Leroy R. Grumman Cadet Squadron

Civil Air Patrol - The official auxiliary of the United States Air Force

Map Terminology and Use

A map is a two-dimensional representation of the three-dimensional world you'll be conducting your search and rescue mission in. All maps will have some basic features in common and map reading is all about learning to understand their particular "language." You may end up using a variety of maps to plan and run your mission, but perhaps the most useful map is a topographic map. A topographic map uses markings such as contour lines to simulate the three-dimensional topography of the land on a two-dimensional map. In the U.S. these maps are usually U.S. Geological Survey (USGS) maps. Other maps that you'll find helpful are local trail maps which often have more accurate and up-to-date information on specific trails than USGS maps do. Here's a brief overview of the basic language of maps.


Latitude and Longitude:

Maps are drawn based on latitude and longitude lines. Latitude lines run east and west and measure the distance in degrees north or south from the equator (0° latitude). Longitude lines run north and south intersecting at the geographic poles. Longitude lines measure the distance in degrees east and west from the prime meridian that runs through Greenwich, England. The grid created by latitude and longitude lines allows us to calculate an exact point using these lines as X axis and Y axis coordinates.

Both latitude and longitude are measured in degrees (°).


1° = 60 minutes
1 minute = 60 seconds

 

Therefore:


7 ½ minutes = 1/8 of 60 minutes = 1/8 of a degree


15 minutes = ¼ of 60 minutes = ¼ of a degree


Scale:

 

All maps will list their scales in the margin or legend. A scale of 1:250,000 (be it inches, feet, or meters) means that 1 unit on the map is the equivalent of 250,000 units in the real world. So 1 inch measured on the map would be the equivalent of 250,000 inches in the real world. Most USGS maps are either 1:24,000, also known as 7 ½ minute maps, or 1:62,500, known as 15-minute maps (the USGS is no longer issuing 15 minute maps although the maps will remain in print for some time).

 

Standard topographic maps are usually published in 7.5-minute quadrangles. The map location is given by the latitude and longitude of the southeast (lower right) corner of the quadrangle. The date of the map is shown in the column following the map name; a second date indicates the latest revision. Photo-revised maps have not been field checked.

 

Map Size

Scale

Covers

Map to Landscape

Metric

7 ½ minute

1:24,000

1/8 of a degree

1 inch = 2,000 feet (3/8 mile)
2.64 inches = 1 mile

(1 centimeter = 240 meters)

15 minute

1:62,500

¼ of a degree

1 inch = ~1 mile

(1 centimeter = 625 meters)


Map Symbols and Colors:

 

  1. USGS topographic maps use the following symbols and colors to designate different features:
  • Black - man-made features such as roads, buildings, etc.
  • Blue - water, lakes, rivers, streams, etc.
  • Brown - contour lines (see page 00)
  • Green - areas with substantial vegetation (could be forest, scrub, etc.)
  • White - areas with little or no vegetation; white is also used to depict permanent snowfields and glaciers
  • Red - major highways; boundaries of public land areas
  • Purple - features added to the map since the original survey. These features are based on aerial photographs but have not been checked on land.

 

 


Map Legend:

The map legend contains a number of important details. The figures below display a standard USGS map legend. In addition, a USGS map includes latitude and longitude as well as the names of the adjacent maps (depicted on the top, bottom, left side, right side and the four corners of the map).

 

 

 

 

  

  1. Map Name
  2. Year of Production and Revision
  3. General Location in State
  4. Next Adjacent Quadrangle Map
  5. Map Scale
  6. Distance Scale
  7. Contour Interval
  8. Magnetic Declination
  9. Latitude and Longitude


Contour Lines: 

Contour lines are a method of depicting the 3-dimensional character of the terrain on a 2-dimensional map. Just like isobars in the atmosphere depict lines of equal atmospheric pressure, contour lines drawn on the map represent equal points of height above sea level.

Look at the three-dimensional drawing of the mountain below. Imagine that it is an island at low tide. Draw a line all around the island at the low tide level. Three hours later, as the tide has risen, draw another line at the water level and again three hours later. You will have created three contour lines each with a different height above sea level. As you see in the figures below, the three dimensional shape of the mountain is mapped by calculating lines of equal elevation all around the mountain, and then transferring these lines onto the map.

 

On multi-colored maps, contour lines are generally represented in brown. The map legend will indicate the contour interval—the distance in feet (meters, etc.) between each contour line. There will be heavier contour lines every 4th> or 5th contour line that are labeled with the height above sea level.

 

 3D View of Mountain showing how contours relate to height

 

 

Top View of Mountain showing contours

 

  

Drawn Contour Lines

  

  • Steep slopes - contours are closely spaced
  • Gentle slopes - contours are less closely spaced
  • Valleys - contours form a V-shape pointing up the hill - these V's are always an indication of a drainage path that could also be a stream or river.
  • Ridges - contours form a V-shape pointing down the hill
  • Summits - contours forming circles
  • Depressions - are indicated by circular contour with lines radiating to the center

Animated Topographic Features

(15 second intervals)


Measuring Distances: 

There are a number of ways to measure distance accurately on a map. One is to use a piece of string or flexible wire to trace the intended route. After tracing out your route, pull the string straight and measure it against the scale line in the map legend. Another method is to use a compass (the mathematical kind) set to a narrow distance on the map scale like ½ mile and then "walk off" your route.

 

It is a good idea to be conservative and add 5-10% of the total distance to take into account things like switchbacks that don't appear on the map. It's better to anticipate a longer route than a shorter one.


Getting to Know the Metric System:

 

If the metric system gives you heartburn, here are a few tips to help you out.

 

 The Truth
(to within 3 or 4 significant digits)

 What you can remember
(You'll be about 10% too short.)

 1 meter

= 3.280 feet
= 1.094 yards

  1 meter

~= 3 feet
~= 1 yard

100 m

= 109 yards

  100 m

~= 100 yards
~= length of a football field

 1000 m

= 1 kilometer
= 1 km
= 0.621 miles
~= 5/8 mile

  1000 m

~= 1/2 mile

 


Map Datums:

A datum describes the model that was used to match the location of features on the ground to coordinates and locations on the map. Maps all start with some form of survey. Teams of surveyors on the ground using transits and distance measuring “chains” carried out early maps and surveys. Surveyors start with a handful of locations in "known" positions and use them to locate other features. These methods did not span continents well. Frequently they also did not cross political borders either. The "known points" and their positions are the information that the map datum is based. As space based surveying came into use, a standardized datum based on the center of the earth was developed.

 

Every map that shows a geographic coordinate system such as UTM or Latitude and Longitude with any precision will also list the datum used on the map.

 

The Global Positioning System uses an earth-centered datum called the World Geodetic System 1984 or WGS 84. WGS 84 was adopted as a world standard from a datum called the North American Datum of 1983 or NAD 83. For all practical purposes there is no difference between WGS 84 and NAD 83.

 

Most USGS topographic maps are based on an earlier datum called the North American Datum of 1927 or NAD 27. (Some GPS units subdivide this datum into several datums spread over the continent. In the Continental United States use NAD27 CONUS.)

 

In the Continental United States the difference between WGS 84 and NAD 27 can be as much as 200 meters.

 

On a USGS topographic map the datum information is in the fine print at the bottom left of the map. The datum will always be NAD 27. There may be information on how many meters to shift a position to convert it to NAD 83. Think of this as the error that will be introduced if you leave your GPS unit set to WGS 84. A dashed cross in the SW and NE corners of the map gives a visual indication of the difference between the two datums.

 

 

 

If you are coordinating with aircraft, they will likely have their datum set to WGS 84, as most aviation charts now use WGS 84. Should you worry about the difference in datums? Typically a pilot will not have any difficulty locating you on the ground if you can get them within several hundred meters of your location. If you are engaged in a mission that requires more precision, then your datums should match.

UTM Coordinate System

About the UTM coordinate system

The U.S. Army adopted the Universal Transverse Mercator projection and grid system in 1947 for designating rectangular coordinates on large-scale military maps. UTM is currently used by the United States and NATO armed forces. With the advent of inexpensive GPS receivers, many other map users are adopting the UTM grid system for coordinates that are simpler to use than latitude and longitude.

 

The UTM system divides the earth into 60 zones each 6 degrees of longitude wide. These zones define the reference point for UTM grid coordinates within the zone. UTM zones extend from a latitude of 80° S to 84° N. In the Polar Regions the Universal Polar Stereographic (UPS) grid system is used.

 

UTM zones are numbered 1 through 60, starting at the international date line, longitude 180°, and proceeding east. Zone 1 extends from 180° W to 174° W and is centered on 177° W.

 

Each zone is divided into horizontal bands spanning 8 degrees of latitude. These bands are lettered, south to north, beginning at 80° S with the letter C and ending with the letter X at 84° N. The letters I and O are skipped to avoid confusion with the numbers one and zero. The band lettered X spans 12° of latitude.

 

A square grid is superimposed on each zone. It's aligned so that vertical grid lines are parallel to the center of the zone, called the central meridian.

 

UTM grid coordinates are expressed as a distance in meters to the east, referred to as the "easting", and a distance in meters to the north, referred to as the "northing".

 


Eastings:

 

UTM easting coordinates are referenced to the centerline of the zone known as the central meridian. The central meridian is assigned an easting value of 500,000 meters East. Since this 500,000m value is arbitrarily assigned, eastings are sometimes referred to as "false eastings".

 

An easting of zero will never occur, since a 6° wide zone is never more than 674,000 meters wide.

 

Minimum and maximum easting values are:

 

160,000 mE and 834,000 mE at the equator

465,000 mE and 515,000 mE at 84° N


Northings:

 

UTM northing coordinates are measured relative to the equator. For locations north of the equator the equator is assigned the northing value of 0 meters North. To avoid negative numbers, locations south of the equator are made with the equator assigned a value of 10,000,000 meters North.

 

 

Some UTM northing values are valid both north and south of the equator. In order to avoid confusion the full coordinate needs to specify if the location is north or south of the equator. Usually including the letter for the latitude band does this.

 

If this is your first exposure to the UTM coordinate system you may find the layout of zones to be confusing. In most land navigation situations the area of interest is much smaller than a zone. The notion of a zone falls away and we are left with a simple rectangular coordinate system to use with our large-scale maps.

 

Frequently, in land navigation, the zone information and the digits representing 1,000,000m, and 100,000m are dropped. The 1m, 10m and 100m digits are used only to the extent of accuracy desired. Note that it's the smaller digits that are dropped in the notation used by the USGS on the edges of their maps. For example 4282000 mN. becomes 82.

 

Because pilots and sailors navigate over much greater distances they still favor the latitude longitude coordinate system.


UTM Coordinates on USGS Topographic Maps:

 

All USGS topographic maps printed in the last 30 years or so include UTM grid tick marks, in blue, on the margin of the map. For a short time period after 1978 the USGS was printing a fine lined UTM grid on their topographic maps. They have since discontinued this practice.

 

Since most USGS 1:24,000 scale topographic maps do not have grid lines printed on them, you will need to draw them in by hand.

 

Start by finding a flat surface to work on. Use a straightedge that is long enough to draw a line across your map. Two to three feet long is a good length.

 

Line the straightedge up between two corresponding UTM tick marks along the neat line (the edge) of the map. Remember that UTM grid lines are not exactly North-South or East-West anywhere but in the center of a zone. This means that the grid lines will not be parallel to the neat lines.

 

Using a mechanical pencil or a fine pointed pen draw a line between the two tic marks. If you are using a pen, select one that has waterproof ink. In addition, you will want to use a straightedge that has the edges lifted off of the paper. This will help keep from leaving an ink smudge when you move the straightedge. High quality straightedges will often have a thin piece of cork stuck to the bottom. This helps keep the rule from slipping, and keeps the edge off of the paper. A piece of masking tape centered on the bottom of your straightedge will work also. Occasionally wipe of the edge of the straightedge to avoid any ink build up.

 

Gridding maps is tedious work. We all wish the USGS would go back to printing the grid on the map. But even then, we would still need to grid our existing maps. As you can see this is not the kind of thing you want to do on the hood of a truck or using a flat rock. Grid your maps before you need them in the field! In a pinch you can fold the map over on itself and use the edge of the paper as a straightedge.


Photocopies of Maps:

 

Frequently, you may use a photocopy of a small portion of a map rather than the entire map. This cuts down the wear and tear on the original map and allows several copies to be distributed among a group.

 

Make sure you transfer at least the large-print portion of the UTM grid markings onto the photocopy. It's also helpful to provide scale and contour information. Preprinted scale bars on Post-It notepaper are available or just make a copy of the scale bars and "cut and paste".

 

Avoid the temptation to change the scale of the map with the zoom on the copier. If you use maps often you will have a good sense of distance. Alter the scale and it will be harder to judge distances. Plus your overlay tools will no longer be useful.

 

If you do change the scale using the copier, be sure and copy the scale bars at the same time, so they will correctly reflect the new scale.

 

If you are marking roads, trails or boundaries on the photocopied map, avoid obscuring the underlying feature with the mark. Pencil lines will usually allow the feature to show through, as will highlighter pens.

 

There is nothing more frustrating than needing to know what is under a big black mark on your copy of the map.


 Why Use UTM Coordinates

 

The UTM coordinate system offers the following benefits:

 

A square grid

 

UTM Provides a constant distance relationship anywhere on the map. In angular coordinate systems like latitude and longitude, the distance covered by a degree of longitude differs as you move towards the poles and only equals the distance covered by a degree of latitude at the equator. Since land navigation is done in a very small part of the world at any one time using large scale maps. The UTM system allows the coordinate numbering system to be tied directly to a distance measuring system.

 

No negative numbers or East-West designators

 

Grid values increase from left to right and bottom to top

 

This is just like the X Y Cartesian coordinate system you learned high school math class. Simple Cartesian coordinate mathematics can be used. No spherical trigonometry is required!

 

Coordinates are decimal based

 

Ones, tens, hundreds and so on. No more minutes and seconds to convert.

 

Coordinates are measured in metric units

 

All UTM coordinates are measured in meters. Most of the world has already adopted the metric system. Now you won't need to remember how many feet are in a mile. And what's that in yards?


 

A Quick Guide to Using UTM Coordinates

 

 

Standing at the road junction marked with the star on the topographic map pictured above, a GPS unit set to display position in UTM coordinates, would report a location of:

 

10 S 0559741
    4282182

 

The 10 S represents the zone you are in. The zone is necessary to make the coordinates unique over the entire globe.

 

The top set of numbers, 0559741, represents a measurement of East-West position, within the zone, in meters. It's called an easting.

 

The bottom set of numbers, 4282182, represents a measurement of North-South position, within the zone, in meters. It's called a northing.

 

The map has Universal Transverse Mercator (UTM) grid lines spaced every kilometer or 1000 meters. The vertical grid lines determine East-West position and the horizontal grid lines determine North-South position.

 

Look along the bottom edge of the map at the labels for the vertical grid lines.

 

559 and 560000 mE.

 

The label, 560000 mE., reads "five hundred and sixty thousand meters East." The label, 559, is an abbreviation for, 559000 mE. The two grid lines are 1000 meters apart. The horizontal grid lines are labeled in a similar manner.


 

Shorthand for UTM Coordinates

 

Most land navigation activities focus on a very small portion of the globe at any one time. Typically the area of interest to an outdoorsman is less than 20 miles on a side. This focus on a small area allows us to abbreviate UTM coordinates.

The zone information and the digits representing 1,000,000m, and 100,000m are dropped. The 1m, 10m and 100m digits are used only to the extent of accuracy desired.

 

A GPS unit might read:

10 S 0559741
    4282182

Using a notation similar to the one found on a USGS topographic map, this would be written as:

 

Zone 10 S 559741 mE. 4282182 mN.

 

An abbreviated format for the same coordinates would look like:

 59 82

 Describes a 1000m by 1000m square.

 597 821

 Describes a 100m by 100m square.

 5974 8218

 Describes a 10m by 10m square.

 59741 82182

 Describes a 1m by 1m square.

The 100m abbreviated format, 597 821, and the 10m abbreviated format, 5974 8218, are the most commonly used.

 

Notice that the easting is reported first, followed by the northing. Remember the phrase "read right up" to help you remember to read the easting from left to right, followed by the northing from the bottom up.

 

Also notice that when you abbreviate coordinates you should not do any rounding. 0559651 becomes 596 not 597. This ensures that your position is still within the reported square. As accuracy decreases, the square gets bigger.


Using a UTM grid overlay tool

 

If you want to find your location with more precision than is available from the grid lines on the map, you will need a tool that is marked in finer divisions. One such tool is a grid overlay. The grid overlay is placed on the map with its edge aligned with the grid lines. Then the position of the mark can be determined using the tool's additional precision. Additional precision is available by either by "eyeballing" or by using a UTM Corner Ruler with finer markings. For many land navigation situations 100m precision is quite adequate.

 

 

 

The example shown here locates the   to a precision of 100m. The 10,000m and 1,000m digits of the coordinate are taken from the map. Thus the coordinates 59 82 locate the 1,000 meter square containing the star. The grid overlay is placed over the grid and the 100m digit is determined. Remember to read the Easting followed by the Northing. 

 

In 100m abbreviated format the coordinates of the  are 597 821.


Using a UTM Corner Ruler

 

A UTM Corner Ruler consists of two scales at right angles to each other. UTM Corner Rulers will typically provide an additional digit of precision beyond a UTM Grid Overlay. On a 1:24,000 scale map you will be able to determine a position to within a 10m square. The trade off is that the Corner Ruler is somewhat harder to use.

 

 

Start by placing the top right corner of the Corner Ruler on the SW corner of the UTM grid that contains the feature. The ruler edges should extend to the West and South. To find the UTM coordinates of a feature marked on your map slide the rulers North and East until the corner is on top of the feature to be measured. Read the UTM coordinate values from the starting grid lines. To locate a UTM coordinate on the map slide the ruler North and East until the desired distances are indicated at the grid lines.

 

If the grid square you are using is on the edge of your map, you may need to start from a corner other than the southwestern one. You can still use the corner ruler, remember that UTM coordinate values increase from West to East and from South to North.

 

 

 

If you are using an odd scaled map or if you left your UTM tools behind, you can quickly make a simple corner ruler using the scale bars on the map.

 

Start with the corner of a scrap of paper. Mark off a one-kilometer distance and the 100m subdivisions using the metric scale bar. Repeat this process along the other edge. Number both rules starting from the corner that would be zero.

 

 

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