This presentation, together with the next few presentations, outline the procedures for measuring and calculating verticaldistances.

Accurately

It is important that we understand and always considerthe required and achievable accuracy.

Errors

Gross Errors Systematic Errors Random Errors

Precision - represents the repeatability of a measurement and is concerned only with random errors. Observations closely grouped together with a small deviation from the sample mean (small standard error) are said to be precise.

Measurement

Pro

babi

lity

of M

easu

rem

ent

Most probable value

Small standard deviation High Precision

Large standard deviation Low Precision

This presentation, together with the next few presentations, outline the procedures for measuring and calculating verticaldistances.

Accurately

It is important that we understand and always considerthe required and achievable accuracy.

Errors

Gross Errors Systematic Errors Random Errors

Precision - represents the repeatability of a measurement and is concerned only with random errors. Observations closely grouped together with a small deviation from the sample mean (small standard error) are said to be precise.

Accuracy is considered to be the overall estimate of the errors including systematic effects.

Most probable value

Precise and accurate results

True value

Most probable value

Precise and inaccurate results

True value

Systematicerror

Vertical Distances - Levelling

Measuring the height

Measuring and calculating the height of a point relative to another point

Level Spirit level Water level Optical level

A surveying optical telescope

eyepiece

object lensobject lensfocusing lensfocusing lens

diaphragm

Typical diaphragms - in different makes of instrument

cross hairs

focusing screw

Focusing

1. Rotate eyepiece to give a sharp,clear image of the cross hairs

2. Rotate focusing screw to give asharp, clear image of the objectbeing observed.The aim of focusing is to remove (eliminate) PARALLAX

line of collimation

Pond Bubble

When pond bubble is centred the instrument’s standing axisis approximately vertical.

The compensators in the instrument take over and adjustthe optical Line of Collimation so that it is horizontal (hopefully)

When the instrument is rotated the compensators ensure thata horizontal plane of collimation is swept out (hopefully)

Parallax

When focussing any optical instrument it is vitally important that we eliminate Parallax.

Move the eye up and down (or from left to right) over theeyepiece of the telescope.

If the cross hairs move relative to the object being observedthen Parallax exists and the focussing is not satisfactory.

Elimination of Parallax

Focus the crosshairs(using the Eyepiece)Focus the object(using the Focussing screw)

Move eyeup and downover the eyepiece

Images appear to moveParallax exists and must be removed by better focussingParallax still exists and must be removed by better focussing

Parallax has been removedTherefore focussing is goodParallax has been removedTherefore focussing is good

Levelling

AB

Measured and CalculatedLevel of A ReducedLevel of A RL A (known)

ReducedLevel of BRL B(unknown)

the Plane of CollimationHeight of

DATUM

DATUM

DATUM

(HPC)

HPC = RL A + S1

S1

Levelling Staff

HPC = RL A + S1

S2

RL B = HPC - S2

Datum

Could be our own Datum - Assumed Datum

- Arbitrary Datum

- Site Datum Or

A National Datum

In the UK we have a national organisation known as The Ordnance Survey (O.S.)

The O.S. has established a ZERO Datum at Newlyn in Cornwall.

- Ordnance Datum

A D

O D

Based on the Ordnance Datum - points of known height aboveor below Zero height have been established around the U.K.

These points around the country are known as Bench Marks

Above Assumed Datum A A D

Above Ordnance Datum A O D

O.S. Bench Marks (OBM)

Arrow or Crowsfoot

mark

Reduced Level

Section throughwall

Rivet

Bottom ofLevel Staff

OBM

TBM

Transferred orTemporary BM

A BC

Some Terminology

RL A RL BRL C

S1

Level staff on A Back Sight (BS) reading is first reading

BS

RL A RL B

A BC

RL C

Level staff on A Back Sight (BS) reading is first reading

S2

Level staff on B Fore Sight (FS) reading is last reading

FS

Move instrument to new position

Move instrument to new position

RL A RL BRL C

A BC

Level staff stays on B

The instrument has changed its position about point B

Point B is known as a Change Point (CP)

CP

S3BS

2nd instrument position starts with BS to B

and finishes with

FS

FS to C

S4S3BS

RL A RL BRL C

A BC

RL A RL B

A BC

RL C

BS FSBS FS

RL A is known

HPC =

HPC

RL A + BS RL B = HPC - FS

(CP)

Now the RL B is known So we can repeat the process

HPC =

HPC

RL B + BS RL C = HPC - FS

Generally : HPC = Known RL + Back SightUnknown RL = HPC - Fore Sight

Summary of Levelling Procedure

When the level has been set up we always start with a BS toa point whose RL is known

- such as an OBM or a TBM

The last reading at any instrument position is always a FS

Either the instrument moves or the staff moves -

never move both

We must always finish levelling at a point of known RL value

- such as an OBM or a TBM

always close your levelling

Reading an E-type levelling staff

Read value atthehorizontalcross hair

The value is ?

1.9001.9101.9201.9301.932

1.1001.1101.1201.1301.133

Introduction to Levelling

We have covered the following

What is meant by RL The basics of an Instrumentknown as a Level

DumpyLevel

Tilting Level Automatic Level Digital Level

How to transfer RL s Datums - OBM - TBM

It is necessary to practice with a real level and staff

Practical session - Meet at Survey store - 2nd Door on Left inside main entrance

Collect - Level and Tripod

N.B. Please - ALWAYS GET YOURSELF ORGANISED BEFOREENTERING THE STORE i.e. pencils - notes - field books etc.

You can leave bags etc inside the survey store -

Carry out : Setting up an automatic level

Eliminating PARALLAX Reading level staff

Reducing levels for given points

Next week

RL A RL B

A BC

RL C

BS FSBS FS

RL A is known

HPC =

HPC

RL A + BS RL B = HPC - FS

RL B + BS RL C = HPC - FS HPC =

HPC

Generally : HPC = Known RL + Back SightUnknown RL = HPC - Fore Sight

(CP)

Levelling

Now the RL B is known So we can repeat the process

When the level has been set up we always start with a BS toa point whose RL is known

The last reading at any instrument position is always a FS

Either the instrument moves or the staff moves -

never move both

- such as an OBM or a TBM

We must always finish levelling at a point of known RL value

always close your levelling

Summary of Levelling Procedure

- such as an OBM or a TBM

i.e.always end with a FS