Nearby as used in 51.100(ii) of this part is defined for a specific structure or terrain feature and
(1) For purposes of applying the formulae provided in § 51.100(ii)(2) means that distance up to five times the lesser of the height or the width dimension of a structure, but not greater than 0.8 km ( 1/2 mile), and
(2) For conducting demonstrations under § 51.100(ii)(3) means not greater than 0.8 km ( 1/2 mile), except that the portion of a terrain feature may be considered to be nearby which falls within a distance of up to 10 times the maximum height (Ht) of the feature, not to exceed 2 miles if such feature achieves a height (Ht) 0.8 km from the stack that is at least 40 percent of the GEP stack height determined by the formulae provided in § 51.100(ii)(2)(ii) of this part or 26 meters, whichever is greater, as measured from the ground-level elevation at the base of the stack. The height of the structure or terrain feature is measured from the ground-level elevation at the base of the stack.
40 CFR § 51.100
As used in this part, all terms not defined herein will have the meaning given them in the Act: