National Geographic : 1914 Jun
to build high towers if the distance be long and to overtop the trees if they interfere. In a perfectly level country, with a line of 20 miles, the towers must be 60 feet high to over come the curvature of the earth and to make the line clear between the light and the theodolite. THE GREAT ARCS OF TRIANGULATION The progress of the Coast and Geodetic S u r v e y is graphically shown in the map on page 664, which reveals the fact that a band of triangulation has been stretched from the frontier of Maine along the coast to the mouth of the Rio Grande. Here a line has been run due north to the Canadian border, where it intersects with the bound ary between North Dakota and Minnesota. From this arc, at a point not far west of Fort Worth, a line swings westward to the Cali fornia coast and thence to Puget Sound, and thence along the Ca nadian boundary to the line extend ing north from the mouth of the Rio Grande. Another great line cuts across the country from San Francisco due east to the Blue Ridge and Alle gheny Mountains, where it inter sects another line running from the Maryland line to Mobile, Alabama. This latter line has a greatly ex tended area in the vicinity of the approximate juncture of the bound aries of Tennessee, North Carolina, Alabama, and Georgia. A short line has also been run across the northern neck of Florida, one up the Mississippi, one from a point south of Denver to the Canadian line, and one from the North Da kota-South Dakota-Minnesota in tersection to the western shore of Lake Superior. In addition to these, there are a number of disconnected surveys here and there over the country and a number of reconnaissances or pre liminary surveys. Upon these lines of triangulation are based all the exact knowledge we have concern ing the exact location of surface of the United States as a geographic entity. THE GREAT CASPAR SIGNAL When surveying in Mendocino County, California, a signal tower was built round a great tree, the trunk of which was used to support the top of the tower, and in this way the instruments were elevated to the height of 135 feet above the ground.