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ContoursCE 221 ‐ Higher SurveyingCE 221 Higher Surveying

Types of contoursTypes of contours

• Index contoursIndex contours• Intermediate contoursS l l• Supplemental contours

• Depression contours• Approximate contours

3Prepared by: Andre‐Paul C. Ampong

Index contoursIndex contours

• Shown as heavy lines usually twice the gaugeShown as heavy lines, usually twice the gauge of the standard contours

• Normally shown every fifth (5th) contour• Normally shown every fifth (5th) contour• Carry the number or elevation designation


Index contoursIndex contours


Intermediate contoursIntermediate contours

• Lines of lighter weight between index contoursLines of lighter weight between index contours• Usually not labeled, except when the terrain I relatively flatrelatively flat

• Conform to the contour interval of the mapI t f th h i t di t t• In parts of the map where intermediate contours are closely spaced as to nearly unite or merge into a single line it is customary not to portrayinto a single line, it is customary not to portray the lines for short distances for readability (the technique is called feathering)technique is called feathering)


Supplemental contoursSupplemental contours

• Used when the terrain is so flat and theUsed when the terrain is so flat and the intermediate contours are too far apart

• Drawn as dashed lines or lines of dots• Drawn as dashed lines or lines of dots• Usually drawn at one‐half the specified

i l f hcontour interval of the map


Depression contoursDepression contours

• Used to show low spots around whichUsed to show low spots around which contours close

• Index or intermediate contours are used to• Index or intermediate contours are used to which ticks drawn perpendicular to the contour lines on the downhill sidecontour lines on the downhill side


Depression contoursDepression contours


Approximate contoursApproximate contours

• Used when contours cannot be drawnUsed when contours cannot be drawn accurately possibly due inaccessibility of the areaarea

• Based on the educated guess of the maker of the mapthe map


The contour intervalThe contour interval

• The constant vertical distance between two adjacent contour lines

• Small intervals are appropriate for areas with light slopes and with small features such as hillslight slopes and with small features such as hills

• Large intervals are used for areas with steep slopesslopes

• Commonly fixed at multiples of 0.5, 1, 2, 5, 10, 20, 50 and 100 meters

• Interrelated with the map scale; generally, the smaller the scale the larger the contour interval


Selection of contour intervalSelection of contour interval

• Relative costsRelative costs– The cost of the map will be higher as the contour interval is reducedinterval is reduced

• Purpose of the mapN t f th t i• Nature of the terrain

• Scale of the map


Characteristics of contoursCharacteristics of contours



• All points on any oneAll points on any one contour have the same elevation



• Every contour closes onEvery contour closes on itself, either within or beyond the limits of the map. The closure may occur within the

d b t ftmapped area, but often happens outside the area and thus will notarea and thus will not appear on the map sheet.



• A contour which closesA contour which closes within the limits of the map indicates either a summit or depression



• Contours on the groundContours on the ground cannot cross one another except where an overhanging cliff, a vertical ledge or wall is

t d threpresented on the map.



• Contours are evenlyContours are evenly spaced on a uniform slope



• Contours are straightContours are straight and parallel to each other on a plane surface.



• Irregular contoursIrregular contours signify rough, rugged terrain



• The horizontal distanceThe horizontal distance between adjacent contours indicates the steepness of the slope of the ground; the l th t thcloser the contours, the steeper the slope



• Contours cross curbsContours cross curbs and a crowned sloping street in typical U‐shaped curves



• Contours cross streamsContours cross streams in a V pattern pointing upward


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