To determine distance between two inaccessible points using Prismatic Compass.
Chain, Tape, Ranging Rods, Arrows, Prismatic Compass, Tripod.
It is a compass where the graduated ring is attached with the needle and does not rotate with line of sight. As the name implies a prism is provided at the Eye Vane end so that the readings on graduated ring are read through the prism. Graduations are engraved inverted since the graduated ring is read through the prism.
The minimum angle between two adjacent graduations is 300.
The advantage of this compass is that both sighting and reading can be done simultaneously.
Temporary Adjustments of Compass:
A tripod is placed over the station with its legs spread well apart so that it is at a workable height. The compass is fixed on the tripod. It is then centered over the station where the reading is to be taken. A plumb bob is hung from the centre of compass. In case the arrangement for a plumb bob is not provided, a stone is dropped from below the compass and it should fall on the peg marking the ground station.
The compass is levelled by eye judgment. This is essential so that the graduated ring swings freely.
Focusing the Prism:
The prism is moved up or down till the figures and graduations are seen clearly.
When two points are too far away, unreachable and the chaining between them is difficult, the distance between these two points is called Inaccessible Distance. But the two points are visible to each other.
Taking a Reading with Prismatic Compass:
The compass is rotated until the point or object and the cross hair at object vane coincide. Now the reading on the graduated ring is taken by observing through the prism which is provided just below the eye vane. The reading that coincides with the cross hair should be taken. The break pin which is provided below the object vane should be pressed down while taking reading to avoid oscillations of graduated ring.
Measuring Angle between Two lines:
Let ABC be a traverse of which the angle at B to be measured (Fig 1). The compass is set up at point B and then the point A is sighted and the reading on graduated ring is noted down. Now the instrument is rotated towards point C and the reading on graduated ring is noted down. The difference of those two reading gives the angle at B which is an angle between line BA and line BC.
Let A and B be the two inaccessible points whose distance to be measured (Fig 2). A point C is established at a reasonable distance from A. Let a, b, c be the distances of sides CB, AC, AB respectively out of which c is the inaccessible length.
Now distance b is measured as it is accessible to point A. The angles QA and Qc are measured with a compass as described before.
The angle QB can be calculated from,
QB = 180 – (QA + Qc)
The inaccessible length c can be calculated from Sine Rule.
a / Sin QA = b / Sin QB = c / Sin Qc
b / Sin QB = c / Sin Qc
Therefore, c = (b X Sin Qc) / (Sin QB)
Note: All calculations should be shown here.
The distance between the two inaccessible points A and B,
c = m