Stratified sampling compared with two-phase stratified cluster sampling for timber volume estimation.

Abstract

Two sampling methods were compared on orthophotos (scale 1:10 000) of a rectangular 12 855-ha area of intensively managed Norway spruce forest in Upper Austria. Trees other than spruce >40 yr old were ignored.

'Method 1' used stratified cluster sampling to determine the area proportion of spruce in the total area: 2 clusters of 5 circular 0.05-ha photo plots were randomly selected within each of 8 rectangular sub-blocks in each of 72 blocks into which the area was 'stratified' (systematically divided). Vol. was determined by 2-phase sampling: (i) 2 clusters were randomly selected in each block, and the % crown cover of spruce was determined in each photo plot; (ii) one photo plot was selected in each block (all crown cover classes being represented) for subsequent vol. determination on 72 plots in the field.

'Method 2' used areas of mature spruce on forest management maps, which were copied on to the orthophotos and then divided into 2 nearly equal strata of higher and lower density whose areas were measured by planimeter. Ten field plots were located in each stratum and enumerated in order to determine the s.d. of each stratum and hence the number of additional random field plots required; the total number of field plots was 55.

The 2 methods required the same number of man-days (36), method 1 requiring less office work and more field work and computation. If the sampling intensity was reduced, method 1 became less time-consuming than method 2 (except in the field) for the same vol. error. However, method 2 with sampling errors only in vol., is preferred over method 1 which has sampling errors in both area and volume. (Abstract retrieved from CAB Abstracts by CABI’s permission)