This page deals with the effect of clearance on magnetic fields. See the companion page for the effect on electric fields.
The actual field produced by an overhead line depends on several factors. This page illustrates this for one standard line, a 400 kV L12 transmission line with typical loads. Our detailed calculations of fields all specify the conditions they were calculated for.
Both electric and magnetic fields depend on the clearance of the line. The minimum ground clearance of a 400 kV line is 7.6 m, dropping to 5.2 m for low-voltage distribution lines. See a full listing of minimum clearances for high-voltage lines under different circumstances. But in reality it is rare for lines to be this low, and the ground-level field falls rapidly with the height of the line above ground.
The maximum fields that are produced by a line occur directly underneath the line, underneath the lowest point of the conductors, which is usually towards the middle of each span. Actual conductor clearances above ground would generally be higher than this (and therefore the fields produced near ground level would be lower) for two main reasons. Firstly, for most of the length of a span, the conductor clearance is higher than it is at the lowest point. Secondly, the actual ground clearance of the conductors depends on their temperature. For the vast majority of the time they operate at less than their rated maximum temperature and therefore sag less, resulting in higher ground clearances.