# How maximum elevation figures are calculated for an Operational Navigation Chart

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:

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

A resultant figure is not rounded up to the next higher 100-foot level when it is an even multiple of 100.

### Example

Elevation of vertical obstruction (top, above mean sea level) = 6,265 ft. Possible vertical error =+ 500ft. Sum elevation and vertical error = 6,765 ft. Round to next higher 100 ft. level = 6,800 ft. Maximum Elevation Figure (MEF) = 6^{8}

## 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:

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

A resultant figure is not rounded up to the next higher 100-foot level when it is an even multiple of 100.

### Example

Highest natural terrain elevation (above mean sea level) = 9,815 ft. Allowance = 200 ft. Possible vertical error =+ 500ft. Sum elevation, allowance, and vertical error = 10,515 ft. Round to next higher 100 ft. level = 10,600 ft. Maximum Elevation Figure (MEF) = 10^{6}

## 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:

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

A resultant figure is not rounded up to the next higher 100-foot level when it is an even multiple of 100.

### Example

Highest terrain elevation (above mean sea level) = - 360 ft. Allowance = 200 ft. Possible vertical error =+ 500ft. Sum elevation, allowance, vertical error = 340 ft. Even hundred - Do not round = 300 ft. Maximum Elevation Figure (MEF) = 0^{3}