Page:The National geographic magazine, volume 1.djvu/124

 square miles. These sheets upon the scale of publication are about 17½ inches by 13 in dimensions. In two or three cases along the coast it seemed to be in the interest of economy to vary from this arrangement slightly, in order to avoid the multiplication of sheets. Many of the sheets upon the borders of the state project over into other states, and, in cases where the area lying without the state was small, the survey was extended beyond the limits of the state, in order to complete the sheets.

Every map is a sketch, which is corrected by the geometric location of a greater or less number of points. Assuming entire accuracy in the location of the points, that is, assuming that the errors of location of the points are not perceptible upon the map, the measure of accuracy of the map consists in the number of these geometric locations per unit of surface, per square inch, if you will, of the map. The greater the number of these locations the greater the accuracy of the map, but however numerous they may be the map itself is a sketch, the points located being simply mathematical points. Whatever method be employed for making these geometric locations, the sketching is substantially the same everywhere. The methods of making these locations must differ with the character of the country, as regards the amount and form of its relief, the prevalence of forests and other circumstances. There are two general methods of making the geometric locations used in surveying; one, by triangulation; the other by the measurement of a single direction and a distance, which is the method employed in traverse surveying. In practice, the two methods are often combined with one another. Both methods have been employed in Massachusetts. The fundamental basis of the work was the triangulation which had been carried over the state by the U. S. Coast and Geodetic Survey. By this survey points were located at wide intervals over the state. Besides this there was executed between 1830 and 1840, at the expense of the state, a triangulation known as the "Borden Survey." This located a much larger number of points, but less precisely. The Coast and Geodetic Survey kindly undertook the adjustment of this triangulation to an agreement with its own work, and, as many of the lines were common to the two pieces of work, the locations made by the Borden Survey were by this adjustment greatly strengthened. Even after this work was done, however, there remained considerable areas which were destitute of located points, and it became necessary to sup-