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CONFORMALITY - 14071_196
LAMBERT CONFORMAL CONIC PROJECTION - 14071_198

Engineering Aid 2 - Intermediate Structural engineering guide book
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As for the transverse Mercator, the conic, and the gnomonic projections, a glance at the appearance of meridians  and  parallels  on  any  one  of  these  indicates not only that direction is different in different parts of the map, but that the direction of North (for example) in one part of the map may be precisely opposite to that of north in another. Let’s call the two types of conformality  we  have  mentioned  directional  con- formality  and  distance  conformality.    Some authorities hold that directional conformality is all that is  required  for  a  conformal  projection.  A  Mercator projection  has  this  type  of  conformality,  and  this  fact makes that type of projection highly advantageous for navigational  charts.  A  navigator  is  primarily interested  in  determining  geographical  location  of  his ship;  and  the  principal  disadvantage  of  Mercator projection—the  north-south  compared  to  east-west distance  distortion  (which  increases  with  latitude)–is negligible  in  navigational  practice.  This  statement applies  only  to  navigation  in  customary  latitudes, however,   since   Mercator   projection   of   the   polar regions   (above   about   80-degrees   latitude)   is impossible. For surveying and other purposes in which dis- tance  measurements  must  be  consistent  in  every  direc- tion,  Mercator  projection  presents  disadvantages.  To understand these, you have only to reflect on the fact that no distance scale could be consistently applied to all parts of a Mercator projection, which means that no square grid system could be superimposed on a Mercator    projection;   however,   the   transverse Mercator projection, as it is used in conjunction with the  UTM  military  grid,  provides  relatively  small-area maps that are virtually conformal, both direction-wise and  distance-wise. POLYCONIC  PROJECTION In polyconic   projection   a  near  approach  to direction  conformality  is  obtained  in  relatively  small- area maps by projecting the area in question onto more than  one  cone.  A  central  meridian  on  the  map  is straight; all the others are slightly curved and not quite parallel. Similarly, the parallels are slightly curved and not quite parallel; therefore, they are not precisely perpendicular  to  the  meridians.  An  example  of  a polyconic map projection is shown in figure 9-24. Polyconic  projection  is  extensively  used  for  the quadrangle maps (familiarly called  quad sheets) of areas of the United States published by the Geological Survey. For most of the built-up areas of the States, these  maps  are  available  on  a  scale  of  1:24,000, Figure 9-24.—Polyconic projection of North America. showing  areas  extending  for  7°30’  of  latitude  and longitude. An  index map  is available, which gives you the quadrangle divisions and the name of the map that covers a particular area. That   polyconic   projection   is   not   conformal distance-wise is indicated by the fact that one of these quad sheets, though it shows an area that is square on the ground, is oblong rather than square. The vertical or latitudinal length of the map is always greater than the horizontal or longitudinal length. The reason is that latitude is measured along a meridian, which is always  a  great  circle,  while  longitude  is  measured along a parallel; and every parallel other than the equator is less than a great circle. An understanding of the concept of the great circle is essential to a thorough understanding of map and 9-21







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