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Topographic Survey:

– Determine position of natural and man made features

– Features drawn to scale on plan or map

– Determine ground elevations (contours, cross-sections and profiles)

– Vast majority done by aerial survey

– EDM and total station (x-y zontal location) and vertical location (elevation) by one sighting

– Rectangular and polar surveying techniques

– Rectangular technique: – Right angle off sets for location detail

– Cross section for elevation and profiles

– Polar technique use stadia or electronic techniques

Topographic survey

 

Scales and Precision:

Scale: Ratio between plan distance and ground distance

Consistent through the plan

Equivalences e.g. 1” = 50’

Fractions e.g. 1 : 500

Table 8.1

Small scale and intermediate scale done by aerial survey

– Reason for survey determine appropriate precise technique

– If points to be plotted on at scale 1:500 –> precision 0.25 m

– If points to be plotted on at scale 1:20,000 –> precision 10 m

– Some details can be precisely determined –>e.g. bldg corner

– Some details cannot precisely determined –>e.g. stream banks

– Some details can be determined with moderate precision ® e.g. single large tree

Details that can be well defined is located with more precision then is required just for plotting because:

– It take little effort

– Uniform practice

– Some details are shown as layout dimensions

– If area contain only natural feature, stadia is used

– All topographic surveys are tied into both horizontal and vertical control (Benchmark)

– Horizontal control could be:

· – Closed transverse

– Transverse from coordinate grid monuments

– Close to another coordinate grid monuments

– Route centerline

– Assumed baseline

– Measurement taken to establish control are more precise than other measurements

– Control should be accurate and well references

– Control should be used for additional work (e.g. layout)

Location by Right Angle Offset

– Used in all topographic survey except mapping

– Provide location of details and area elevation taken by X-section

– Measure distance to base line and station on baseline

– Baseline laid by stakes (nails) each 100’ or 20/30 in,

– Sketch in note book

– Tape can be laid on baseline if terrain is smooth

– Details on both sides of baseline or make split baselines

– Penta prism or (swing-arm technique (Appx.)

– SAT good result for short of test 15 m otherwise use penta prism or transit

Cross Section and Profile

– Cross section to he baseline

– Profile along the baseline

– Elevation plotted as spot elevation, contours or end area for construction quantity estimation

– Intervals 20/30 in. – in changing terrain 10-15 m + any sudden change in terrain (top, bottom of slops)

topographic-mapping

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