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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8-9. IDENTIFICATION OF PHOTOGRAPH FEATURES

The identification of features on a photograph is not difficult if the following facts are remembered. The view that is presented by the aerial photograph is from above and, as a result, objects do not look familiar. Objects that are greatly reduced in size appear distorted. Most aerial photography is black and white, and all colors appear on the photograph in shades of gray. Generally speaking, the darker the natural color, the darker it will appear on the photograph.

a.   The identification of features on aerial photographs depends upon a careful application of five factors of recognition. No one factor will give a positive identification; it requires the use of all five.

(1)   Size. The size of unknown objects on a photograph, as determined from the scale of the photograph or a comparison with known objects of known size, gives a clue to their identity. For example, in a built-up area the smaller buildings are usually dwellings, and the larger buildings are commercial or community buildings.

(2)   Shape (Pattern). Many features possess characteristic shapes that readily identify the features. Man-made features appear as straight or smooth curved lines, while natural features usually appear to be irregular. Some of the most prominent man-made features are highways, railroads, bridges, canals, and buildings. Compare the regular shapes of these to the irregular shapes of such natural features as streams and timber lines.

(3)   Shadows. Shadows are very helpful in identifying features since they show the familiar side view of the object. Some excellent examples are the shadows of water towers or smoke stacks. As viewed directly from above, only a round circle or dot is seen, whereas the shadow shows the profile and helps to identify the object. Relative lengths of shadows also usually give a good indication of relative heights of objects.

(4)   Shade (Tone or Texture). Of the many different types of photographic film in use today, the film used for most aerial photography, except for special purposes, is panchromatic film. Panchromatic film is sensitive to all the colors of the spectrum; it registers them as shades of gray, ranging from white to black. This lighter or darker shade of features on aerial photographs is known as the tone. The tone is also dependent on the texture of the features; a paved highway has a smooth texture and produces an even tone on the photograph, while a recently plowed field or a marsh has a rough, choppy texture and results in a rough or grainy tone. It is also important to remember that similar features may have different tones on different photographs, depending on the reflection of sunlight. For example, a river or body of water appears light if it is reflecting sunlight directly toward the camera, but appears dark otherwise. Its texture may be smooth or rough, depending on the surface of the water itself. As long as the variables are kept in mind, tone and texture may be used to great advantage.

(5)   Surrounding Objects. Quite often an object not easily recognized by itself may be identified by its relative position to surrounding objects. Large buildings located beside railroads or railroad sidings are usually factories or warehouses. Identify schools by the baseball or football fields. It would be hard to tell the difference between a water tower next to a railroad station and a silo next to a barn, unless the surrounding objects such as the railroad tracks or cultivated fields were considered.

b.   Before a vertical photograph can be studied or used for identification of features, it must be oriented. This orienting is different from the orienting required for the construction or use of the point designation grid. Orienting for study consists of rotating the photograph so that the shadows on the photograph point toward yourself. You then face a source of light. This places the source of light, an object, and its shadow in a natural relationship. Failure to orient a photograph properly may cause the height or depth of an object to appear reversed. For example, a mine or quarry may appear to be a hill instead of a depression.

8-10. STEREOVISION

One of the limitations of the vertical aerial photograph is the lack of apparent relief. Stereoscopic vision (or as it is more commonly known, stereovision or depth perception) is the ability to see three-dimensionally or to see length, width, and depth (distance) at the same time. This requires two views of a single object from two slightly different positions. Most people have the ability to see three-dimensionally. Whenever an object is viewed, it is seen twice--once with the left eye and once with the right eye. The fusion or blending together of these two images in the brain permits the judgment of depth or distance.

a.   In taking aerial photographs, it is rare for only a single picture to be taken. Generally, the aircraft flies over the area to be photographed taking a series of pictures, each of which overlaps the photograph preceding it and the photograph following it so that an unbroken coverage of the area is obtained (Figure 8-20). The amount of overlap is usually 56 percent, which means that 56 percent of the ground detail appearing on one photo also appears on the next photograph. When a single flight does not give the necessary coverage of an area, additional flights must be made. These additional flights are parallel to the first and must have an overlap between them. This overlap between flights is known as side lap and usually is between 15 and 20 percent (Figure 8-21).

Figure 8-20. Photographic overlap.

Figure 8-20. Photographic overlap.

 

Figure 8-21. Side lap.

Figure 8-21. Side lap.

b.   The requirement for stereovision can be satisfied by overlapping photographs if one eye sees the object on one photograph and the other eye sees the same object on another photograph. While this can be done after practice with the eyes alone, it is much easier if an optical aid is used. These optical aids are known as stereoscopes. There are many types of stereoscopes, but only the two most commonly used are discussed in this manual.

(1)   Pocket Stereoscope. The pocket stereoscope (Figure 8-22), sometimes known as a lens stereoscope, consists of two magnifying lenses mounted in a metal frame. Because of its simplicity and ease of carrying, it is the type used most frequently by military personnel.

Figure 8-22. Pocket stereoscope.

Figure 8-22. Pocket stereoscope.

(2)   Mirror Stereoscope. The mirror stereoscope (Figure 8-23) is larger, heavier, and more subject to damage than the pocket stereoscope. It consists of four mirrors mounted in a metal frame.

Figure 8-23. Mirror stereoscope.

Figure 8-23. Mirror stereoscope.

c.   A method to orient a pair of aerial photographs for best three-dimensional viewing is outlined below:

(1)   Arrange the selected pair of photos in such a way that the shadows on them generally appear to fall toward the viewer. It is also desirable that the light source enters the side away from the observer during the study of the photographs (Figure 8-24).

Figure 8-24. Placement of stereoscope over stereopair.

Figure 8-24. Placement of stereoscope over stereopair.

(2)   Place the pair of photographs on a flat surface so that the detail on one photograph is directly over the same detail on the other photograph (Figure 8-24).

(3)   Place the stereoscope over the photographs so that the left lens is over the left photograph and the right lens is over the right photograph (Figure 8-24).

(4)   Separate the photographs along the line of flight until a piece of detail appearing in the overlap area of the left photograph is directly under the left lens and the same piece of detail on the right photo is directly under the right lens.

(5)   With the photograph and stereoscope in this position, a three-dimensional image should be seen. A few minor adjustments may be necessary, such as adjusting the aerial photographs of the stereoscope to obtain the correct position for your eyes. The hills appear to rise and the valleys sink so that there is the impression of being in an aircraft looking down at the ground.

(6)   The identification of features on photographs is much easier and more accurate with this three-dimensional view. The same five factors of recognition (size, shape, shadow, tone, and surrounding objects) must still be applied; but now, with the addition of relief, a more natural view is seen.


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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