Surveying Errors and Adjustment -...

Surveying Errors and Adjustment

A short Training Course

Prepared By:Prepared By:

Dr. Maher A. El-HallaqLecturer of SurveyingFaculty of Engineering

The Islamic University of Gaza

April, 2011

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Part One:

Surveying Errors (sources and types)

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Sources of Errors در ا;���ء� �

Measure a distance several times and compare

the obtained measurements????

They are not equal and the reason:They are not equal and the reason:

The imperfections of the instruments

The fallibility of the human operator

The uncontrollable nature of the environment

Errors always exist in measurements, where:

The error (The error (eeii)) = the measured value (x= the measured value (xii) ) –– the true value (x)the true value (x)

But since the true value can never be determined, we But since the true value can never be determined, we

can use instead the most probable value (xm). Thus:

eeii = x= xi i –– xxmm

Where,

Types of Errors ا;���ء > ? @

Blunders (Mistakes) أ���ء $� ��

Random Errors �B5اA � أ���ء

Systematic Errors �?���1 أ���ء

Blunder Errors (Mistakes) أ���ء $� ��

caused by human carelessness, fatigue and haste

can be positive or negative, large or small and theiroccurrence is unpredictable

recording 43.18 instead of 34.18 and sighting a wrongtarget when measuring an observation

Blunders are disastrous if left in the surveyingmeasurements

must be eliminated by careful work and by using fieldprocedures that provide checks for blunders

Random Errors � Aا�B5 أ���ء

caused by imperfections of the measuring instruments,the surveyor to make an exact measurement, and thevariations in the environment

can be minimized by using better instruments andcan be minimized by using better instruments andproperly designed field procedures and by makingrepeated measurements

have small magnitudes

Positive and negative errors of the same magnitude occur with the same frequency. Cancel each other

Consider the mean value

Systematic Errors ���1 � أ���ء

behave according to a particular system or physical lawof nature, which may or may not be known

When the law of occurrence is known, systematic errorscan be calculated and eliminatedcan be calculated and eliminated

always occurs with the same sign and magnitude and istherefore often referred to as a constant error

Examples: assuming occupy point coordinates, earth curvature and temperature or pressure corrections

Correction doesn’t require field re-measurements

Example:

Given 12 measurements of a certain distance, as follows:

58.78، 58.83، 58.80، 58.85، 58.18، 58.77، 58.79، 58.80

58.81 & 58.82، 58.79 ،58.82

First iteration (n = 12)

= 58.75 m

= ± 0.18 m

Second iteration (n = 11):

xm = 58.81 m σx = ± 0.02 m max. error= ± 0.06 m

Precision and Accuracy

Precision and Accuracy

In general, to obtain high precision and high accuracy in surveying, the following strategies must be followed:

Follow techniques that will help detect andFollow techniques that will help detect andeliminate all the blunders.

Eliminate or correct all systematic errors byfrequent calibration and adjustment of theinstruments

Minimize the random errors by using goodinstruments and field procedures.

Part Two:

Control points Selection

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Control points

Control points may be:

Benchmark points (z)Benchmark points (z)

Horizontal coordinate points (x, y)

Combined (x, y, z)

Benchmark points \ (�?� [��@ ط���

The following conditions should be considered:

Its elevation should be accurately measured

Evenly distributed to enable surveyors to use themto determine elevations every where in the region

Well marked and fixed in the ground

Have precise description to locate them easily

Benchmark points \ (�?� [��@ ط���

Positioning before GPS

Surveys took on the form of networks of triangles. The vertices of the triangles often became horizontal control points. Initial positions were points. Initial positions were determined using astronomical observations.

Horizontal coordinates (latitude and longitudes) were determined separately form vertical coordinates (elevation)

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The following conditions should be considered:

Suitable location (away from multistory buildings,electric wires, traffic, changeable area, etc)

Enough distributed points, see at least two otherpoints (backsight and foresight)

Well marked and fixed in a firm ground

Have precise description to locate them easily, welldocumentation

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GPS points 9��3( 3( أس � ���ط @��] ا�&ا/ �ت أ*� Increasing the accuracy of GPS coordinate calculation:

Point location

The specification of the used receiver

Number of used receivers

Surveyor and GPS operator

� Before measuring

� During measuring

� After measuring

Property surveys to establish boundaries -Traverse

Opentraverse

Closedtraverse

Property surveys to establish boundaries -Traverse

The following conditions should be considered:

Minimum number of stations, as needed

lines should be as close as possible to the details to be surveyedlines should be as close as possible to the details to be surveyed

Distances between traverse stations should be approximatelyequal

Stations should be chosen on firm ground, or monumented in away to make sure that they are not easily lost or damaged

easy to see the backsight and foresight stations from onestation

Part Three:

Adjustment of Surveying Networks

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Leveling Networks

Ordinary levelingK=30 mm

Precise levelingK=5 mm

D= Distance in Kmmm

Coordinate Networks (Traversing)

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Coordinate Networks (Traversing)

Adjustment process includes:

Angle correction

Distance correction

1. Angles Adjustment

True sum of internal angles = (n – 2)×180˚

Determine the measured sum

Angle error = measured sum – true sumAngle error = measured sum – true sum

If angle error ≤ (60 – 90)˝ , accept the error, else, reject.

Correct each angle by adding to it

Compute the corrected azimuth for each traverse side using:

2. Position Adjustment

Starting from the known point, calculate thecoordinate of all points ending to the start point

Error in the easting direction(εx) = xmeasured – xtrue

Error in the northing direction(εy) = ymeasured – ytrue

Accept or reject errors according to specifications

Correct positions according to compass rule

Allowable errors in traverse surveying

Example:

Angle correction step

Position correction step

∆x= ∆y= yx

Position correction step

Thank You !!!!Thank You !!!!