ic observation.

The primary triangulation having been made, the topography is executed by
a variety of methods, adapted to the peculiar conditions found in various
portions of the country. To a large extent the plane-table is used. In the
hands of the topographers of the Geological Survey, the plane-table is not
simply a portable draughting table for the field; it is practically an
instrument of triangulation, and all minor positions of the details of
topography are determined through its use by trigonometric construction.

The scale on which the map is made is variable. In some portions of the
prairie region, and in the region of the great plains, the topography and
the geology alike are simple, and maps on a comparatively small scale are
sufficient for practical purposes. For these districts it is proposed to
construct the sheets of the map on a scale of 1-250,000, or about four
miles to the inch. In the mountain regions of the West the geology is more
complex, and the topography more intricate; but to a large extent these
regions are uninhabited, and to a more limited extent uninhabitable. It
would therefore not be wise to make a topographic or geologic survey of
the country on an excessively elaborate plan. Over much of this area the
sheets of the map will also be constructed on a scale of 1-250,000, but in
special districts that scale will be increased to 1-125,000, and in the
case of important mining districts charts will be constructed on a much
larger scale. In the eastern portion of the United States two scales are
adopted. In the less densely populated country a scale of 1-125,000 is
used; in the more densely populated regions a scale of 1-62,500 is
adopted, or about one mile to the inch. But throughout the country a few
special districts of great importance, because of complex geologic
structure, dense population, or other condition, will require charts on
still larger scales. The area of the United States, exclusive of Alaska,
is about three million square miles, and a map of the United States,
constructed on the plan set forth above, will require not less than 2,600
sheets. It may ultimately prove to require more than that, from the fact
that the areas to be surveyed on the larger scale have not been fully
determined. Besides the number of sheets in the general map of the United
States, there will be several hundred special maps on large scales, as
above described.

Such is a brief outline of the plan so