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Plane table surveying is a graphical method of surveying where field observations and plotting are done simultaneously on a drawing board called a plane table. The plane table provides a stable, level surface on which a drawing sheet is fixed and maps or plans are directly sketched in the field. The surveyor uses an alidade to sight objects and draw lines on the sheet that correspond to directions of features on the ground. This method allows for real-time creation of scaled drawings by aligning the alidade with ground features and sketching their relative positions accurately.

Key steps in plane table surveying include:

• Centering the plane table exactly over a ground point using a plumb bob
• Leveling the table horizontally with a spirit level
• Orienting the table so directions on the sheet match directions on the ground, often using a compass or by back-sighting known points
• Fixing the drawing sheet securely on the table
• Taking sightings through the alidade and drawing rays/lines on the sheet corresponding to directions of visible objects or points

The method is especially suitable for small-scale surveys, industrial areas where compass surveys may fail, and for filling in detail between stations fixed by triangulation or traversing methods. There are several methods within plane table surveying including radiation, intersection, traversing, and resection, used for locating points or the plane table itself on the drawing.

This technique is advantageous because it avoids errors from omitting details or misinterpretation by allowing simultaneous observation and plotting. However, accuracy depends on proper setup and careful handling of instruments.

Instruments commonly used in plane table surveying include the plane table (drawing board on tripod), alidade (sighting and drawing rule), spirit level, compass, plumb bob or plumbing fork for centering, and drawing sheets or tracing paper.

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Levelling in Surveying

Types of Maps and Topographical Maps

Topographical maps are specialized maps that provide detailed representations of both natural and man-made features of the terrain. ​

• Topographical maps show relief using contour lines, which connect points of equal height. ​
• Contours are spaced at regular intervals, typically 10 meters on a 1:50,000 scale map. ​
• The shape and spacing of contours indicate the slope and elevation of the terrain. ​

Interpretation of Contours and Relief

Understanding contour lines is essential for interpreting the shape and elevation of the ground. ​

• Contours indicate the shape of the land; close contours signify steep slopes, while distant contours indicate gentle slopes. ​
• Contours are continuous, except in cases of cliffs, and can form convex or concave slopes based on their spacing. ​
• The innermost closed loop of contours represents the highest elevation. ​

Grid References and Navigation

Grid references are crucial for accurately locating points on a topographical map. ​

• A grid consists of Eastings (vertical lines) and Northings (horizontal lines) that help pinpoint locations. ​
• Eastings are read first, followed by Northings, and the origin is at the southwest corner of the map. ​
• Four-figure and six-figure grid references provide varying levels of location accuracy. ​

Representation of Relief on Topographical Maps

Topographical maps utilize various methods to represent the relief of the Earth’s surface. ​

• Relief can be depicted through contour lines, spot heights, triangulation points, and hachuring. ​
• Hachuring indicates slope direction but does not provide height information. ​
• Hill shading adds shadows to create a three-dimensional effect, enhancing the visual representation of terrain. ​

Drainage Patterns in Topographical Maps

Drainage patterns illustrate how water flows across the landscape and are categorized into distinct types. ​

• Dendritic patterns resemble tree branches and form in areas with uniform rock resistance. ​
• Trellis patterns develop in regions with alternating geology, where streams cut along weak rock zones. ​
• Radial patterns occur around high elevations, with water flowing outward from a central peak. ​

Significance of Colors in Topographical Maps

Colors on topographical maps convey specific information about various features of the terrain. ​

• Black represents names, roads, and surveyed trees; blue indicates water bodies; green shows wooded areas. ​
• Yellow denotes cultivated land, while white patches indicate uncultivable land. ​
• Brown is used for contour lines, and red marks grid lines and settlements. ​

Understanding Settlements and Occupations

Topographical maps provide insights into human settlements and their associated occupations. ​

• Settlements are represented by red symbols, with size indicating the population or significance of the area. ​
• Dense settlements are typically found in fertile plains, while sparse settlements occur in less hospitable regions. ​
• Occupations such as agriculture, mining, and tourism are inferred from the map features and land use. ​

Approximate or Relative Height Representation

Relative height is used to describe elevations in relation to surrounding areas rather than sea level. ​

• Heights can refer to structures like dams or natural features like hills. ​
• Examples include a relative height of 3 meters for a well or 5 meters for a dry tank. ​
• The context of the height is crucial for accurate interpretation.

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Map Scales of Topographic Maps

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