Measurement
Stratification
Stratification divides a project area into zones that are alike inside and different from each other, by soil, slope, water, previous use or planting model, and samples each zone separately. It gives a more precise estimate from the same number of plots.
An unstratified sample averages a ridge and a valley floor into a figure that describes neither. Sampled separately, each zone gives a tight reading and the total is built from the parts.
Strata are set before sampling and kept stable between rounds, so that change can be read zone by zone.