Published on January 11, 2008
MEASURE BEARINGS USING PRISMATIC COMPASS: PO/EO - 401.01 INST - REF’S - B-GL-306-008/FP-001 B-GL-318-008/FP-001 TIME - MEASURE BEARINGS USING PRISMATIC COMPASS EO 401.01 TP A-G MTP’S: MTP’S A. Purpose and description B. Compass card C. Index glass D. Prism E. Definitions F. Accuracy G. Reading; and Purpose: Purpose The prismatic compass c-5 is a pocket-sized instrument used for measuring magnetic bearings. Description: Description When closed the compass forms a circular brass disc. A compass point scale, used for night marching, is engraved around the outer edge of the compass. Description cont.: Description cont. When the compass is closed, north direction can be obtained by viewing, through the window, the luminous white triangle marked on the compass card. This triangle homes on magnetic north. To open, press up on prism guard. The compass is opened for reading through the prism. The compass card: The compass card Made of synthetic translucent material. Has two scales, marked in mils, and the cardinal points of the compass. Outer mil scale is inverted and can be viewed through the prism. It is divided into 320 equal parts, each division representing 20 mils. Every tenth division is accentuated and marked with a figure. The compass card cont.: The compass card cont. Inner scale is read against the day lubber. It is divided into 64 equal parts, each division representing 100 mils. Every fourth division is accentuated and marked with a figure Questions?: Questions? The index glass: The index glass Held in a milled slipping ring known as the index ring. May be turned to any desired bearing and secured in position by a clamp. Line graduated near the outer edge every 200 mils with a dot indicating each 100 mils The index glass cont.: The index glass cont. A setting vane, coincident with the 6400 mil mark on the index glass, is engraved on the milled edge of the index ring. May be read day or night, against the lubber line scribed on the luminous patch of the compass inner body The index glass cont.: The index glass cont. A luminous source, forming part of the night lubber line, is fixed on the under side of the index glass coincident with the 6400 mil mark. Questions?: Questions? The prism: The prism The prism assembly is hinged. When the compass is opened, the prism must be swung into the reading position over the index glass. Prism is also fitted to slides which allow it to be raised slightly for focusing. The prism cont.: The prism cont. Outer scale of the compass card is seen through the viewing aperture of the prism. Within the compass, directly below the prism and beneath the compass card, is a luminous source against which the compass card can be read at night. Atop viewing aperture is a sighting slit for aligning the sighting line. Questions?: Questions? Definitions: Definitions Bearing: a bearing is the angle measured clockwise from a fixed line (north) to any other line (reference object). Back bearing: the bearing from an object to the observer. Definitions cont.: Definitions cont. Accuracy: bearings to an accuracy of 20 mils may be read directly from the outer mil scale. Questions?: Questions? Reading the compass: Reading the compass May be read in the open posn. Using the prism or it may be kept closed and read directly through the window. In the closed posn, the day lubber line is read against the inner scale on the compass card. Reading the compass cont.: Reading the compass cont. At night, turn index ring until setting vane indicates the bearing of the object on the external compass point scale, or the corresponding index glass graduation is under the lubber line on the luminous patch. Clamp the index glass at this bearing. Reading the compass cont.: Reading the compass cont. The axis of the compass will be on the required bearing when the luminous north point on the compass card coincides with the luminous night lubber line on the index glass. Questions?: Questions? Q: what is the accuracy of the prismatic compass? A: 20 mils Q: why is the x100 symbol on the compass card? A: a reminder that all readings must be multiplied by 100. Questions?: Questions? Q: what is the purpose of the clamp on the index glass? A: to set the compass at a desired bearing.