98 HYDROGRAPHICAL SURVEYING [ctiap. tv. 



great circle drawn between the two positions makes with the 

 meridian of the observing position. As meridians are not 

 parallel, but converge at the poles, the great circle will cut 

 each meridian it passes at a different angle, the amount of 

 difference, for equal meridians, depending on the latitude. 



To further the comprehension of this, let us consider tlie 



method of projection of the sphere used when graduating a 



map, made from the original data of angles and measurements. 



Projec- It will be evident to anyone who considers the subject that, 



tiig as our globe is a sphere (speaking roughly), a portion of its 



Sphere, surface cannot be shown on a flat piece of paper without 



distortion, more or less, according to the extent so shown. 



There are a variety of methods used to deUneate a portion 

 of the earth's surface on a map, which are called " projec- 

 tions." Into this variety it is not proposed h^re to enter, as 

 but one can be used when actually making a survey, which 

 is the " Gnomonic Projection." 

 Gnomonic This projection is the only one on which great circles of the 



Projec- earth are shown as straight lines. As it is on the chord of a 

 tion. . ^ . 



great circle that we see one object from another, it is evident 



that in graduating a map on which we have laid down, or are 



going to lay down, one position from another by drawing 



straight lines, we ynust use this projection. 



A chart on the Gnomonic Projection is supposed to be drawn 

 on a flat surface laid against the earth, touching it at the 

 central point of the flat surface, and there only. From the 

 centre of the earth lines are supposed to be drawn, passing 

 through the different points to be shown on the map, until 

 they pierce the flat surface. 



The positions so indicated on the upper side of the flat 

 surface are those corresponding to the points required. 



Here, in Fig. 15, P Q S is the globe, and A B C D a flat 

 surface laid against it, touching at the point J, the centre of 

 the flat surface, the under side of which is shown. P is the 

 pole. M, F, are points taken on the same meridian as J. 

 Imaginary lines drawn from the centre of the earth through 

 these points will touch the flat surface in N and G, and the 

 Hne joining them, the central meridian of the chart, will be a 

 straight one. K, another point on the globe east of the central 

 meridian, ^^^ll be projected at L, by the same method of 



