Understanding the Telescopic Alidade in Surveying

A telescopic alidade is an essential instrument used in plane tabling, a method of surveying. It is primarily used for sighting objects and drawing lines on the plane table sheet along the sighted direction. The term "telescopic" indicates that it is fitted with a telescope.

Let's look at the options provided regarding what a telescopic alidade provides:

• Accurate angle
• Inaccurate line of sight
• Inaccurate angle
• Accurate line of sight

Compared to a simple alidade, which uses simple sights (like pinnules or vanes) for sighting, a telescopic alidade is equipped with a telescope. The telescope provides magnification, allowing the surveyor to clearly view distant objects. This ability to view distant objects with clarity and focus is crucial for precisely aligning the alidade's edge with the target point.

The edge of the alidade is then used to draw a ray on the plane table sheet, representing the line from the station point (where the alidade is pivoted on the table) to the sighted object. The accuracy of this drawn ray directly depends on how accurately the alidade was aligned with the target.

Here's why the telescope enhances accuracy:

• Clearer Vision: Magnification from the telescope makes distant or small objects visible and distinct.
• Precise Alignment: Using crosshairs within the telescope allows for very precise alignment of the line of sight onto the target point, minimizing parallax errors that can occur with simple sights.
• Longer Sight Lines: Accurate sightings can be made over much longer distances compared to a simple alidade.

While the plane table itself is used for graphical triangulation and plotting angles directly on the sheet, the alidade's primary function is to establish an accurate line of sight from the station point to any other point of interest in the field. The telescope on a telescopic alidade significantly improves the accuracy of this line of sight compared to a simple alidade.

Therefore, a telescopic alidade provides an accurate line of sight, which is fundamental for accurate plotting in plane tabling.