In addition to the identifications described above, the margin of a map contains other information important to the user in evaluating and interpreting the map (table I).
a. Credit Note. The credit note aids in evaluating the map and contains interpretive information. The note describes the method of preparation, identifies the source material used in compilation, gives the dates of aerial photography, and lists the source of horizontal and vertical control. It notes whether the map conforms with national map accuracy requirements and whether the map has been field checked. It includes any special information pertinent to the particular sheet.
b. Symbol Legend. The symbol legend defines and illustrates the symbols most commonly used such as populated places, roads, and rail roads. It also contains symbols for items peculiar to the area being mapped.
c. Index to Adjoining Sheets. The index to adjoining sheets, or on 1:250,000 scale maps the location diagram, identifies the surrounding sheets.
d. Index to Boundaries. The index to boundaries identifies the political areas appearing in the body of the map. The boundaries in the diagram are schematic but serve as aids in locating the boundaries on the map. On the 1:250,000 scale maps this information is shown in the location diagram.
e. Coverage Diagram. The coverage diagram, shown on large-scale maps, portrays in graphic form the methods of compilation, notes the dates of any photography used, and identifies and evaluates any maps used as bases.
f. Reliability Diagram. The reliability diagram, shown on medium-and small-scale maps, contains graphic references to the reliability of the sources used and identifies the scale, method of survey, and date of the basic sources.
g. Datum Notes. The horizontal, vertical, and hydrographic datum notes identify the controls used for these items on the map. Generally, horizontal and hydrographic datum notes are not shown on medium-and small-scale maps.
h. Grid Notes and Data. Maps of 1:1,000,000 and larger scale contain grid notes and a grid reference box with sample reference, to explain the grid data on the map. Maps carrying 1,000-unit-interval grid lines also show a declination diagram and a protractor scale in the margin. The declination diagram shows the relationship bet wee n true north, magnetic north, and grid north for the major grid at the center of the sheet. Maps carrying 10,000-unit-interval grid lines show a magnetic declination note. This note indicates the variation in the east and west map edges. It also shows the mean annual change.
