464 



SCIENGJEJ. 



[Vol. VIII., No. 198 



work. This work of location serves only to cor- 

 rect the map ; it forms no part of the map itself. 

 The matter of the map is obtained by the second 

 part of the topographer's work, that of sketching. 

 This is artistic work. Here we come to a defini- 

 tion of a map from the constructive point of view. 

 It is a sketch, corrected by locations. The more 

 locations per square inch of the map, other things 

 being equal, the better the map ; but however 

 numerous the locations may be, the map itself is 

 none the less a sketch. 



Locations are effected in two ways, as noted 

 above. First, by angular measurement, starting 

 from a carefully measured base. These measure- 

 ments may be made with the theodolite or plane- 

 table. Second, by direction and distance measure- 

 ments ; the former by compass, theodolite, or 

 plane-table, the latter by chain, steel tape, odome- 

 ter, or stadia. These two methods are frequently 

 used in combination, as in the ordinary work of 

 the U. S. coast and geodetic survey, where the 

 plane-table stations and many unoccupied points 

 are located by angular work, while other points 

 are fixed in position by direction and distance, 

 from the stations, using the plane-table and stadia 

 for this purpose. 



Among topographers of wide experience in the 

 use of instruments and methods of work, there is 

 no question, where the conditions are favorable, 

 as to the advisability of using the method of loca- 

 tion by angulation, rather than that by direction 

 and distance measurements. It is without doubt 

 the most rapid and the most accurate method of 

 cuntroUing the sketches for a topographic map. 

 Its employment requires, however, that the coun- 

 try shall present some relief, that it contain an 

 adequate number of points, natural or artificial, 

 suitable for being 'cut in,' and that it be not too 

 generally covered with forests, in order that a 

 sufficient number of cleared summits, well dis- 

 tributed for stations, may be obtained. The pri- 

 mary advantage in the use of the plane-table with 

 this method, as with all other methods, is, that 

 the work is plotted directly upon the station, and 

 the sketch is then made upon a correct frame- 

 work, — in other words, the map is made upon 

 the station, with the country in view. The prin- 

 cipal disadvantage, if it be a disadvantage, is that 

 the angles, being recorded graphically, cannot be 

 used subsequently for a map upon a larger scale. 

 Objections to this instrument on account of its 

 weight and cumbrousness have no force, as the 

 plane-table may ^be made very light and simple 

 without reducing its accuracy. 



The theodolite may be used, the angles recorded, 

 and the map plotted upon the station. This com- 

 bines the advantages of the plane-table and the 



theodolite. It requires, however, more men, more 

 instruments, and more time devoted to work upon 

 the station, where time is of especial value. An- 

 other method of combining the advantages of 

 both instruments is in extensive use in the western 

 work of the U. S. geological survey. Here a light 

 and simple plane-table is used, in conjunction 

 with a theodolite. The map is made upon the 

 former instrument, while with the latter, angles 

 for location are read upon all important points. 

 These are subsequently plotted in the office, and 

 the plane-table sheets corrected accordingly. 



The methods of contmuous location by direc- 

 tion and distance measurements are known gen- 

 erally as traverse or meander methods. They are 

 primarily applicable to the survey of lines and not 

 of areas. The essential feature of these methods 

 is, that one station is located from another in con- 

 tinuous series. There is necessarily an accumula- 

 tion of error in such a series, which may be cor- 

 rected by connecting the line with points in a 

 triangulation. All these methods are imperfect in 

 several respects. First, they are inaccurate, be- 

 cause of the liability to an accumulation of error. 

 Second, they are ill adapted to the survey of areas, 

 inasmuch as while the line (usually a road) and its 

 immediate neighborhood are surveyed in the 

 greatest detail, the areas between lines of survey 

 are, in practice, comparatively poorly surveyed, 

 and the resulting map is unequal in quality in dif- 

 ferent parts. Third, the traverse is necessarily 

 made upon a much larger scale than is required by 

 the scale of the map, and so the work is more ex- 

 pensive than it need be. The more hilly the 

 country, the more force there is in the second of 

 these objections, as the lines of survey, following 

 the roads and trails, necessarily pass through the 

 lowest parts of the country, and, therefore, the 

 topographer, instead of being free to select the 

 best points for overlooking his area, is obliged to 

 content himself with the poorest outlooks. The 

 result is seen in meaningless hill-forms, which 

 were evidently sketched from below in the valleys. 

 By experienced topographers, traverse methods 

 are avoided whenever practicable, but in flat or 

 timbered regions it often becomes necessary to 

 adopt them. 



Of the instruments used, the plane-table is ill 

 adapted to this work, being difficult to manipulate 

 quickly. The chain and tape are generally dis- 

 carded in the survey of areas, as being more 

 accurate than the requirements, and proportion- 

 ately slower. The odometer attached to a revolv- 

 ing wheel measures distances with ample accuracy 

 for almost any scale, and, in connection with the 

 compass, allows the most rapid work of any of 

 these methods. Measurements of height, how- 



