Part One
  Map Reading


  Reading Topographical Maps Introduction 1. HOME

  Topographical Maps - Definition, Purpose and Categories 2. Maps

  Information in the margins of an army map 3. Marginal Information    and Symbols

  Latitude, Longitude and Other Methods to Locate Points on Topographic Maps 4. Grids

  Translating Distance on a Topographic Map to Distance on the Ground 5. Scale and Distance

  Grid North, Azimuth, Declination And Other Concepts Used To Find Direction With Topographic Maps 6. Direction

  Overlays - Used Primarily In Army Map Reading 7. Overlays

  Aerial Photographs - Supplements And Substitutes For Topographic Maps 8. Aerial Photographs

 Part Two
  Land Navigation


  Using Compass, GPS, Sun, Shadows, and Stars in Land Navigation 9. Navigation Equipment    and Methods

  Reading The Shape Of The Land In Topographic Maps 10. Elevation and Relief

  Orienting and Navigating With Topographic Maps 11. Terrain Association

  Mounted Land Navigating With Motorized Vehicles 12. Mounted Land    Navigation

  Land Navigation In Different Types of Terrain 13. Navigation in    Different Types of    Terrain



  Sketching Topographic Maps A. Field Sketching

  Folding Topographic Maps B. Map Folding     Techniques

  Units of Measure and Conversion Factors Used in Reading Topographic Maps C. Units of Measure and      Conversion Factors

  Units of Measure and Conversion Factors Used in Reading Topographic Maps D. Joint Operations      Graphics

  US Army Training Material for Map Reading and Land Navigation E. Exportable Training      Material

  Orienteering F. Orienteering

  US Army M2 Compass G. M2 Compass

  Additional Aids such as Night Vision Goggles and Global Positioning System or GPS H. Additional Aids      (GPS, Night Vision)

  Global Positioning System -  GPS J. Global Positioning      System - GPs

 

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10-3. CONTOUR INTERVALS

Before the elevation of any point on the map can be determined, the user must know the contour interval for the map he is 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—

a.   Determine the contour interval and the unit of measure used, for example, feet, meters, or yards (Figure 10-2).

Figure 10-2. Contour interval note.

Figure 10-2. Contour interval note.

b.   Find the numbered index contour line nearest the point of which you are trying to determine the elevation (Figure 10-3).

Figure 10-3. Points on contour lines.

Figure 10-3. Points on contour lines.

c.   Determine if you are going from lower elevation to higher, or vice versa. In Figure 10-3, point (a) is between the index contour lines. The lower index contour line is numbered 500, which means any point on that line is at an elevation of 500 meters above mean sea level. The upper index contour line is numbered 600, or 600 meters. Going from the lower to the upper index contour line shows an increase in elevation.

d.   Determine the exact elevation of point (a), start at the index contour line numbered 500 and count the number of intermediate contour lines to point (a). Locate point (a) on the second intermediate contour line above the 500-meter index contour line. The contour interval is 20 meters (Figure 10-2), thus each one of the intermediate contour lines crossed to get to point (a) adds 20 meters to the 500-meter index contour line. The elevation of point (a) is 540 meters; the elevation has increased.

e.   Determine the elevation of point (b). Go to the nearest index contour line. In this case, it is the upper index contour line numbered 600. Locate point (b) on the intermediate contour line immediately below the 600-meter index contour line. Below means downhill or a lower elevation. Therefore, point (b) is located at an elevation of 580 meters. Remember, if you are increasing elevation, add the contour interval to the nearest index contour line. If you are decreasing elevation, subtract the contour interval from the nearest index contour line.

f.   Determine the elevation to a hilltop point (c). Add one-half the contour interval to the elevation of the last contour line. In this example, the last contour line before the hilltop is an index contour line numbered 600. Add one-half the contour interval, 10 meters, to the index contour line. The elevation of the hilltop would be 610 meters.

g.   There may be times when you need to determine the elevation of points to a greater accuracy. To do this, you must determine how far between the two contour lines the point lies. However, most military needs are satisfied by estimating the elevation of points between contour lines (Figure 10-4).

Figure 10-4. Points between contour lines.

Figure 10-4. Points between contour lines.

(1)   If the point is less than one-fourth the distance between contour lines, the elevation will be the same as the last contour line. In Figure 10-4, the elevation of point a will be 100 meters. To estimate the elevation of a point between one-fourth and three-fourths of the distance between contour lines, add one-half the contour interval to the last contour line.

(2)   Point b is one-half the distance between contour lines. The contour line immediately below point b is at an elevation of 160 meters. The contour interval is 20 meters; thus one-half the contour interval is 10 meters. In this case, add 10 meters to the last contour line of 160 meters. The elevation of point b would be about 170 meters.

(3)   A point located more than three-fourths of the distance between contour lines is considered to be at the same elevation as the next contour line. Point c is located three-fourths of the distance between contour lines. In Figure 10-4, point c would be considered to be at an elevation of 180 meters.

h.   To estimate the elevation to the bottom of a depression, subtract one-half the contour interval from the value of the lowest contour line before the depression. In Figure 10-5, the lowest contour line before the depression is 240 meters in elevation. Thus, the elevation at the edge of the depression is 240 meters. To determine the elevation at the bottom of the depression, subtract one-half the contour interval. The contour interval for this example is 20 meters. Subtract 10 meters from the lowest contour line immediately before the depression. The result is that the elevation at the bottom of the depression is 230 meters. The tick marks on the contour line forming a depression always point to lower elevations.

Figure 10-5. Depression.

Figure 10-5. Depression.

i.   In addition to the contour lines, bench marks and spot elevations are used to indicate points of known elevations on the map.

(1)   Bench marks, the more accurate of the two, are symbolized by a black X, such as X BM 214. The 214 indicates that the center of the X is at an elevation of 214 units of measure (feet, meters, or yards) above mean sea level. To determine the units of measure, refer to the contour interval in the marginal information.

(2)   Spot elevations are shown by a brown X and are usually located at road junctions and on hilltops and other prominent terrain features. If the elevation is shown in black numerals, it has been checked for accuracy; if it is in brown, it has not been checked.

NOTE: New maps are being printed using a dot instead of brown Xs.

Return to Elevation and Relief
 



 

Books

Map Reading and Land Navigation Buy the book this website is based on: Map Reading and Land Navigation

This website is based on the US Army Field Manual: "Map Reading and Land Navigation" Buy a copy from Amazon.com to take with you out in the field.

 

Book Review - Be Expert with Map and Compass

One of the best ways to learn and become proficient in any subject is to find a way to make a game or sport of it. That's exactly what orienteering does! Orienteering began to develop almost 100 years ago in the Scandinavian countries as a fun and effective method for military training in land navigation. Bjorn Kjellstrom was closely involved with the early development of orienteering, and he is the person who introduced the sport to North America. He, along with his brother Alvar, and a friend named Gunnar Tillander, invented the modern orienteering compass. They manufactured and marketed it as the Silva Protractor compass. This compass, along with Bjorn's book Be Expert with Map and Compass, made it much easier for anyone to learn how to use a map and compass.

This book has become the most widely read classic on the subject of map reading, compass use, and orienteering. Over 500,000 copies have been sold in the english language editions alone. There have been very successful editions published in French, Italian, and other languages as well. It is a short (just over 200 pages), easy to read, enjoyable book that can help you to have fun while you learn the subject quickly and effectively.

The book is organized into four main parts, plus a short, useful introduction. Part 1 covers having fun with maps alone. Then, Part 2 covers having fun with a compass alone. Part 3 puts it together and shows you how to have fun with a map and compass together. This section also introduces the game or sport of orienteering. Part 4 covers competitive orienteering for those who would like to compete with others in the sport.

A reproduction of a segment of an actual topographic map is included as a fold-out in the back of the book. It is used together with the "how-to" instructions the book provides. For example, one of the exercises in Part 3 is an imaginary orienteering "hike" that uses the sample map.

If you would like to have one of the best books available on map reading and using a compass, Be Expert with Map and Compass is hard to beat. You can buy a copy from Amazon.com today.

Read a book review of Agincourt

Boat Navigation For The Rest of Us
  Boat Navigation For The Rest of Us

Basic Coastal Navigation
  Basic Coastal Navigation