PUBLlt) LAND SUKVEYS. 105 



ments have extended westward, while Wisconsin, Illinois, and Iowa present 

 conditions almost as favorable. 



The public-land surveys are corrected either by extending over 

 them belts of triangulation or by primary traverses. When the former is 

 employed, it is unnecessary to cover the area with triangulation. It is 

 sufficient to restrict it to belts of simple figures, such as triangles or quadri- 

 laterals, such belts being 75 to 100 miles apart. 



Each triangulation station should be connected by the simplest and 

 most direct method with the nearest section corner of the land surveys. This 

 is done generally by measuring the direction and chaining the distance, 

 although it ma}' be necessary to run a short traverse, or even a bit of minor 

 triangulation, in order to reach the section corner. In this way connection 

 is made with the land surveys at intervals of 10 or 15 miles along the belt of 

 the triangulation. These locations" are of course supplemented by any other 

 accurate locations which may have been made in the region under survey. 



When primary traverses are employed for control, connection should 

 be made with all section and township lines crossed, the distance along the 

 line to the nearest corner should be measured, and the direction of the line 

 relative to the courses of the traverse should be measured. 



In open country, where the public-land surveys are of good quality, 

 as above described, the work of the topographic parties is reduced to the 

 measurement of heights, and sketching. All the roads are matters of public 

 record and are obtained from the county officers. The same is true of the 

 plats of all towns and the plans and profiles of all railroads. These are 

 obtained and placed upon outline plats of the townships, upon a scale double 

 that of which the maps are to be published. 



Heights are measured with the vertical circle and by aneroid, except in 

 Illinois, where, the contour interval being 10 feet, the vertical circle only is 

 used. 



Where both are used, the vertical angle lines are run at intervals of 4 

 or 5 miles in one direction, while roads at intervals of a mile are run in the 

 other direction with aneroids, checking them upon the crossings of the 

 vertical angle lines. Sketching goes on coincidently with the measurement 

 of heights. 



