Study the properties of Contour Map
Contour lines, or isohypses, connect points of equal elevation.
• Consider a receding water level.
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Index Contours are used to label the elevation of the their associated points. • Bolded to facilitate reading of the map. • Often occur in intervals of five contour lines.
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Contour Interval: Horizontal distance between two contours • Close together: Steep slope • Far apart: Gradual slope.
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• Valleys: Lines form a “V” pattern along valleys • “Vs” point upstream; indicating the direction of the stream flow. • Streams always flow downhill • Contours can and do cross streams.
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Concurrency: Contour lines never cross or divide. • Cliffs: May appear to merge on vertical cliffs, but are stacked. • Caves: May appear to cross in caves, but go under one another.
• Hills and knobs are shown as closed, concentric contours. • Closed depressions and basins are shown as closed contours with hatches pointing downslope.
Other geologic features commonly expressed by contour maps include: • Drumlins • Sink Holes • Sand Dunes • Volcanos • Dissected Plateaus • Rolling Hills • Folds
Contour Map Generation:-
In the field, points and their elevations are measured using automatic levels, total stations, or GPS devices • Geotechnical engineers use these points to generate topographic maps.
Objective:
To plot the contour map for a given land by direct method.
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Eqipments:
Dumpy Level, Levelling Staff, Tripod, Staff bubble, Chain or Tape.
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Procedure:
1. First, ensure that an appropriate bench mark (BM) is available near the site of the survey. If a B.M is not available, then one should be located near the site by fly leveling.
2. Once a benchmark is available, set up the instrument (level) at a suitable position covering a large part of the area to be surveyed.
3. The area is divided into a number of squares and all grid points are marked (Ref. Fig. 1). Commonly used size of square varies from 5 m × 5 m to 20 m × 20 m.
4. Levels of all grid points are established by leveling.
5. Then grid square is plotted on the drawing sheet. Reduced levels of grid points marked and contour lines are drawn by interpolation [Ref. Fig. 1].
