LEC15 Mapping (Contour)

Contours

  • A contour is a line connecting points of equal elevation.
  • Water assumes a level surface, so the shoreline of a lake is a visible contour.
  • Contours are generally not seen in nature.
  • On maps, contours represent the planimetric locations of the traces of level surfaces for different elevations.
  • Engineers and surveyors use contours to illustrate relief.
  • Relief represents the difference in altitude within a given area.

Contour Intervals

  • The vertical distance between consecutive level surfaces forming the contours on a map is called the contour interval.
  • Contour intervals depend on:
    • Diversity of relief or nature of the ground (flat or steep).
    • Scale of the map.
    • Purpose of surveying.
  • Reducing contour intervals requires expensive and precise fieldwork.

Supplementary Contours

  • Used in regions with both flat coastal areas and mountainous terrain.
  • Drawn at one half or one fourth of the basic contour interval.
  • Shown with dashed lines.

Spot Elevations

  • Used on maps to mark unique or critical points such as peaks, potholes, valleys, streams, and highway crossings.
  • May also be used in lieu of contours for defining elevations on relatively flat terrain that extends over a large area.

Topographic Map Convention

  • Drawing only those contours that are evenly divisible by the contour interval.
  • Elevations are shown in breaks in the contour lines.
  • At least every fifth contour is labeled using a heavier line.

Features of Contours

  • Contour lines must close on themselves, either on or off a map. They cannot dead end.
  • Contours are perpendicular to the direction of maximum slope.
  • The distance between contours indicates the steepness of a slope:
    • Wide separation denotes gentle slopes.
    • Close spacing denotes steep slopes.
    • Even and parallel spacing denotes uniform slope.
  • Irregular contours signify rough, rugged country. Smooth lines imply more uniformly rolling terrain.
  • Concentric closed contours that increase in elevation represent hills.
  • A contour forming a closed loop around lower ground is called a depression contour.
  • Contours of different elevations never meet except on a vertical surface such as a wall, cliff, or natural bridge.
  • They cross only in the rare case of a cave or overhanging shelf.
  • Knife-edge conditions are never found in natural formations.
  • A contour cannot branch into two contours of the same elevation.
  • Contour lines crossing a stream point upstream and form V’s.
  • Contour lines point down the ridge and form U’s when crossing a ridge crest.
  • Contour lines go in pairs up valleys and along the sides of ridge tops.
  • A single contour of a given elevation cannot exist between two equal height contours of higher or lower elevation. For example, an 820-ft contour cannot exist alone between two 810- or two 830-ft contours.
  • Cuts and fills for earth dams, levees, highways, railroads, canals, etc., produce straight or geometrically curved contour lines with uniform, or uniformly graduated spacing.
  • Contours cross sloping or crowned streets in typical V- or U-shaped lines.

Direct and Indirect Methods of Locating Contours

  • Contours can be established using either:
    • Direct method (trace-contour method).
    • Indirect method (controlling-point method).
  • The controlling-point method is generally more convenient and faster.
  • The controlling-point method is the most frequent choice when data is entered into a computer for automated contouring.

Indirect Method for Locating Contours

  • In the indirect method, the rod is set on “controlling points” that are critical to the proper definition of the topography.
  • They include high and low points on the terrain and locations where changes in ground slope occur.
  • Channels of drainage features and ridgelines must be included.
  • Elevations are determined using total station equipment and trigonometric leveling technique.
  • Horizontal distance and azimuth are also observed to locate the point.
  • The positions of controlling points are then obtained and plotted, and contours interpolated between elevations of adjacent points.

Steps of Indirect Method (Controlling Points)

  • Suitable for large areas and small-scale map:
    • Depending on the area, develop a traverse to cover up the area.
    • Make sure you can see the whole area from stations of traverse.
    • Use total station to find coordinate points.
    • Shot to every feature in the area including streams, ridge lines, valleys, top hills, depressions, and manmade permanent objects.
    • Do not forget to draw a sketch of area and assign codes to points.
    • Using calculated coordination, plot points and draw ridges/streams.
    • Again, interpolation is used to find the place of contour lines.