Take measurements to ensure dose hniv rate is representative over a scale of up to 5-10 m. Normally, a single dose rate measurement(e.g. Mini 6-80) will be representative at this scale, although dose rate can be measured at 2-3 locations at distances of 10 m Y apart over the same ground type and an average result reported.
Note that where geology is changing rapidly, it may be difficult to choose a reference background dose rate for comparison.