Scale. For American and British charts the scale is usually expressed by the inches or fractions of an inch to the minute or degree of latitude, or by the fractional proportion of a distance on the map to the corresponding distance on the earth. These fractions are sometimes stated on the British charts, and nearly always on those of the United States Coast Survey. The chart catalogues give the scale in one or the other form. A familiarity with the meaning of scales is of value in selecting the most suitable chart, in judging of the relative uses of charts, and in estimating distances. Where the fractional scales are stated they furnish a simple means of comparing charts, as, for instance, a chart on 150000 scale will show all distances just twice as long as a chart on 1100000 scale.
The following are scale equivalents:
| Scale | 110000 | is equivalent to 7.30 inches | to one nautical mile. |
| Scale | 120000 | is equivalent to 3.65 inches | to one nautical mile. |
| Scale | 140000 | is equivalent to 1.82 inches | to one nautical mile. |
| Scale | 150000 | is equivalent to 1.46 inches | to one nautical mile. |
| Scale | 180000 | is equivalent to 0.91 inch | to one nautical mile. |
| Scale | 1100000 | is equivalent to 0.73 inch | to one nautical mile. |
| Scale | 1200000 | is equivalent to 0.36 inch | to one nautical mile. |
| Scale | 1400000 | is equivalent to 0.18 inch | to one nautical mile. |
| Scale | 11000000 | is equivalent to 0.07 inch | to one nautical mile. |
| Scale | 11200000 | is equivalent to 0.06 inch | to one nautical mile. |
For use in measuring distances on large scale charts the length of one or more nautical miles is usually drawn on the chart, and sometimes scales are also given in other units. On British charts the nautical mile scale is divided into tenths (that is, cables of 100 fathoms or 600 feet length); on the American charts into quarters and eighths. Where the scale covers more than one mile the fractional divisions are shown only for the left-hand mile and the zero of the scale is placed between this and the full mile scale, so that with dividers the full miles and fraction may readily be taken off. The nautical mile in the United States is taken to be the length of a minute of arc of a great circle on a sphere whose surface equals that of the earth; this definition makes the nautical mile equal 6080.27 feet. Lecky adopts 6080 feet as the nautical mile. The length of the actual minute of latitude on the earth's surface increases from 6046 feet at the equator to 6108 feet at the poles, an increase of about one per cent. It is, however, this somewhat variable unit of length which is ordinarily used in scaling distances on the sailing charts.
On small scale charts there is usually a border scale entirely around the chart, conveniently subdivided; this serves the double purpose of facilitating the plotting or reading of positions by latitude and longitude and of furnishing a scale of minutes of latitude for use in measuring distances. On a mercator chart this scale of course varies with the latitude and it must be referred to in the mean latitude of the distance to be measured. In general practice the minute of latitude is taken as equal to the nautical mile.
Projection. On only a few charts is there a statement of the projection used. Practically all general sailing charts are on the mercator projection, which can be readily recognized by the rectangular network of meridians and parallels and the increase with latitude of the distance between the parallels. On large scale local and harbor charts the kind of projection used is not of importance to navigation, as for such limited areas the difference between projections would not affect the use of the chart. On certain small scale charts of the United States Coast Survey which are on the polyconic projection this fact is stated on the chart, and can also be readily recognized by the convergence of the meridians and curvature of the parallels. Gnomonic charts intended for taking off great circle courses are always described in their titles and are also easily recognized by the increased scale and distortion toward all the borders. Charts of the polar regions are published on several different projections, which are distinguished from the mercator by their circular or curved parallels.
Geographic position. For large scale and harbor charts the latitude and longitude of some point marked on the chart are sometimes stated on the face of the chart. For others of these, however, and for smaller scale and general charts, positions are obtained by reference to the border scale. There is a latitude scale down either side of the chart, and a longitude scale across the top and bottom. These scales are conveniently subdivided into degrees, minutes, or fractions of a minute. The minute is divided into tenths (6´´), sixths (10´´), quarters (15´´), or halves (30´´) on various charts.
Directions are indicated on charts both by the projection lines and by compass roses. Nearly all charts are now oriented with the meridian, that is, north is the top of the chart, and on a mercator chart the east and west border lines are parallel with the meridians and the north and south border lines with the parallels. Formerly many charts were not so oriented. Some of these are still in use and can readily be recognized by the diagonal or inclined direction of the projection lines with respect to the border of the chart. Of course directions must not be referred to the border lines of these diagonal charts, and scales along such border lines must not be used. Directions with respect to true north may always be referred to the projection lines of the chart, but on a polyconic or polar chart a direction must not be carried so far from any projection line as to introduce error on account of convergence of the meridians. Compass roses are placed on charts to facilitate the taking off or laying down of directions, though in some respects their use is less accurate and convenient than the use of protractors, referring to the projection lines. The British charts and many of those of the United States Coast Survey have only magnetic compasses, with degrees outside and points inside, the former graduated to 90°. These are engraved on the chart with the magnetic variation for the date of publication, or for a few years in advance, and give the annual change in the variation. Because of expense of engraving they can be changed on the charts only at intervals of some years, and until this is done allowance for the change in variation is to be made if important. The German charts and those of the United States Hydrographic Office now have a threefold compass, the outer one degrees true, the middle degrees magnetic and the inner points magnetic; the degrees in both cases are graduated to 360°, reading from north through east, south, and west; thus northwest would be stated as 315° instead of N. 45° W. Small scale charts covering extensive areas have no magnetic compasses. They sometimes have true compasses, and usually have the isogonic lines, or lines of equal magnetic variation, marked on them, from which the variation at any intermediate point can be estimated.