practice, our site and the benchmark might be very far apart. In order to transfer the benchmark elevation to our site, a process called differential leveling is used.

With differential leveling, we make use of turning points (TP’s). Recall that turning points are selected locations where we use a foresight to establish the elevation.

We then move the instrument to a new location and take another backsight to establish a new height of instrument.

We can then repeat the “foresight/move the instrument/backsight” sequence until we establish the elevation at our desired location.

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Field Assignment #1Instrument Tests.

Complete the following tests and document them fully in your field book.

1) Instrument Test #1 (level tube check)2) Instrument Test #2 (cross hair check)3) Instrument Test #3 (2 peg test)

Field Survey

Use standard differential leveling to establish the elevation at three stations defining a closed traverse. Document your level survey in your field book.

Your fieldwork must include at least four turning points (TP’s) and eight separate instrument setups as shown on the following figure.

You must get your fieldbook signed by an instructor before you can leave.

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Preparation for your first field survey

1) Carefully read pages 1-36 of your CIVE125/ENVE100 Measurement and Surveying Notes.

Know how to care for your instrument.

Understand how to level your instrument.

Understand the three instrument tests.

Understand how to correct your closure error.

2) Check the weather forecast. (appropriate dress, sunscreen, etc.)

3) Meet at the equipment shed located beside Conrad Grebel College.

4) Be on Time !!

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Group Assignments

E1, E2, E3 Mike MuffelsE4, E5, E6 Jennifer SonE7, E8, E9 Terry RidgwayE10, E11, E12 Alison SherkE13, E14 Prof. Cascante

C1, C2, C3 Mike MuffelsC4, C5, C6, C7 Jennifer SonC8, C9, C10, C11 Alison SherkC12, C13, C14, C15 Don FraserC16, C17, C18 Prof. CascanteC19, C20, C21, C22 Terry Ridgway

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