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.