TOPOGRAPHIC SURVEYING AND MAPPING



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