02 c horizdist w
DESCRIPTION
SurveyTRANSCRIPT
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Horizontal Distance Methods ApproachesPacingOptical rangefindersOdometersTacheometry (stadia or subtense)TapingElectronic Distance Measurement (EDM)Global Positioning System (GPS)IssuesOften measure surface or slope distancePotential error or need to reduce to horizontal
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Incorrect Length of TapeConcept
Solution
Correction Example
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Variations in Tape TemperatureConcept
For steel coefficient of thermal expansion (k)CorrectionIssues
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Incorrect TensionConcept
For steelCorrection
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SagConcept
Correction
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Tape Not HorizontalConceptOccurs when tape is not kept levelSolutionCorrected using Pythagorean TheoremOr by approximate correction
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How Far Off Can You Be?ProblemNo matter how hard you try, never have a horizontal tapeQuestionWhen does departure from horizontal cause significant error in result? In fieldTry to keep tape horizontal within this limit Or compute a correction
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Incorrect Alignment/Tape Not StraightConcept
Correction
Incorrect alignmentTape not straight
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Taping Correction ProblemBackgroundSteel tape calibrated at 68F (supported throughout) at 5 kg tension is found 30.006m between 0 m and 30 m marksTape has cross-sectional area of 0.003 in2 and weighs 0.7 kgProblemCrew is taping by laying tape along ground down a uniform grade from point A to point BPoint A is 190.45m AMSL and point B is 175.68m AMSLTemperature is 88F and tension is 7 kgFinal recorded distance is 148.47mWhat is corrected length of tape?
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TacheometryConcept
MethodsStadia
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Tacheometry: StadiaL1
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Stadia PrinciplesNeed simple method for establishing
Method
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Stadia Readings
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Stadia PrinciplesA,B rod interceptsa, b stadia hairsS = rod intercept F = principal focus of objective lensCdDicfC = stadia constantK = f/i = stadia interval factord = distance from focal point to rodD = distance from instrument center to rodbaa'b'FBASf = focal lengthi = stadia hair spacing c = distance from instrument center to objective lens center
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Stadia EquationsHorizontal sightsInclined sightsFrom similar triangles
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Issues with Stadia MeasurementsStadia common for map data collection
Accuracy of stadia
Current situation
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TacheometryConceptDetermine distances indirectly using triangle geometryMethodsStadiaEstablish constant angle and measure length of opposite sideLength increases with distance from angle vertex
Subtense
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Tacheometry: Subtense
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Subtense PrinciplesNeed simple method for establishing
Issues
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Subtense EquationDerive equation for computing distance by subtense LWhat value would you choose for L?
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Issues with Subtense MeasurementPrinciples still useful in many situationsAccuracy of subtense
Current situation
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ReadingsChapter 6 sections 6.6, 6.7, 6.14 6.16Chapter 16 section 16.9.2