Contour Line

1. Contour  Line

Contour line is a false line drawn upon geographic space of the map, passing those points at the same height of geographic map. The contour lines are represented with brown color, and dark brown for an index contour. The properties of contour lines are as follows:

- Every contour displays the height values vertically.
- Every contour stretches horizontally and is in the same plane.
- Each contour presents geographical types and characteristics.
- Each contour is a closed line, that is, it creates a bounding circle. However, in a single section map, the circle line will not be completed unless each section of the map is connected to each other.
- Each contour may have different spacing, depending on geographical characteristics. In a steep sloped area, the contours will lie closer to each other than in a less sloped area.
- Normally, contours do not overlap one another, except in the cliff areas.
- Each contour usually turns its rugged side to the water source.
- Every position on the same contour has the same height.

As the Earth’s surface has different geographical features. The steep slope areas have a number of contours adjacent to each other, which causes confusion. Therefore, an index contour is set to make it easier to read. For the less steep areas, the contour lines are displayed with wider spacing, while some sink areas require a special type of contour to make it noticeable. As a result, there are several types of contours. Geographers have classified the characteristics and symbols of contours into five categories, so that map users can study the geographical features faster and more conveniently.

- Index contour is the main contour that indicates the height with divisible numbers such as 100, 200 and 300, etc.  The line is thicker and bigger than other contours, making it easily noticed. Index contour is normally marked with numbers.

- Supplemental contour is a contour line placed between regularly spaced contours to display additional height since that area has a wide contour spacing. The supplemental contour is represented with dashed line. In the 1:50,000 scale geographic map, the supplemental contours are placed in 10-meter spacing. Thus, each contour is marked with such numbers as 30, 50, 70, 90, 110, 130 and 150 to indicate the height.

- Approximate contour is a contour line that is approximately determined. It is created because map makers did not obtain data on the real height of certain areas. This is probably because an aerial image which is used for crafting the geographic map is clouded. As a result, the dashed line is used in that area to represent the estimated height in continuation of  index contour or regularly spaced contours.

- Depression contour is a contour line which is lower than the surrounding areas of other contour lines. The special feature of depression contour is a dash which is perpendicular to the contour, with its end pointing to the downslope.




2. Geographical features

The geographical features appearing on the map are represented by different types of contour as follows:

2.1  Mountain/Hill and Peak are areas where many contour circles overlapping one another.  The central area of the innermost contour is at the highest level, which represents the mountain. Most importantly, the top of the mountain is always marked with the height.
2.2  Ridge is a chain of mountains or hills that form a continuous elevated crest for some distance. Ridge can be found in a map by drawing a line along the tops of the mountains, which can be distinguished by the contour.





2.3 Spur is part of the mountain that is separated from the main ridge sloping downward the great valley. The contour in the spur area is a curved line extending to the main river. Both sides of the spur are braced with branch rivers.



2.4  Saddle is a notch or the low point in a ridge. The contour lines in that area are separated circles.

2.5 Valley is an area where the height of the contours gradually decrease to the lowest point which is a water course of a river or a stream. The contour is a sharp angular notch pointing up towards the water source.



2.6  Gorge is the contour lines in a valley. They are parallel and very close to each other.
2.7  Cliff is represented by the very-close-interval contour lines. The lines will overlap each other if it is a steep cliff.



2.8 Sink is represented by contour lines that form a circle or many circles.

2.9 Plain has a very distant contour spacing. In other words, the contour lines seldom pass in this area.
- Karst topography is a landscape resulting from the excavating effects of underground water on massive soluble limestone. In a humid subtropical area such as Thailand, a single mountain top normally appears alternately with the sinkhole. The contour lines in that area are in circular forms, scattering over the surface alternately with depression contour circles. There is usually no water course on the surface because the water soaks into the limestones, and merged  into underground waterway.
- There are several types of slope in the topographic map, such as uniform slope, concave slope and convex slope. From the 1:50,000 scale geographic map, which display the height of a landscape by using the 20-meter spaced contour lines, the three types of slope can be interpreted from contour spacing as follows:

  Uniform slope can be noticed in the area with equally spaced contours. If the contour lines are equally yet widely spaced, it indicates that the area is less steep. But if the contour lines are equally and closely spaced, it means that area has a very steep slope.


     Concave slope is a geographical feature with inconstant slopes. The upper part is steeper than the lower part. The concave slope is represented by the closely spaced contour in the upper locations and gradually become widely spaced in the lower areas.



    - Convex slope is a geographical feature with inconstant slopes. The upper part is less steep than the lower part. The convex slope is represented by the contour lines that have opposite features to the concave slope. In other words, the lower contours are closely spaced, and become more distant in the upper level.



3. Map Reading and Usage in

3.1 Map Reading and Usage in

The first step of map usage in geography is map orientation, which is extremely crucial to correct orientation for map reading and application in searching for accurate geographical positions. The map orientation is simply processed by putting a compass on the map grid, in which north-south direction is straight along the grid. When the arrow directs to the north, it indicates that the direction of map is also the north in geography. However, if more details are required, it is possible to adjust the directions by comparing grid north with magnetic north identified out of the map border because the north we actually use is the magnetic north while the north in the map is the grid north.

3.2 Resection
The resection is a positioning method that needs at least two prominent geographical positions. It can be processed by several methods such as resection with compass and protractor and graphical resection.

 - Resection with compass and protractor can be processed by selecting the target prominent geographical positions clearly appearing on the map and having enough space for creating angles. The observer is at the peak of angle; then the compass directs to both target positions and the magnetic azimuth record is done. After that the protractor is used to measure the value of back azimuth at the target positions, and the line is drawn from both angles to intersect each other. The intersect position is the position of observer.


– Graphical resection does not measure the angles, but the ruler or another device having similar functions is used to direct the position by opening the map in the correct direction, observing at least three prominent target geographical positions appearing on the map and supposing that A, B and C geographical positions appear at a, b and c positions on the map. Both types of positions should be less than 90 degrees from the position of observer.

3.3 Intersection

The intersection is a method of positioning a specific geographic positions on the map, which has at least two known target positions. The intersection is processed by using the compass and protractor as follows:

– Select obvious position of the observer, which appears in both geography and on themap, and it can be intersected when drawing a line from the first position. Point the compass to the target position and record the azimuth values; then go to the second position and follow the same steps.

– Open the map and build up the north line to touch the first position; then use the protractor to build up azimuth angle. Follow the same steps with the second position and draw a line from a side of azimuth angle from both positions to intersect at the target position.

- The graphical intersection is processed as follows:

1) Select a geographical position having the same details with the position on the map at

least two positions. Place the map down on the flat floor. Manage the map in the correct direction from position 1. Use the ruler pointing to the target position; then draw a line from the observing position to the target position.

2) From position 2, put the ruler at position 2; then point to the target positions. Draw a

straight line from this position directing to the target position, which intersects the line drawn from the first position at the target position.



ที่มา : สรรค์ใจ กลิ่นดาว (2531)

Admin 14/9/2015 6955 0
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