PLANE TABLE SURVEYING. 
107 
plane table will be accurately oriented for the true north and south. 
This should be tested by drawing rays from the other fixed points, and 
it will very probably be found that they do not exactly meet at the 
point indicating the position of the plane table. It may be possible, by 
twisting the plane table a little to the right or left, that all the rays 
may be made to fall on the same point, in which case this point will be 
the position of the plane table on the map; but should this not be the 
case, then recourse must be had to the method shown pp. 100 to 103. If 
care has been taken with the projection, it is not at all likely that 
anything will be wrong with that, and therefore too great care cannot be 
taken in plotting the fixed points on the map. 
Having the plane table thus fixed and oriented in the true meridian, 
place the compass on the sheet and move it until the needle points to 
magnetic north while the plane table is in this position; this will enable 
the surveyor to approximately orient his table in the true meridian 
should it be set up in a position where he is not able to orient it by 
points previously fixed. It must, however, be borne in mind that there 
are countries, such as portions of South Africa, where the local 
deviation is so variable and so great that this method cannot be 
depended on. 
In many countries which the explorer may visit there are no fixed 
points, in which case it will be necessary for him to determine by 
astronomical observation the latitude and longitude of each end of a 
base, and from these fix the positions of a certain number of prominent 
points by triangulation. This being done, he must proceed to fix other 
points by moving his table to different stations, orienting his table, and 
drawing rays to them; the intersections of the rays drawn from any two 
stations to the same point will fix the position of that point provided 
the angle of intersection is well chosen, i.e. a neither too obtuse nor too 
acute. 
Broken Survey .—The directions given above comprise briefly the 
fundamental rules of more accurate plane-tabling. 
A map, however, may be, and often must be, constructed without the 
continuous connection of fixed points from sheet to sheet, as is above 
suggested, and which, in the rough work of an ordinary journey, is 
frequently impossible. 
The traveller may often find that the station from which he wishes 
