Experience has shown that submissions often contain elevation and/or location errors. Current directives require the FAA to apply accuracy standards to obstacles when evaluating effects on instrument procedures. These accuracy standards typically require a 4D adjustment of 250 feet horizontally and 50 feet vertically to be applied in the most critical direction. Normally, these adjustments are applied to those structures that may become the controlling obstructions and are applicable until their elevation is verified by survey.

In these instances, the acceptable accuracy verification method will be requested by the FAA to pursue a favorable determination. A licensed engineer or surveyor must certify the provided survey accuracy and include the plus or minus accuracy required, as well as, the signature of the engineer/surveyor and the appropriate seal. A final determination based on improved accuracy shall not be issued until after the certified survey is received.

The following Obstacle Accuracy Codes are applied in accordance with FAA Order 8260.19J, Appendix C.

HORIZONTAL VERTICAL

Code Tolerance Code Tolerance

1 +20 ft (6 m) A +3 ft (1 m)

2 +50 ft (15 m) B +10 ft (3 m)

3 +100 ft (30 m) C +20 ft (6 m)

4 +250 ft (75 m) D +50 ft (15 m)

5 +500 ft (150 m) E +125 ft (38 m)

6 +1,000 ft (300 m) F +250 ft (75 m)

7 +1/2 NM (900 m) G +500 ft (150 m)

8 +1 NM (1800 m) H +1,000 ft (300 m)

9 Unknown I Unknown

During the aeronautical study process, the FAA may request a certified survey with an accuracy of either 1A (+20 ft horizontally +3 ft vertically) or 2C (+50 ft horizontally +20 ft vertically).