62 THE UNITED STATES GEOLOGICAL SURVEY. [bull.227. 
at least two permanent bench marks may be established in each town- 
ship or equivalent area 6 miles square. Levels of a less degree of 
accuracy are then run over every road, and where these are too far 
apart, say a mile or more, the levels are run over trails or across 
country, either by spirit level or by vertical angulation with stadia. 
Meantime the topographer is extending a graphic triangulation on 
his plane-table board, the object of which is to locate precisely, on his 
sheet, points all over the area under survey. These include signals 
placed upon prominent hills, church spires, lone trees, and other defi- 
nite objects; or in case the country is of such nature that it can not be 
controlled b} T triangulation, he has careful lines of traverse, by stadia 
or steel tape, run at intervals of about 6 miles and connected with the 
lines of primary traverse previously run across the sheet. At the same 
time the traversemen, equipped with lighter plane-table boards on 
which are smaller sheets representing about one-fourth of the whole 
quadrangle, are running graphic traverse lines over all roads and paths 
and across country, with a view to locating the cultural features, the 
courses of streams, the outlines of lakes, etc. On highways the dis- 
tances are measured by counting the revolutions of a wheel, the cir- 
cumference of which is known, or by stadia, and in dense woods by 
tape. 
On the completion of this primary control and coincident with its 
preparation the topographer transfers to a clean sketch sheet the pri- 
mary control points and adjusts to these the positions determined by 
secondary plane-table triangulation or stadia or tape traverse. The 
less accurate traverse lines are then adjusted betw r een these more accu- 
rate positions and the elevations procured by spirit leveling or vertical 
triangulation are then added to the map. 
Equipped with this control sheet, on which are shown approximately 
one trigonometric location per square inch of paper, two or three linear 
inches of traverse per square inch of paper, and from two to ten 
elevations per square inch of paper, the topographer walks or rides 
over all roads and paths and about the margins of lakes, across country, 
etc., selecting routes so near one another that he may be able to see 
all portions of the land. If the country is rough he uses an aneroid, 
setting it at one of the fixed elevations and frequently checking it by 
others of like kind; with it he is able to determine the crossing of 
each contour line on his route of travel. In a relatively flat country 
he uses a hand level, or reads vertical angles from his plane table and 
ascertains like facts. Stopping frequently, he sketches in, by means 
of contour lines, the relief of the country, and corrects as he goes the 
positions of roads, houses, streams, etc., located by his assistants. 
For locations of topographic parties in 1903-4 see PI. IV. 
At the close of the field season the temporary employees are dis- 
charged and the party chief returns to the office. Here, during the 
