ACTIVITIES 37 



over eight million dollars. Thus, it seems safe to say that 

 the total cost of these surveys and of the preparation of the 

 topographic maps has been not less than twelve million dol- 

 lars. 



Of the 3,026,789 square miles in the United States proper, 

 1,257,623 square miles have now been mapped, covering about 

 2,225 sheets. The accompanying map, taken from the annual 

 report of the Director of the Survey for 19 17, shows the pro- 

 gress that had been made up to June 30 of that year in the 

 execution of the topographic survey of the country. 



In addition, there have been mapped 202,361 square miles 

 in Alaska, comprising over 34 per cent, of the total area ; and 

 1,393 square miles in Hawaii, comprising 22 per cent, of the 

 total area. 



Erection of Bench Marks. In connection with its topo- 

 graphic work, the Survey erects bench marks in areas sur- 

 veyed, a bench mark being a monument upon which is noted 

 its elevation. This work has been carried on since 1896, in 

 pursuance of an act of Congress passed in that year which re- 

 quired that thereafter in topographic surveys "west of the 

 ninety-fifth meridian, elevations above a base-level located in 

 each area under survey shall be determined and marked on the 

 ground by iron or stone posts or permanent bench marks, at 

 least two such posts or bench marks to be established in each 

 township or equivalent area, except in the forest-clad and 

 mountain areas, where at least one shall be established, and 

 these shall be placed, whenever practicable, near the township 

 corners of the public land surveys; and in the areas east of 

 the ninety-fifth meridian at least one such post or bench mark 

 shall be similarly established jn each area equivalent to the 

 area of a township of the public land survey." 



A bench mark serves two purposes — that ordinarily served 

 by a monument of establishing an exact point of reference in 

 the use of the topographic maps (the location of the bench 

 mark being indicated on the map), and that of furnishing for 

 all purposes a permanent record on the land itself of its eleva- 



