The vertical interval between contours depends principally upon the scale of the map although it is also conditioned by the type of country. Thus the same vertical interval could hardly be applied to Switzerland and to Flanders. A good idea as to the proper interval in feet for average country will be obtained by dividing so by the scale in inches to the mile. Thus a map on the scale of 2 inch to one mile would have contours at zoo feet intervals.
Field Sheets.—As explained in the article on plane tabling the board is covered with paper, canvas backed, or mounted previously on a thin sheet of aluminium. Some form of celluloid is also used on occasion. This covering is called the Field Sheet. On it are plotted the control and the boundary lines of the individual task, and on it the plane tabler subsequently maps. A plane tabler should be "within his points." The area of his work should then be substantially less than that of the board itself in order to allow of plotting control points in the margins, which are also wanted for notes and lists of names. On the other hand, a plane tabler starts a new field sheet slowly and gathers pace as he gets to know his control and his country. A good mean figure for the area of work on a board of 18X 24 in. is from 6o to 8o sq. inches.
The plane tabler's area should be bounded by definite, ruled in lines. It sometimes happens that a river or other topo graphical obstacle makes it imperative for neighbouring topogra phers on either side to work to this natural and therefore irregu lar line, but the accuracy of subsequent adjustment suffers. The
dividing line should be one across which roads, rivers and other details pass naturally. An overlap between well trained plane tablers is neither necessary nor advisable, nor is it necessary for them to meet at any stage of their work. The correct conven tional signs and colours will have been laid down in advance and each plane tabler will ink in his work as and when he considers it final. No colour which is not photographically opaque should be used. It is not generally advisable to map at scales larger than the final map. The plane tabler judges more accurately the amount of generalisation necessary if his field sheet is plotted at the map scale and as names are generally written in the margin, there is ample space available in which to show all the required detail.
On conclusion of the survey, the field sheet will be available for checking. Every party should have some proportion of its strength earmarked, for revision. After, or during, revision the edges common to the finished field sheets should be compared. Differences on the edges should be small, but occasional errors in orientation ("swing") may arise if there has been difficulty in seeing the control points. Differences should be traced back wards to the nearest resections and for this reason resected points and their heights should be left in pencil on the field sheet until comparisons are finished.