The instrument used in plane table surveying for the measurements of horizontal and vertical distances directly is the plane alidade.

Plane table surveying:
Plane table surveying is a graphical method of surveying in which the field work and plotting are done simultaneously. The instrument used in plane table surveying is a board called a plane table. The plane table is fixed at the point of survey and leveled carefully. The board is oriented with respect to the true north using a magnetic compass.

Plane alidade:
A plane alidade is an instrument that is used to measure horizontal and vertical distances directly. It is attached to the plane table and is used to sight the object under survey. The plane alidade consists of a telescope and a horizontal and vertical scale. The horizontal scale is used to measure the horizontal distance while the vertical scale is used to measure the vertical distance.

Uses of plane alidade:
The plane alidade is used in plane table surveying for the following purposes:

1. Measuring horizontal distances directly.
2. Measuring vertical distances directly.
3. Measuring angles between two objects.
4. Setting out right angles by measuring from a base line.

Advantages of plane table surveying:
The advantages of plane table surveying are:

1. It is a rapid method of surveying.
2. The accuracy of the survey is high.
3. The survey results can be plotted directly on the board.
4. It is suitable for small areas.

Disadvantages of plane table surveying:
The disadvantages of plane table surveying are:

1. It is not suitable for large areas.
2. It requires skilled personnel to operate the instrument.
3. The instrument is bulky and heavy.
4. It is affected by atmospheric conditions such as wind, rain, and fog.