432 HOW MAPS ARE MADE. 



From the datuui great lines and cross lines of levels are rnu all over 

 tlio CDuntry, covering" it with a network; and at all convenient spots 

 the heights are permanently recorded by the well known broad arrow, 

 and called bench marks. Wherever the broad arrow is fonnd engraved 

 on the gronnd its height from the datnm line will be fonnd in tlie ord- 

 nance map of that part of the gronnd. A very common spot to find an 

 ordnance bench mark is the keystone of the arch of a bridge, which 

 wonld naturally be the last thing to be removed. These levels are got 

 by spirit levelling. Wiien the main levelling operations have been 

 completed the surveyor fixes at what intervals of height his contours 

 are to be drawn. 



The surveyor starts, let us say, to determine the line at 100 feet 

 above datum. He goes to the nearest bench mark he has to this height, 

 say it is 105 feet. He levels down until i;e finds a point 5 feet below 

 this bench mark. There he leaves a flag or a peg and goes on finding 

 point after point at the same level; that is, he must read the same 

 figure on the leveling staff. These j)olnts he then surveys as he would 

 any natural feature, and permanently marks the imaginary lines join- 

 ing them on the map, thereby showing a line of equal heights. 



Military sketch or reconnoissance is a form of map which ought not to 

 pass entirely undescribed. The object of a staft officer in making a 

 sketch is to give such a representation of the nature of the ground as 

 will give useful information to his general. It may take any amount of 

 elaboration, may be as complete as a cadastral survey taken with 

 instruments of precision, or it may be merely the roughest indication 

 of the nature of the ground, taken with such instruments as may be 

 carried in the pocket, or even imi)r()vised without instruments, and be 

 a mere eye sketch of the features of the ground. A s the military infor- 

 nmtion generally desired is the nature of the ground, whether suitable 

 for maneuvering, for artillery, for cavalry, the nature of the roads, of 

 the hills, of the rivers, should all be looked to, and rough (contouring 

 and hill shading is a very important part of the officer's work. He 

 must also get information of defensible spots, of the water supplies, 

 the food supplies, and the resources of the country, and this should be 

 modified as much as possible on the i^lanor on the report attached to it. 



Though any degree of elaborateness may be used, any instruments of 

 l)recision employed, the typical military sketch is nuide with a sketch- 

 ing case, which is merely an improvised plane table. The nmin lines 

 or traverses are taken from the bearings of the j^rismatic compass laid 

 down on the sketch itself. The lines are generally paced or guessed, 

 distant objects fixed by bearings from the station i)oints, and the con- 

 touring measured angularly by Abney\s level, or sketch by the eye. 

 The shading of the hills shows steepness by the lines used to indicate 

 them being drawn closer or farther apart. 



Cart(>(/r(q)]tey. — The plans and maps having been drawn, and all 

 notes made of information, tliey reach the cartographer or atlas-maker. 

 His duty is first to compare all new iiit'or unit ion. with Mliat is (ilready 



