# Page:EB1911 - Volume 17.djvu/679

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[MAP PROJECTIONS
MAP

Mercator's projection, although indispensable at sea, is of little value for land maps. For topographical sheets it is obviously unsuitable; and in cases in which it is required to show large areas on small scales on an orthomorphic projection, that form should be chosen which gives two standard parallels (Lambert's conical orthomorphic). Mercator's projection is often used in atlases for maps of the world. It is not a good projection to select for this purpose on account of the great exaggeration of scale near the poles. The misconceptions arising from this exaggeration of scale may, however, be corrected by the juxtaposition of a map of the world on an equal-area projection.

It is now necessary to revert to the general consideration of conical projections.

It has been shown that the scales of the projection (fig. 23) as compared with the sphere are p'q'/pq=dp/dz==¢r along a meridian, and p'r'/pr' =ph/sin z=a' at right angles to a meridian. Now if aa' = I the areas are correctly represented, then hpdp =sin zdz, and integrating %hp'=C -cos z; (i.) this gives the whole group of equal-area conical projections. As a special case let the pole be the centre of the projected parallels, then when

z=o, p =o, and const=I, we have p=2sin sz/6h (ii.) Let zi be the co-latitude of some parallel which is to be correctly represented, then 2h sin ézl/6h=sin zl, and h=cos' ézl; putting this value of h in equation (ii.) the radius of any parallel =p=2 sin sz sec § z1 (iii.)

This is Lambert's conical equal-area projection with one standard parallel, the pole being the centre of the parallels. If we put Z1 =0, then h= I, and the meridians are inclined at their true angles, also the scale at the pole becomes correct, and equation (iii.) becomes

p = 2 sin éz; V (iv)

this is the zenithal equal-area projection. Reverting to the general expression for equal-area conical projections p=/{2(C-cos z)/h} (i.)

we can dispose of C and h so that any two selected parallels shall be their true lengths; let their co-latitudes be zi and zz, then ~ 2h(C -cos zi) = sin' z, (v)

2h(C -cos zz) = sin' zz ' (vi.)

from which C and h are easily found, and the radii are obtained from (i.) above. This is H. C. Albers' conical equal-area projection with two standard parallels. The pole is not the centre of the parallels.

Projection by Rectangular Spheroidal Co-ordinates. If in the simple conical projection the selected parallel is the equator, this and the other parallels become parallel straight lines and the meridians are straight lines spaced at equatorial distances, cutting the parallels at right angles; the parallels are their true distances apart. This projection is the simple cylindrical. If now we imagine the touching cylinder turned through a right-angle in such a way as to touch the sphere along any meridian, a projection is obtained exactly similar to the last, except that in this case we represent, not parallels and meridians, but small circles parallel to the given meridian and great circles at right angles to it. It is clear that the projection is a special case of conical projection. The position of any point on the earth's surface is thus referred, on this projection, to a selected meridian as one axis, and any great circle at right angles to it as the other. Or, in other words, any point is hxed by the length of the perpendicular from it on to the fixed meridian and the distance of the foot of the perpendicular from some fixed point on the meridian, these spherical or spheroidal co-ordinates being plotted as plane rectangular co-ordinates. The perpendicular is really a plane section of the surface through the given point at right angles to the chosen meridian, and may be briefly called a great circle. Such a great circle clearly diverges from the paralle; the exact difference in latitude and longitude between the point and the foot of the perpendicular can be at once obtained by ordinary geodetic formulae, putting the azimuth=9o°. Approximately the difference of latitude in seconds is x' tan ¢> cosec I'/2pv where x is the length of the perpendicular, p that of the radius of curvature to the meridian, v that of the normal terminated by the minor axis, qi the latitude of the foot of the perpendicular. The difference of longitude in seconds is approximately x sec p cosec I'/v. The resulting error consists principally of an exaggeration of scale north and south and is approximately equal to sec x (expressing x in arc); it is practically independent of the extent in latitude.

It is on this projection that the 1/2,500 Ordnance maps and the 6-in. Ordnance maps of the United Kingdom are plotted, a meridian being chosen for a group of counties. It is also used for the 1-in., 5 in. and i in. Ordnance maps of England, the central meridian chosen being that which passes through a point in Delamere Forest in Cheshire. This projection should not as a rule be used for topographical maps, but is suitable for cad astral plans on account of the convenience of plotting the rectangular co-ordinates of the very numerous trigonometrical or traverse points required in the construction of such plans. As regards the errors involved, a range of about 150 miles each side of the central meridian will give a maximum error in scale in a north and south direction of about o-1 %.

Elliptical Equal-area Projection.

In this projection, which is also called Mollweide's projection the parallels are parallel straight lines and the meridians are ellipses, the central meridian being a straight line at right angles to the equator, which is equally divided. If the whole world is represented on the spherical assumption, the equator is twice the length of the central meridian. Each elliptical meridian has for one axis the central meridian, and for the other the intercepted portion of the equally divided equator. It follows that the meridians 9o° east and west of the central meridian form a circle. It is easy to show that to preserve the property of equal areas the distance of any parallel from the equator must be / 2 sin 5 where 'rr sin ¢>=25-4-sin 25, qi being the latitude of the parallel. The length of the central meridian from pole to pole=2 /2, where the radius of the sphere is unity. The length of the equator =4 /2.

The following equal-area projections may be used to exhibit the entire surface of the globe: Cylindrical equal area, Sinusoidal equal area and Elliptical equal area.

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 N

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FIG. 26.-Globular Projection.,

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 1o°); 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 