HOW MAPS ARE MADE. 431 



Of the methods of filling- up. wliich are several, I Mill briefly describe 

 two or three of the principal: 



Trdfcrslno irith the chain and throdolitc. — A traverse is detiued as a 

 ciicnitons route performed ou leaving any place on the earth's surface 

 by stages in different directions and of various lengths with a view of 

 arriving at any other place. The angles which the stages (or station 

 lines) form with the meridian (/. c, tlie north and south line) are called 

 bearings. In other words, it is a walking from point to point in 

 straight lines, always recording your distance and ycnir direction. 



These traverse lines are measured with the chain. They are gener- 

 ally laid out round the country to be surveyed, and are as multifarious 

 as the necessities of the ground require. The bearings in a good per- 

 manent survey are measured m ith the theodolite, and when the traverse 

 is complete it should be ch>sed where begun, when, if no error is made, 

 the bearing of the first line will read on the theodolite exactly as it 

 read in the beginning. Cross checks and connecting lines are con- 

 stantly taken to test the accuracy of the work, and while the survey is 

 going on the measurements of all the features of the country are set 

 down in what is called the field book. Where the line does not cross 

 the natural features perpendiculars, called offsets, are set off and meas- 

 ured from the traverse line to the bends and angles of all surface details, 

 bends of streams, fences, houses, roads, and so on, and so the map gets 

 filled in bit by bit, P^ither it is set off at the beginning from the ord- 

 nance triangulation or subsequently joined to it by trigonometrical 

 measurements. Such detail may be made piecemeal and fitted in like 

 a Chinese puzzle to the main map of the country and altered or more 

 minutely surveyed, according to requirements. For rapid and not very 

 accurate purposes exactly the same methods may be adopted as for a 

 military reconnoissance or sketch map by pacing the traverse lines and 

 taking the bearings with the prismatic compass, and this is what is gen- 

 erally done in military sketches. All these operations and measure- 

 ments are noted in a field book and are afterwards taken to the office 

 and "plotted" on a sheet or sheets of paper. 



There is also a contrivance for filling in a survey, with which no 

 field book is used, but by which very fairly accurate work may be 

 obtained. It is very little used in this country except for military pur- 

 poses, and then generally in a modified form shortly to be noticed; but 

 it is much used for topographical work in India and on the continent 

 and the United States. This instrument is the jj^rw^ table. It serves 

 itself as a theodolite, and the plan actually grows on the ground with- 

 out after office work. 



Contour Hues. — A very important part of cadastral survey is the 

 plotting on the map of contour lines, or lines of equal height. This is 

 done after the features of the surface have been mapped. To draw the 

 contour lines we must have a starting point, or, as it is called, a datum 

 level. In our Ordnance Survey this is the level of the mean tide at 

 Liverpool. 



