CONTOURS Engr. Randy G. Policarpio

Definitions: Contour – an imaginary line of constant elevation on the ground surface. Contour line – a line on the map representing a contour.

CONTOURS

 TYPES OF CONTOURS 1. Index Contours 2. Intermediate Contours 3. Supplemental Contours 4. Depression Contours 5. Approximate Contours Intermediate Contour Lines

CONTOURS 1.INDEX CONTOURS  Index contour is shown by a heavier line at regular intervals on a topographic map. These heavier lines which are normally twice the gauge of the standard contours.  They are usually drawn every fifth contour and carry the contour number or the elevation designation.  The index contour is broken along portions of the line and the contour number indicated accordingly.  They are numbered on the upper side of each line, which indicates at first sight uphill and downhill directions, placing the figures in a row above one another.

CONTOURS 2.INTERMEDIATE CONTOURS  The four lighter weight contours found between the index contours.

Intermediate Contours  These lines are not usually labelled except where the terrain is relatively flat and elevations are not readily obvious

Intermediate Contours  Intermediate contours are so closely spaced as to nearly unite or merge into a single line, it is standard practice for readability not to portray the lines for short distances. This technique is called feathering (see Fig. 16.2)

CONTOURS 3. SUPPLEMENTAL CONTOURS  A supplementary contour is used when the elevation change is minimal, like in flat floodplains.  They are used to represent a smaller change in elevation than the intermediate contour.

Supplemental Contours  Supplementary contours are drawn as dashed lines or lines of dots.  These lines are drawn at all one elevation, but they differ from the previous two types of lines in that their spacing or change in elevation that they represent is different. They almost always represent half the elevation change that is found between intermediate and index contour lines.

CONTOURS 4.DEPRESSION CONTOUR  Depression contour are drawn to show low spots such as excavations around which contour close.  The symbol used is the index or intermediate contour to which ticks are drawn perpendicular to the contour line on the downhill side.

DEPRESSION CONTOUR

CONTOURS 5.APPROXIMATE CONTOURS  In some instances contour accuracy cannot be definitely determined. The area maybe inaccessible on the ground or it may be difficult to interpret contours from aerial photographs when heavy cloud or shadows occur. To portray a reasonable idea of relative elevation, the map maker has to make an educated guess rather than have a blank area in the map.

CONTOURS THE CONTOUR INTERVAL  The contour interval is the constant vertical distance between two adjacent contour lines.

CONTOURS  In the design of topographic maps, the contour interval is commonly fixed at multiples of 0.5, 1, 2, 5, 10, 20, 50, and 100 meters  For general types of terrain, the contour interval used maybe as follows:  Flat to gently rolling: 1 to 5 meters  Hilly: 5 to 20 meters  Mountainous: 25 to 100 meters  According to Map Scale SCALEINTERVALSCALEINTERVAL 1/ m1/25, /2,00011/50, /5,00021/100, /10,0005 or 101/250,00050

CONTOURS SELECTION OF CONTOUR INTERVAL The following factors to be considered: 1.RELATIVE COST  The time and expense of field and office work is given important consideration.  The smaller the interval, the greater is the amount of field work, reduction, and plotting required in the preparation of the map.  The cost of the map will be higher as the contour interval is reduced. 2.PURPOSE OF THE MAP  Close contouring will be required when the map is to be utilized for the detailed design of engineering construction or for the measurement of earthwork quantities.  A wider interval will be required for surveys of reservoirs, drainage areas, and lines of communication. 3.NATURE OF THE TERRAIN  Rugged terrain will require a larger interval than gentle and rolling country.  To portray adequately flat ground a relatively small interval must be specified. 4.SCALE OF THE MAP  The contour interval should be in inverse ratio of the scale of the map.  If the map is reduced, the interval must be increased; otherwise line are crowded, confuse the map user, and may possibly obscure some important map details.

CONTOURS CHARACTERISTICS OF CONTOURS 1.All points an any one contour have the same elevation.

CHARACTERISTICS OF CONTOURS 2.Every contour closes on itself, either within or beyond the limits of the map. 3.A contour which closes within the limits of the map indicates either a summit or a depression. Contours which increase in elevation represent hills; those which decrease in elevation portray valleys or excavations.

CHARACTERISTICS OF CONTOURS 4.Contours on the ground cannot cross one another except where an overhanging cliff, a vertical ledge or wall is represented on the map. Wrong

CHARACTERISTICS OF CONTOURS 5.Contours are spaced evenly on a uniform slope. 6. Contours are straight and parallel to each other on a plane surface.

CHARACTERISTICS OF CONTOURS 7.Irregular contours signify rough, rugged terrain.

CHARACTERISTICS OF CONTOURS 8.The horizontal distance between adjacent contours indicates the steepness of the slope of the ground. Where the contours are relatively close together, the slope is comparatively steep; where the contours are far apart, the slope is gentle.

CHARACTERISTICS OF CONTOURS 9.Contours cross curbs and crowned sloping street in typical U- shaped curves.

CHARACTERISTICS OF CONTOURS 10.Contour lines form a V pattern when crossing streams. The apex of the V always points upstream (uphill).