576 



SCIENCE 



[N. S. Vol. XLIS. No. 1277 



In collecting data for a map tliose survey- 

 ing methods must be adopted in any particu- 

 lar case that suit the requirements. If one 

 should wish only a route map running from 

 one village to another, it would be perfectly 

 satisfactory to use a compass for direction and 

 the pacing of a horse or the readings of an 

 odometer on a wheel for the distance between 

 the two points. But maps are usually not so 

 simple as that. 



TYPES OF MAPS 



There are several types of high grade maps 

 needed in this country. One must be made 

 along the coasts to show the location of the 

 actual shore line and the character of the 

 ground immediately back of the coast in order 

 that the navigator may be able to locate him- 

 self from, topographic features along the shore, 

 should he be driven off his course during a 

 storm. In addition the depths of the water 

 and all obstructions to navigation must be 

 indicated on this map or chart, and the eleva- 

 tion and shape of the ground on islands and 

 near the shore line must be shown by contours. 



A second class consists of maps on which 

 the features other than elevations are shown 

 in their correct horizontal positions. This 

 type of map would be practically the same as 

 the third type where the area covered is very 

 level like the coastal plain of Louisiana. 



The third class covers maps of the interior 

 or of large islands on which all features, cul- 

 tural and natural, are located in their proper 

 horizontal positions and contours are shown to 

 give the elevations of the ground and the shape 

 of the hills, ridges, valleys, etc. This map 

 would be used by engineers in laying out rail- 

 roads and highways, and in conducting vari- 

 ous' classes of engineering work. 



These three classes of maps are the ones in 

 which we are most directly interested. 



The map which shows the horizontal posi- 

 tions of cultural and natural features on the 

 surface of the earth, but no contours, can be 

 made more rapidly than the one which re- 

 quires contouring. All that is needed in the 

 former case is some method of obtaining the 

 direction and distance between each two fea- 



tures in the area to be surveyed. The usual 

 method of making such a map is by compass 

 and chain, transit and tape, transit and stadia, 

 or by the plane table. These methods are all 

 very closely allied and such accuracy as may 

 be demanded may be obtained by varying the 

 methods used. 



M.\PS CONTROLLED BY FIXED POINTS 



In lany event there must be within the area 

 to be surveyed, if it is a large one, a number 

 of control stations. These control stations 

 consist of triangulatdon stations placed on the 

 highest parts of the ground or traverse sta- 

 tions along the roads, accurately located in 

 latitude and longitude and accurately and sub- 

 stantially marked with concrete or rock in 

 order that they may be recovered and identi- 

 fied by the surveyors or engineers who may 

 wish to see them. 



There are now many thousands of such sta- 

 tions in the United States, established prin- 

 cipally by the Coast and Geodetic Survey, 

 available for the fundamental control of sur- 

 veys and maps. Prom these stations control 

 of the same or of a lower grade of accuracy 

 may be extended in any direction for the im- 

 mediate control of topographic maps. 



OVERLAPS, GAPS AND OFFSETS TO BE AVOIDED 



It is readily seen that without the funda- 

 mental control, which extends over the whole 

 area of the United States, there would be 

 great confusion. If the control in any one 

 state is not properly coordinated and corre- 

 lated with that of any other state near it, the 

 result will be that when different toi)ographic 

 surveys and maps are joined there will be 

 overlaps, gaps and offsets which cause no end 

 of trouble and confusion to the cartographer 

 and map maker. When there is a single sys- 

 tem of control for the whole country we avoid 

 this unfortunate condition. 



3,000,000 SQUARE MILES, LESS THAN ONE HALF 

 MAPPED 



There is to-day only about 40 per cent, of 

 the 3,000,000 square miles of the United States 

 mapped both as to horizontal positions of the 



