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%Surveying for nonsurveyors}National Reference points whose Coordinates are known are used to determine coordinate value of points ( or stations) on site~Z~XNational Reference points connected to site stations by angular and distance measurementYY(
_The Coordinates of the site stations are determined from the angular and distance measurements ``(
3The survey measurement can never be without errors 44(So we do calculations to distribute the errors evenly around all our measurements
In the hope that this will mean smaller errors on any particular set of coordinates.KAngles and distances are measured with an instrument called a Total StationLL(This may be thought of as a particularly expensive protractor!
The angles can be measured to the 5th decimal place of accuracy.
The angles in this country are measured in Degrees, Minutes and seconds.
Distances are measured electromagnetically (i.e. by light reflection)*b
Consider the angular measurement!!(
:To reduce errors angles are measured twice at each station;;(These are known as Face 1 and Face 2 readings the average value is used
( = the mean included angle)
The readings are simply the values pointed to on the protractor and we need to subtract one reading from another to determine the angle.
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8The survey usually closes on itself for greater accuracy99(xHence we can check that the sum of the internal angles are correct
And distribute around the survey any errors present.
Correcting Angular ErrorsInternal angles of polygon = (2N4)90
Dist. Coeff. = closing error / N
Corrected angle = Measured angle Dist. Coeff.
Where closing error = Theorectical sum actual sum of internal angles
N = Number of sides or angles in polygon, `) AN.Distance are often also measured several times//(XAgain the average value is used to help determine the coordinates of the site stations.
TrigonometryA recap of your knowledge of trigonometry may be useful here.
Refer to Trig. File
This knowledge will be used to determine the North and East vectors for the distance between stations
These are known are North and East Partials.Partials
Whole Circle BearingsThe angle used in the Partial calculations is the angle between the line from station to station and the North or East axis.
A more general solutions is to use the Whole Circle Bearing (WCB) of the line.
A WCB is the angle a line makes with the North Measured in clockwise direction.
Only the WCB of the first line is determined on site. The rest are calculated from the known relationship between adjacent lines.
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Partials from WCB value
CoordinatesvThe coordinates of stations are then calculated from adjacent stations, starting with the first known reference point.Site Measurement errorsOf course the coordinates we end up with have errors and if we do a closed survey ( returning to the starting point) we will notice that the coordinates for the end point ( which is in fact the starting point) do not match the starting point values the closing error.
We do a correction called the Bowditch correction to compensate for this.6[T"8&/$Closing errors
Bowditch CorrectionThis suggests that the greater the length of a measured line the greater the error in measurement is present.
The amount of correction to a line therefore depends on it s length. The longer the line the more correction is needed.Formula in Bowditch CorrectionThe closing error in the North and East directions are found first by summing all the partials.
Correction to Partials:
= (Line length / line lengths) x closing error in east or north direction
i.e. a fraction x closing error
the partials are corrected in accordance with this premise.
The corrected coordinates are then computed.
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ArialDefault DesignSurveying for nonsurveyorsYNational Reference points connected to site stations by angular and distance measurement`The Coordinates of the site stations are determined from the angular and distance measurements 4The survey measurement can never be without errors LAngles and distances are measured with an instrument called a Total Station!Consider the angular measurement;To reduce errors angles are measured twice at each stationExample$Do not be fooled by these readings!9The survey usually closes on itself for greater accuracyCorrecting Angular Errors/Distance are often also measured several times
Trigonometry PartialsWhole Circle BearingsPartials from WCB valueCoordinatesSite Measurement errorsClosing errorsBowditch CorrectionFormula in Bowditch CorrectionFonts UsedDesign Template
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