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AC 150/5300-4B CHG 3 12/5/78 Appendix S * c. After tentative selection of a site through visual application of appropriate criteria contained herein, make a thorough magnetic survey of the site. This is necessary because many sites which meet all visually applied criteria regarding distances from structures, etc., still are unsatisfactory because of locally _generated or natural magnetic anomalies. At locations near heavy industrial areas, intermittent magnetic variations may be experienced and sufficient surveys at various periods of time are necessary to ascertain if this situation exists. d. The difference between magnetic and true north must be uniform in the vicinity of the site. Make sufficient surveys to determine that the angular difference between true and magnetic north measured at any point does not differ from the angular difference measured at any other point by more than one-half degree within a space between 2 and 10 feet (0.6 and 3 m) above the surface of the base and extending over an area within a 250-foot (75 m) radius from the center. 6. CONSTRUCTION OF COMPASS CALIBRATION PAD. For pavement construction, the applicable portions of AC 150/5320-6 should be used. The following additional information is considered important: ' a. Do not use magnetic materials, such as"reinforcing"steel or ferrous aggregate, in the construction of the calibration pad or of any pavement within a 300-foot (90 m) radius of the center of the site. If drainage pipe is required within 300 feet (90 m) of the center of the site, use nonmetallic or aluminum culvert. Appropriate quality control procedures must be employed during the construction phase to insure that only nonferrous materials are used throughout b. Each of the radials must be oriented within one minute of the magnetic bearing indicated by its markings. c. Mark the date of observation and any annual change in direction of magnetic north durably and legibly on the surface of the calibration pad near the magnetic north mark. It would be well to establish a permanent monumant at some remote location on the true north radial for future reference. d. The U.S. Geological Survey of the Department of Interior is available to conduct the necessary surveys to determine the difference between true and magnetic north and the uniformity of this difference. The cost for this service is that necessary to cover the expense to the U.S. Geological Survey. Requests for this service should be made to the following: Branch of Electromagnetism and Geomagnetism, U.S. Geological Survey, Mail Stop 964, Box 25046, Denver Federal Center, Denver, Colorado 80225, Telephone (303) 234-2588. There are also many other competent registered surveyors or engineers who are capable of performing these, surveys. It is recommended that a Page 4 Par 5