Short Answer:

An alidade is a simple surveying instrument used in plane table surveying to sight objects and draw straight lines (rays) on a drawing sheet. It helps in measuring directions from the plane table to ground features. Alidades come in two main types: plain alidade and telescopic alidade.

To use an alidade, it is placed on the drawing sheet with one edge touching the point where the observer is standing. Then, the surveyor sights the object through the alidade and draws a line along its edge. This line shows the direction of the object on the map.

Detailed Explanation:

Alidade and its use

An alidade is an essential instrument in plane table surveying used for sighting distant objects and drawing accurate lines of sight on the map. It plays a key role in determining the direction of various points in the field. This instrument ensures that the drawn lines (called rays) correctly represent the direction from the survey station to the observed ground feature.

Types of alidade:

1. Plain Alidade
   • Made of wood or metal.
   • Has two vertical sight vanes at both ends.
   • One vane has a narrow slit and the other a thin wire or vertical opening.
   • Commonly used for general surveys.
2. Telescopic Alidade
   • Has a small telescope mounted on a base.
   • Provides higher accuracy for longer distances.
   • Includes crosshairs inside for precise targeting.
   • Used in engineering or detailed mapping surveys.

How an alidade is used:

1. Fix the plane table:
   Set up the plane table at the survey station. Center and level it properly using a plumb bob and spirit level.
2. Position the alidade:
   Place the alidade on the drawing sheet with one of its straight edges passing through the plotted point representing the instrument station.
3. Sight the object:
   Rotate the alidade until the object in the field (like a tree, pole, or corner) is visible through the slit and wire (in a plain alidade) or through the telescope (in a telescopic alidade).
4. Draw the ray:
   Once the object is aligned, draw a straight line (ray) along the edge of the alidade. This line represents the direction from the station to the object on the drawing.
5. Plot the distance:
   If the distance to the object is measured using tape or chain, mark it along the ray to scale. This gives the exact position of the object on the map.
6. Repeat for other objects:
   Continue the process for all other visible objects from the same station. This method is especially used in the radiation method of plane table surveying.

Importance of the alidade:

• Ensures accurate direction marking on maps.
• Helps in plotting features without taking angular readings.
• Easy to use and does not require complex setup.
• Allows immediate correction in case of alignment mistakes.
• A must-have tool for every plane table surveyor.

Conclusion

An alidade is a key instrument in plane table surveying used for sighting and drawing directional lines to ground objects. It is placed on the map, aligned with a feature, and helps the surveyor draw rays that represent directions. With accurate use, the alidade helps produce correct and clear survey maps directly in the field.