CHAPTER 8 : Contouring
Introduction
- Imaginary lines that connect points of equal elevation/height/altitude
- Obtain by joining points of equal elevation on the map
- Found in contour maps and topographic maps
Types of line
Index contour
Contour line
Contour interval
Purpose of Contour survey
- every 5th contour line is printed darker or thicker
- elevation printed on it
- every 50m, 100m elevation interval
- four intermediate contour lines fall between the index contour
- no printed elevation on it
- equal spacing
- 10m, 20m interval
- for preparing contour maps
- to locate the alignment of a canal, road, railways, dam, reservoir, drainage, etc
- to determine cutting and filling in earthwork
- to locate physical features of ground (hill, depression)
Factors in determining contour intervals
- The natural of the ground
- Flat area = small contour interval
- The scale of the map
- Larger scale map = large control interval
- The purpose and extent of the survey
- Accurate calculation earthwork = small contour interval
- Time and expense of field and office work
- Less time = large contour interval
Common values of the contour interval
- building sites and earthworks : 0.5m-1m
- airport and airbase : 0.25m
- for reservoirs and town planning schemes : 1m-2m
- topographical maps : 5m-10m
- small-scale topographical maps : 10m-50m
Characteristics
- All points in a contour line have the same elevation.
- A uniform slope is indicated when the contour lines are uniformly spaced
- A plane surface when they are straight, parallel and equally spaced
- Flat ground & gentle slope is indicated where the contours are widely separated
- Steep slope (cerun curam) is where the contours lines run close together
- A series of closed contour lines on the map represent a hill, if the higher values are inside
- A series of closed contour lines on the map indicate a depression if the higher values are outside.
- If the higher values are inside the bend or loop in the contour, it indicates a ridge (rabung)
- If the higher values are outside the bend, it represents a valley
- Cannot merge or cross one another on map except in case of an overhanging cliff
- Never run into one another except in the case of vertical cliff.
- Depressions between summits is called a saddle. It is represented by four sets of contours as shown. It represents a dip in a ridge.
Method of locating contours
Direct method
- only method to produce true contours
- involves finding points on the ground with the value of the required contour
- By establishing a series of such points, the contour lines
are effectively marked out on the ground
- It is then possible to plot them by conventional
detail surveying.