Determination of stand volume in practical forest inventories based on field measurements and photo-interpretation: The Norwegian experience

The accuracy of determination of stand volume (m(3) ha(-1)) using two practical survey methods, i.e. relascope survey and photo-interpretation, was tested on 333 forest stands of Norway spruce (Picea abies (L.) Karst.), Scots pine (Pinus sylvestris L.), and mixed stands of spruce, pine, and deciduous tree species. The material was collected at 14 different test sites in southern Norway and between 1 and 19 individual surveyors participated at each site. Reference values for stand volume of each stand were obtained from intensive field measurements. On average, the relascope surveyors underestimated the reference volume by 2% to 6%. The average standard deviation for the differences between practically determined volume and reference volume was in the range 15-31%. For stand volume determined by photo-interpretation, an average underestimation of reference volume of 4% to 38% was found. The average standard deviation for the differences was in the range 13-33%. In the relascope surveys the practically determined stand volume was predicted from mean height and basal area. On average, the effect on stand volume of bias in the stand height measurements and in the basal area measurements seemed to be equally serious.