Intro to Levelling
24
resentation, together with the next few presentatio e the procedures for measuring and calculating vert ces. Accurately is important that we understand and always conside e required and achievable accuracy. Errors Gross Errors Systematic Errors Random Errors ion - represents the repeatability of a measureme concerned only with random errors. ations closely grouped together with a small deviat he sample mean (small standard error) are said to b e.
description
Introduction to Leveling
Transcript of Intro to Levelling
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
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
