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

  1. The natural of the ground
  • Flat area = small contour interval
  1. The scale of the map
  • Larger scale map = large control interval
  1. The purpose and extent of the survey
  • Accurate calculation earthwork = small contour interval
  1. 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

  1. All points in a contour line have the same elevation.
  1. A uniform slope is indicated when the contour lines are uniformly spaced
  1. A plane surface when they are straight, parallel and equally spaced
  1. Flat ground & gentle slope is indicated where the contours are widely separated
  1. Steep slope (cerun curam) is where the contours lines run close together
  1. A series of closed contour lines on the map represent a hill, if the higher values are inside
  1. A series of closed contour lines on the map indicate a depression if the higher values are outside.
  1. If the higher values are inside the bend or loop in the contour, it indicates a ridge (rabung)
  1. If the higher values are outside the bend, it represents a valley
  1. Cannot merge or cross one another on map except in case of an overhanging cliff
  1. Never run into one another except in the case of vertical cliff.
  1. 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.