A map is defined as
a graphic representation of a portion of the Earth's surface drawn to scale, as seen from above. It uses colors, symbols, and labels
... [Show More] to represent features found on the ground.
-The ideal representation would be realized if every feature of the area being mapped could be shown in true shape.
-The purpose of a map is to provide information on the existence, location, and distance between ground features, such as populated places and routes of travel and communication. It also indicates variations in terrain, heights of natural features, and the extent of vegetation cover.
-All operations conducted require maps; however, the finest maps available are worthless unless the map user knows how to read them.
Marginal Information
A map is considered equipment. To use it properly, you should read the instructions.
These instructions on the outer edges of the map are called marginal information.
All maps are different so examine the marginal information on each map carefully.
Marginal Information (Top Margin 1-6)
The top margin of the map helps you to identify the specific area covered as well as the age and scale of the map.
1 Sheet Name (Top Margin)
The sheet name is found in bold print at the center of the top and in the lower left area of the map margin. A map is generally named for the settlement contained within the area covered by the sheet or for the largest natural feature located within the area at the time the map was drawn.
2 Sheet Number (Top Margin)
The sheet number is bolded in the upper right and lower left areas of the margin and in the center box of the adjoining sheets diagram, which is found in the lower right margin.
It is used as a reference number to link specific maps to overlays, operations orders, and plans.
For maps at 1:100,000 scale and larger, sheet numbers are based on an arbitrary system that makes possible the ready orientation of maps at scales of 1:100,000, 1:50,000, and 1:25,000.
3 Series Name (Top Margin)
The map series name is found in the same bold print as the sheet number in the upper left corner of the margin.
The name given to the series is generally that of a major political subdivision, such as a state within the United States or a European nation.
A map series usually includes a group of similar maps at the same scale and on the same sheet lines or format designed to cover a particular geographic area.
It may also be a group of maps that serve a common purpose, such as the military city maps.
4 Scale (Top Margin)
The scale is found both in the upper left margin after the series name and in the center of the lower margin.
The scale note is a representative fraction that gives the ratio of a map distance to the corresponding distance on the Earth's surface.
For example, the scale note 1:50,000 indicates that one unit of measure on the map equals 50,000 units of the same measure on the ground.
5 Series Number (Top Margin)
The series number is found in both the upper right margin and the lower left margin. It is a sequence reference expressed either as a four-digit numeral (1125) or as a letter, followed by a three- or four-digit numeral (M661; T7110).
6 Edition Number (Top Margin)
The edition number is found bolded in the upper right area of the top margin and the lower left area of the bottom margin.
Editions are numbered consecutively; therefore, if you have more than one edition, the highest numbered sheet is the most recent.
This date is important when determining how accurately the map data might be expected to match what you will encounter on the ground.
Marginal Information (Bottom Margin 7-14)
This portion of the map includes information that helps to interpret the map. It provides correlation between actual terrain and man-made features and the map's topographic symbols. Some information found in the bottom margin is repeated from the top margin.
7 Boundaries (Bottom Margin)
The index to boundaries diagram appears in the lower or right margin of all sheets.
This diagram, which is a miniature of the map, shows the boundaries that occur within the map area, such as county lines and state boundaries.
8 Adjoining Sheets (Bottom Margin)
Maps at all standard scales contain a diagram that illustrates the adjoining sheets. The diagram usually contains nine rectangles, but the number may vary depending on the locations of the adjoining sheets. All represented sheets are identified by their sheet numbers. Sheets of an adjoining series of the same scale, whether published or planned, are represented by dashed lines. The series number of the adjoining series is indicated along the appropriate side of the division line between the series.
9 Elevation Guide (Bottom Margin)
This is normally found in the lower right margin.
It is a miniature characterization of the terrain shown.
The terrain is represented by bands of elevation, spot elevations, and major drainage features.
The elevation guide provides the map reader with a means of rapid recognition of major landforms.
10 Declination Diagram (Bottom Margin)
This is located in the lower margin of large-scale maps and indicates the angular relationships of true north, grid north, and magnetic north.
On maps at 1:250,000 scale, this information is expressed as a note in the lower margin.
In new maps, there is a note indicating the conversion of azimuths from grid to magnetic and from magnetic to grid next to the declination diagram.
11 Bar Scale (Bottom Margin)
These are located in the center of the lower margin.
They are rulers used to convert map distance to ground distance.
Maps have three or more bar scales, each in a different unit of measure.
Care should be exercised when using the scales, especially in the selection of the unit of measure that is needed.
12 Contour Interval Note (Bottom Margin)
This note is found in the center of the lower margin normally below the bar scales.
It states the vertical distance between adjacent contour lines of the map.
When supplementary contours are used, the interval is indicated.
In new maps, the contour interval is given in meters instead of feet.
13 Legend (Bottom Margin)
The legend is located in the lower left margin.
It illustrates and identifies the topographic symbols used to depict some of the more prominent features on the map.
The symbols are not the same on every map.
Always refer to the legend to avoid errors when reading a map.
14 Grid Reference Box (Bottom Margin)
Grid Reference Box
Colors on a Map
By the fifteenth century, most European maps were carefully colored.
Profile drawings of mountains and hills were shown in brown.
Rivers and lakes were shown in blue.
Vegetation was shown in green.
Roads were shown in yellow.
Special information was shown in red.
A look at the legend of a modern map confirms that the use of colors has not changed much over the past several hundred years. To facilitate the identification of features on a map, the topographical and cultural information is usually printed in different colors.
Black (Colors on a Map)
This color indicates cultural (man-made) features such as buildings and roads, surveyed spot elevations, and all labels.
Blue (Colors on a Map)
Identifies hydrography or water features such as lakes, swamps, rivers, and drainage.
Green (Colors on a Map)
This color identifies vegetation with military significance, such as woods, orchards, and vineyards.
Reddish-Brown (Colors on a Map)
The colors red and brown are combined to identify cultural features such as boundaries and major roads, all relief features, non-surveyed spot elevations, and elevations or contour lines.
Types of North
True North is a line from any point on the Earth's surface to the North Pole. All lines of longitude converge at the North Pole and are true north lines. A star is used to depict true north.
Magnetic North is the direction to the north magnetic pole, as indicated by the north-seeking needle of a magnetic compass. Magnetic readings are used to navigate in the field. A half arrowhead is used to depict magnetic north.
Grid North is the north that is established by using the vertical grid lines on the map. Grid north lines are parallel lines on the map; they do not converge at the North Pole. The letters GN are used to depict grid north.
Declination Diagram
Declination diagram is the angular difference between true north and either magnetic or grid north.
There are two declinations:
A magnetic declination
A grid declination
The declination diagram shows the angular relationship, represented by prongs, among these three types of north.
G-M Angle
G-M angle (grid-magnetic angle) value is the angular size that exists between grid north and magnetic north and the year it was prepared.
This value is expressed to the nearest 1/2 degree, with mil equivalents shown to the nearest 10 mils.
The G-M angle is important to the map reader/land navigator, because it will affect the accuracy of navigation skills in the field.
G-M Conversion
G-M conversion is applying the angular difference between the grid north and the magnetic north to convert compass readings to grid version for use in navigation.
Since the location of this magnetic field does not correspond exactly with the grid-north lines on the maps, a conversion from magnetic to grid or vice versa is needed.
Simply refer to the conversion notes that appear in conjunction with the diagram explaining the use of the G-M angle.
One note provides instructions for converting a magnetic azimuth to a grid azimuth, and the other, for converting a grid azimuth to a magnetic azimuth.
The conversion (add or subtract) is governed by the direction of the magnetic-north prong relative to that of the north-grid prong.
Contour Lines
Contour lines are the most common method of showing relief and elevation on a standard topographic map. A contour line represents an imaginary line on the ground, above or below sea level. All points on the contour line are at the same elevation. The elevation represented by contour lines is the vertical distance above or below sea level.
The three types of contour lines used on a standard topographic map are:
Index lines
Intermediate lines
Supplementary lines
Three types of contour lines used on a standard topographic map:
1. Index
Starting at zero elevation or mean sea level, every fifth contour line is a heavier line.
These are known as index contour lines.
Each index contour line is typically numbered at some point.
This number is the elevation of that line.
2. Intermediate
The contour lines falling between the index contour lines are called intermediate contour lines.
These lines are finer and do not have their elevations given.
There are normally four intermediate contour lines between index contour lines.
3. Supplementary
These contour lines resemble dashes.
They show changes in elevation of at least one-half the contour interval.
These lines are normally found where there is little change in elevation, such as on fairly level terrain.
Contour Interval
Before the elevation of any point on the map can be determined, you must know the contour interval for the map you are using. The contour interval measurement given in the marginal information is the vertical distance between adjacent contour lines.
To determine the elevation of a point on the map, find the numbered index contour line closest to the point you are trying to determine.
Determine if the point is a higher or lower elevation than the index contour line.
Once that is established, you can count the number of contours higher or lower and, by referencing the marginal data, determine your actual elevation.
Elevation and Relief
Terrain features do not normally stand alone.
Major Terrain Features (Elevation and Relief)
1. Hill
A hill is an area of high ground. From a hilltop, the ground slopes down in all directions.
A hill is shown on a map by contour lines forming concentric circles.
The inside of the smallest closed circle is the hilltop.
2. VALLEY
A valley is reasonably level ground bordered on the sides by higher ground. A valley may or may not contain a stream course.
Contour lines indicating a valley are U- or V-shaped and tend to parallel a stream before crossing it.
The closed end of the contour line (U or V) always points upstream or toward high ground.
A valley generally has maneuver room within its confines.
3. RIDGE
A ridge is a sloping line of high ground. Standing on the centerline of a ridge, you will normally have low ground in three directions and high ground in one direction with varying degrees of slope.
Contour lines forming a ridge tend to be U-shaped or V-shaped. The closed end of the contour lines points away from high ground.
Crossing a ridge at right angles, you will climb steeply to the crest and then descend steeply to the base.
When moving along the path of the ridge, depending on the geographic location, there may be an almost unnoticeable slope or a very obvious incline.
4. SADDLE
A saddle is a dip or low point between two areas of higher ground. If you are in a saddle, there is high ground in two opposite directions and lower ground in the other two directions.
Contour lines for a saddle typically resemble an hourglass.
A saddle is not necessarily the lower ground between two hilltops; it may be simply a dip or break along a level ridge crest.
5. DEPRESSION
A depression is a low point in the ground or a sinkhole. It could be described as an area of low ground surrounded by higher ground in all directions, or simply a hole in the ground.
Depressions are represented by closed contour lines that have tick marks pointing toward low ground.
Usually, only depressions that are equal to or greater than the contour interval will be shown.
Minor Terrain Features (Elevation and Relief)
6. DRAW
A draw is a less developed stream course than a valley. Standing in a draw, the ground slopes upward in three directions and downward in the other direction.
The contour lines depicting a draw are U-shaped or V-shaped, pointing toward high ground.
In a draw, there is essentially no level ground and little or no maneuver room within its confines.
7. SPUR
A spur is a short, continuous sloping line of higher ground, normally jutting out from the side of a ridge. The ground is sloped down in three directions and up in one direction.
Contour lines on a map depict a spur with the U- or V-shaped lines pointing away from high ground.
A spur is often formed by two rough parallel streams, which cut draws down the side of a ridge.
8. CLIFF
A cliff is a vertical or near vertical feature; it is an abrupt change of the land.
When a slope is so steep that the contour lines converge into one "carrying" contour of contours, this last contour line has tick marks pointing toward low ground.
Cliffs are also shown by contour lines close together and, in some instances, touching each other. [Show Less]