Contour maps: preparation and understanding.

The theme behind preparation of contour maps, various projections of topographical features, the processor making them and methods used in their making.

1. “CONTOUR MAPS: AN INTRODUCTION” Prepared & Presented By: Vivek Chauhan (16091) Presented To: Dr. Deepa Rawat COLLEGE OF FORESTRY (VCSG UUHF,RANICHAURI) CFT-397:APPLICATION OF GIS IN FOREST RESOURCE MANAGEMENT

2. Maps A  map  is a symbolic depiction emphasizing relationships between elements of some space, The word "map" comes from the medieval Latin  Mappa mundi , wherein  mappa  meant napkin or cloth and  mundi  the world. Thus, "map" became the shortened term referring to a two-dimensional representation of the surface of the world. such as objects, regions, or themes.

3. Types of maps: Cadastral map Climatic map Geologic map Nautical map Physical map Political map Relief map Resource map Star map Street map Thematic map Topographic map Weather map World map Cultural map

4. Contour Maps: A contour map is a kind of topographical map that depicts the elevation of an area using the contour lines. What is a Topographic Map? The  International Cartographic Association  defines  Topographic Maps as  the following: "Topographic maps are maps at large and medium scales which incorporate a huge variety of information. The basic coverage (a country’s largest map series) is based on measurements made in the field and/or from aerial photographs. Derived topographical maps (of medium and small scales ) are prepared by reduction and generalisation from the original basic maps.

5. Things to know about Contour Maps: The rule of the Contour V’s Streams and rivers flow opposite to the point of the V that is formed when a contour line crosses them. Close contour lines means that there is a steep slope. Contour lines that are far apart mean that the land is flatter. Gradient means slope. Bulls eyes represent hills or mountains. Hachured lines on bulls eyes mean depressions or valleys . Each line represents a different elevation in feet or meters . Places on the same contour line are at the same elevation.

6. Characteristics of contours: The variation of vertical distance between any two contour lines is assumed to be uniform. Contours of different elevations cannot cross each other (caves and overhanging cliffs are the exceptions). Contour lines cannot begin or end on the plan. A contour line must close itself but need not be necessarily within the limits of the map. A closed contour line on a map represents either depression or hill . Contours do not have sharp turnings .

7. Types of Contour Lines: 1. Index lines  are the thickest contour lines and are  usually   labeled with a number at one point along the line. This tells you the elevation above sea level. 2. Intermediate lines  are the thinner, more common, lines between the index lines. They usually don't have a number label. Typically one index line occurs for every five intermediate lines. 3. Supplementary lines  appear as dotted lines, indicating flatter terrain.

8. Various Projections Using Contours

9. METHODS OF CONTOURING: There are mainly two methods of locating contours:- Direct Method and Indirect Method.

10. Direct method: In this method, the contours to be located are directly traced out in the field by locating and marking a number of points on each contour. These points are then surveyed and plotted on plan and the contours drawn through them.

11. Direct Method: (Radial Line Method) This method is suitable for small areas, where a single point in the centre can command the whole area. Radial lines are laid out from the common centre by theodolite or compass and their positions are fixed up by horizontal angles and bearings.

12. Contd. Temporary bench marks are first established at the centre and near the ends of the radial lines . The contour points are then located and marked on these lines and their positions are determined by measuring their distances along the radial lines. They are then plotted on the plan and the contours drawn by joining all the corresponding points .

13. Indirect Method: In this method the points located and surveyed are not necessarily on the contour lines but the spot levels are taken along the series of lines laid out over the area . Their positions are then plotted on the plan and the contours drawn by interpolation. This method of contouring is also known as contouring by spot levels . There are three types of indirect method: Square Method Cross-section Method Tacheometric Method

14. Square Method: In this method, the whole area is divided into number of squares, the side of which may vary from 5m to 30m depending upon the nature of the ground and the contour interval. The square need not be of the same size throughout. The important points within the squares may be taken when required and located by measurements from the corners. The squares are plotted and the reduced levels of the corners are written on the plan.

15. Cross-section Method: This method is most suitable for the surveys of long narrow strips such as a road, railway or canal etc. Cross sections are run transverse to the centre line of the work and representative points are marked along the lines of cross-section. The cross-section lines need not necessarily be at right angles to the centre line of the work. This may be inclined at any angle to the centre line if necessary. The spacing of the cross-sections depends upon the topography of the country and the nature of the survey.

16. Tacheometric Method: This method is most suitable in hilly areas as the number of stations which can be commanded by a techeometer is far more than those by a level and thus the number of instrument settings are considerably reduced. A number of radial lines are laid out at a known angular interval and representative points are marked by pegs along these radial lines. Their elevations and distances are then calculated and plotted on the plan and the contour lines are then interpolated.

17. Interpolation of the Contours: The process of spacing the contours proportionally between the plotted ground – points is termed as interpolation of contours . This becomes necessary in the case of indirect contouring as only the spot levels are taken in this method. While interpolation of contours the ground between any two points is assumed to be uniformly sloping.

18. Reference: https://theconstructor.org/surveying/contouring-methods-maps-uses/6451 / http:// www.gitta.info/TopoCart/en/html/NotInt_learningObject1.html https:// www.greenbelly.co/pages/contour-lines https:// www.slideshare.net/gauravhtandon1/contouring-26905323 https:// www.slideshare.net/gokulsaud/contouring-pdf https://en.wikipedia.org/wiki/Map

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