At this point in his estimate the commander utilizes his charts, intelligence reports, and hydrographic publications to make a factual study of the theater. This study is not for the purpose, at this time, of drawing any conclusions as to possible courses of action, but to furnish data which will assist in consideration of later sections of the estimate. The study may be made under several important headings, as follows: (a) Hydrography. A study of the hydrography will determine the depth of water, the existence of shoals, the presence of unusual currents, the rise and fall of the tides, the availability of channels, and other pertinent features. These are recorded for later use. Shallow water may permit mining or may prevent the operation of submarines. On the other hand, the ability to mine in shallow water may be curtailed by strong currents or by the rise and fall of the tide. Again, the depth of water, the strength of currents, and the range of the tide may determine the feasibility of netting the entrance to a port or base. In a tactical action, advantage may be taken of shoals to limit the freedom of action of the enemy, without, however, interfering with that of one's own forces. (b) Topography. The topography of the area is also frequently of interest to the naval commander. In actions close to the shore, the character of the coast may play an important role. A high bluff, combined with considerations as to light, may create a very definite advantage or disadvantage in a naval tactical situation. Topography may be a most important consideration in determining what bases are to be used. The commander makes note of the topography of the various possible bases; later in his estimate, the natural features lending assistance to the defense of the various sites may play an important part in the selection of bases. The use of channels may depend upon the topography of the bordering land. Questions arise as to whether such land can be seized and held, or, if in friendly hands, whether it can afford adequate protection to the channel. In any landing operation, the topography of the area to be occupied may be the controlling factor. (c) Weather. The seasonal weather in the theater will have a direct bearing upon operations. The use of aircraft, the employment of light forces, the habitability of ships over long periods, the use of smoke, the range at which a gun action may be fought, the effect of spray and gases,--these considerations are but some of the matters which will be affected by weather. The possession of, and the position of, meteorological stations within the theater are of growing importance in the successful planning of coordinated air, submarine, and surface operations.