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.
The higher above ground, the closer to the conductors, and so the higher the field. But usually, over the first couple of metres or so, this is a relatively small effect. More detail on how this effect varies with distance from the line.