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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
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)
