TELESCOPIC ALIDADES

possible, then, to obtain relative bearings of objects by merely training the vanes on an object, then reading the graduation on the inner circle alongside the lubber’s line on the pelorus or repeater. Each of the far vanes contains a spirit level to indicate when the circle is level. Bearings taken when the azimuth or bearing circle is not on an even keel are inaccurate. TELESCOPIC ALIDADES Another means of taking bearings is by using an alidade, which, like the bearing circle, is mounted on a repeater. The telescopic alidade (fig. 9-3) is merely a bearing circle with a small telescope attached to it. The image is magnified, making distant objects appear larger to the observer. A series of prisms inside the low-power telescope enables the bearing-taker to read the bearing directly from the compass card without removing the eye from the eyepiece. Bearings and azimuths may be true, per gyrocompass (PGC), magnetic, or per steering compass (PSTCO). When you are helping the navigator in piloting, you probably will report bearings directly from the gyro repeater, and the navigator will correct them to true. Figure 9-3.—Telescopic alidade. CHARTS AND PUBLICATIONS LEARNING OBJECTIVES: Explain the use of navigational charts and publications. Explain chart scales, chart sounding marks, and how to make chart corrections. A map represents pictorially all or part of Earth's surface. Maps specially designed for navigators are called charts. Navigational charts show water depths and the nature of the bottom, together with a topography of adjacent land. A chart is a printed reproduction of a portion of Earth's surface depicting a plan view of the land and water. A chart uses standard symbols, figures, and abbreviations that supply data on water depth, characteristics of the bottom and shore, location of navigational aids, and other information useful in navigation. Figures indicating water depth are placed throughout the water area to indicate the shape of the bottom. Normally the density of sounding on a chart increases as you approach land. A chart is normally lined with a network of parallels of latitude and meridians of longitude, which aid in locating various features. LOCATING POSITIONS ON CHARTS Earth is approximately an oblate (flattened at the poles) spheroid. However, for most navigational purposes, Earth is assumed to be a sphere, with the North Pole and South Pole located at opposite ends of the axis on which it rotates. To establish a feature's location geographically, it is necessary to use two reference lines, one running in a north-south direction and the other in a east-west direction. Numerical designators are applied to these reference lines. The numerical system used is circular and consists of 360°, with 60 minutes or 3,600 seconds in a degree. Lines running in the north-south direction, called meridians, start at one pole and end at the opposite pole. (See fig. 9-4.) Lines running east-west are parallel lines and are called parallels. Meridians The prime (0°) meridian, which is the reference line for all meridians, passes through the Royal Observatory located at Greenwich, England. Earth is divided into Eastern and Western Hemispheres. All meridians are numbered between 0° and 180° east and west of the prime meridian. In addition to the number value, each line is identified by the letter E or W, denoting the proper hemisphere. 9-3