580 



SCIENCE 



[N. S. Vol. XLIX. No. 1277 



gress will respond to our appeals for funds in 

 order that the work may be carried on so 

 fapidly that all mapping operations of fed- 

 eral, state, city, coimty and private organiza- 

 tions, may have their needs met. This is a 

 very urgent matter and I shall do my utmost 

 to persuade the authorities to give this branch 

 of federal surveying ample support, in order 

 that the country may be mapped more satis- 

 factorily and more efficiently. 



When this control is available in any area, 

 the usual method is to have surveying parties 

 in the field place the topographic features on 

 the maps in their proper relation to the con- 

 trol points. Every object on the face of the 

 earth has one, and only one position, and it is 

 the duty of the surveyor to place that object, 

 whether it is a road crossing, a bridge, the top 

 of a hill, or any other object, in its proper 

 position on the map. On the most exact map 

 for military purposes a well-defined feature is 

 placed on the map within thirty feet of its 

 exact relation to the nearest control station. 

 Other maps have larger allowable discrep^ 

 ancles. 



The work involved in the topographic sur- 

 veying consists not only in placing the 

 features on the map in their correct horizontal 

 positions, but also in showing by contours the 

 lines of equal elevation, the slopes of the 

 groimd, the shapes of hills and the exact ele- 

 vations of a number of critical points. 



The elevations are based upon lines of levels 

 run inward from the oceans. The surface of 

 the ocean, if it were at rest, would be a con- 

 tinuous one, and thus the mean position of the 

 surface serves as a datum plane from which 

 to measure heights in the interior of the 

 country. More than 40,000 miles of the high- 

 est grade leveling has already been established 

 in the interior of the cotmtry, and there are 

 more than 20,000 precise leveling bench marks 

 whose elevations are known within a very 

 small portion of a foot. 



In addition to the above there are many 

 thousands of miles of leveling of a lower grade 

 of accuracy which is used for the immediate 

 control of the topographic surveying. 



It is the duty of the Coast and Geodetic 



Survey to extend the lines of precise leveling 

 into the interior of the country for the pur- 

 pose of furnishing starting points for the 

 leveling needed for the immediate use of the 

 surveyor and engineer. What has been said 

 in regard to the fundamental horizontal con- 

 trol is also applicable to the precise leveling. 

 Many more thousands of miles of this grade 

 of leveling are needed in the United States 

 to-day and it is hoped that the bureau may be 

 given the support necessary to complete within 

 the next few years the work which is now 

 needed and should have already been done. 



TOPOGRAPHIC SURVEYING WITH PLANE TABLE 



The topographic surveying is done generally 

 by means of the plane table which consists of 

 a tripod with certain fixtures and a plane board 

 mounted thereon. The board is approxi- 

 mately 24 X 30 inches in horizontal dimen- 

 sions. On this board is placed a sheet of 

 paper on which the topographical features 

 are shovm. On the paper there will have been 

 placed before going to the field, the positions 

 of the control jroints, and with these as start- 

 ing points, the topographer weaves a net show- 

 ing the various features of the earth's surface 

 by means of symbols. These symbols have 

 been standardized by the map users of the 

 United States. Any one wishing to utilize the 

 information given by one of the high-grade 

 maps, should be thoroughly familiar with these 

 symbols. 



As far as present development of airplane 

 photography is concerned, it seems absolutely 

 necessary in making the contoured survey, to 

 do the work vsdth the present methods. One 

 can readily imderstand that it would be dm- 

 possible to show contours at intervals of twenty 

 feet over a wooded area, where trees in difier- 

 ent parts of the forest varied in height. The 

 area photographed will not show the differences 

 of elevations of trees in a wood, for the low 

 trees and bushes not more than twenty feet in 

 height, would show about the same on a photo- 

 graph as a primitive forest where the trees 

 may be seventy to one hundred feet high. 



