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How maximum elevation figures are calculated for an Operational Navigation Chart

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The Calculate Maximum Elevation Figures tool computes the maximum elevation figures for an Operational Navigation Chart (ONC) based on the type of the highest vertical obstruction in a quadrangle. Two obstruction types influence the calculation: man-made and natural terrain. The calculation differs if one type is higher than the other or if the obstruction is below sea level.

Man-made vertical obstruction

If the highest vertical obstruction within a quadrangle is a man-made feature, the Calculate Maximum Elevation Figures tool follows this process:

  1. Determine the elevation of the top of the feature above mean sea level (MSL).
  2. Add the vertical error figure (vertical accuracy factor of data/source).
  3. Round the resultant sum to the next higher 100-foot level for the final figure.

Example

Elevation of vertical obstruction (top, above mean sea level)  =  6,265 ft.
Possible vertical error                                        = +  500 ft.
Sum elevation and vertical error                               =  6,765 ft.
Round to next higher 100 ft. level                             =  6,800 ft.
Maximum Elevation Figure (MEF)                                 =    68

Natural terrain feature vertical obstruction

If the highest vertical obstruction within a quadrangle is a natural terrain elevation, the Calculate Maximum Elevation Figures tool follows this process:

  1. Determine the highest natural terrain elevation.
  2. Add an allowance of 61 meters (200 feet) for nonrepresented natural or man-made features.
  3. Add the vertical error figure (vertical accuracy factor of data/source).
  4. Round the resultant sum to the next higher 100-foot level for the final figure.

Example

Highest natural terrain elevation (above mean sea level)       =    9,815 ft.
Allowance                                                      =      200 ft.
Possible vertical error                                        =    + 500 ft.
Sum elevation, allowance, and vertical error                   =   10,515 ft.
Round to next higher 100 ft. level                             =   10,600 ft.
Maximum Elevation Figure (MEF)                                 =       106

Highest feature is below sea level

If the highest vertical obstruction within a quadrangle is below sea level (BSL), the Calculate Maximum Elevation Figures tool follows this process:

  1. Add the BSL value to the 61-meter (200-foot) allowance for nonrepresented natural or man-made features.
  2. Add the vertical error figure (vertical accuracy factor of data/source).
  3. Round the resultant sum to the next higher 100-foot level for the final figure.

Example

Highest terrain elevation (above mean sea level)               =  - 360 ft.
Allowance                                                      =    200 ft.
Possible vertical error                                        =  + 500 ft.
Sum elevation, allowance, vertical error                       =    340 ft.
Even hundred - Do not round                                    =    300 ft.
Maximum Elevation Figure (MEF)                                 =     03

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