[Illustration: FIG. 26.--Globular Projection.] _Conventional or Arbitrary Projections._ These projections are devised for simplicity of drawing and not for any special properties. The most useful projection of this class is the _globular projection_. This is a conventional representation of a hemisphere in which the equator and central meridian are two equal straight lines at right angles, their intersection being the centre of the circular boundary. The meridians divide the equator into equal parts and are arcs of circles passing through points so determined and the poles. The parallels are arcs of circles which divide the central and extreme meridians into equal parts. Thus in fig. 26 NS = EW and each is divided into equal parts (in this case each division is 10°); the circumference NESW is also divided into 10° spaces and circular arcs are drawn through the corresponding points. This is a simple and effective projection and one well suited for conveying ideas of the general shape and position of the chief land masses; it is better for this purpose than the stereographic, which is commonly employed in atlases. [Illustration: (From _Text Book of Topographical Surveying_, by permission of the Controller of H.M. Stationery Office.) FIG. 27.--Plane Table Graticule, dimensions in inches, for a scale of 4 in. to 1 m.] _Projections for Field Sheets._ Field sheets for topographical surveys should be on conical projections with rectified meridians; these projections for small areas and ordinary topographical scales--not less than 1/500,000--are sensibly errorless. But to save labour it is customary to employ for this purpose either form of polyconic projection, in which the errors for such scales are also negligible. In some surveys, to avoid the difficulty of plotting the flat arcs required for the parallels, the arcs are replaced by polygons, each side being the length of the portion of the arc it replaces. This method is especially suitable for scales of 1:125,000 and larger, but it is also sometimes used for smaller scales. Fig. 27 shows the method of plotting the projection for a field sheet. Such a projection is usually called a graticule. In this case ABC is the central meridian; the true meridian lengths of 30´ spaces are marked on this meridian, and to each of these, such as AB, the figure (in this case representing a square half degree), such as ABED, is applied. Thus the point D is the intersection of a circle of radius AD with a circle of radius BD, these lengths being taken from geodetic tables. The method has no merit except that of convenience. _Summary._ The following projections have been briefly described:-- / 1. Cylindrical equal-area. | 2. Orthographic. | 3. Stereographic (which is orthomorphic). Perspective < 4. General external perspective. | 5. Minimum error " (Clarke's). \ 6. Central.