Training Network: Wilderness Survival | Fitness Training  
Fitness Training
 

The Sport:

  • Orienteering History
  • Orienteering Overview
  • Course Setup
  • Officials
  • Start/Finish Areas
  • Course Safety
  • Control Point Guidelines
  • Map Symbols
  • Orienteering Techniques
  • Civilian Orienteering

    The Skills:

  • Maps
  • Marginal Information and Symbols
  • Grids
  • Scale and Distance
  • Direction
  • Overlays
  • Aerial Photographs
  • Navigation Equipment and Methods
  • Elevation and Relief
  • Terrain Association
  • Navigation in Different Types of Terrain

  • Field Sketching
  • Map Folding Techniques
  • Units of Measure and Conversion Factors

  • Scale Determination

    Before a photograph can be used as a map supplement or substitute, it is necessary to know its scale. On a map, the scale is printed as a representative fraction that expresses the ratio of map distance to ground distance, For example:

    RF = MD
    ——
    GD

    On a photograph, the scale is also expressed as a ratio, but is the ratio of the photo distance (PD) to ground distance. For example:

    RF = PD
    ——
    GD

    The approximate scale or average scale (RF) of a vertical aerial photograph is determined by either of two methods; the comparison method or the focal length-flight altitude method.

    a.   Comparison Method. The scale of a vertical aerial photograph is determined by comparing the measured distance between two points on the photograph with the measured ground distance between the same two points.

    SCALE (RF = Photo Distance
    ————————
    Ground Distance

    The ground distance is determined by actual measurement on the ground or by the use of the scale on a map of the same area. The points selected on the photograph must be identifiable on the ground or map of the same area and should be spaced in such a manner that a line connecting them will pass through or nearly through the center of the photograph (Figure 8-8).

    Figure 8-8. Selection of points for scale determination.

    Figure 8-8. Selection of points for scale determination.

    b.   Focal Length-Flight Altitude Method. When the marginal information of a photograph includes the focal length and the flight altitude, the scale of the photo is determined using the following formula (Figure 8-9).

    Figure 8-9. Computation of scale from terrain level.

    Figure 8-9. Computation of scale from terrain level.

    When the ground elevation is at sea level, H becomes zero, and the formula is as shown in Figure 8-10.

    Figure 8-10. Basic computation of scale from sea level.

    Figure 8-10. Basic computation of scale from sea level.

    Back to Aerial Photographs

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