106 
KIRTLEY F. MATHER 
Plane table orientation. Orientation of the plane-table sheet 
is ordinarily accomplished by means of the magnetic needle 
housed in the compass box attached to the base of the alidade. 
The fiducial edge of the alidade base is placed along a line ruled 
on the plane table sheet, and the needle liberated from its rest 
so that it swings freely in its box; the table is then rotated until 
the needle points to the north and south marks at either end 
of the compass box. Thereafter, whenever the same alidade is 
placed on the same side of the same line so that the same edge 
of its base coincides with the line, and by rotating the table the 
freely vibrating needle is made to come to rest at the same point, 
the sheet is in approximately parallel position. The orientation 
line ruled on the plane table sheet should extend the full length 
of the alidade base, or should consist of two shorter lines, three 
or four inches in length, drawn to each extremity of the base. 
An arrow should be sketched at the north end of the line or 
lines; the compass box is so attached to the alidade that when 
the needle comes to rest in the line of the marks at its ends the 
line of sight through the telescope leads approximately toward 
magnetic north. 
Although of course not necessary, it is very desirable and uni- 
versally customary so to draw the orientation line that the 
sides of the plane-table sheet coincide with the four cardinal 
directions. This may be accomplished in several ways, a few of 
•which will be mentioned here. In many parts of the country, 
the public roads have been accurately surveyed along section 
lines; commonly the fences have been similarly placed in a true 
north-south or east-west direction; or again, section corners 
may be so situated that one corner-post — or a flag placed directly 
above it — may be seen from another. If the plane table can be 
set up in the line thus determined by road or fence or land net, 
the ruler-edge of the alidade should be placed along the margin 
of the plane table sheet, or along the line previously drawn to 
represent the meridian of the map, and the table rotated until 
the line of sight through the telescope falls along the visible line 
thus defined. The table locked in that position, the alidade is 
moved to the desired part of the sheet and turned until the 
