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Fig. 1. Study area of Lapinjärvi research forest, located in southern Finland. Map derived for countries 1:1 M (a)/© QGIS; Back-ground map for sample plot area of interest 1:1 M (b)/© National Land Survey of Finland/ WMS open data licensed under a CC 4.0 license.

Table 1. Quantity of measured sample trees by measurement instrument, measured variable, observer, and tree species. Measured variables are diameter at breast height (dbh) and upper stem diameter (d6).
Instrument Sonar Criterion Calliper
Variable dbh dbh d6 d6
Observer 1 or 2 1 2 1 2 1 2
Scots pine 112 108 107 108 107 59 49
Norway spruce 142 130 101 130 101 52 78
Birch 64 61 35 61 35 25 35
Aspen 4 4 4 4 4 3 1
Grey alder 4 3 3 3 3 1 2
Sample trees total 326 306 250 306 250 140 165
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Fig. 2. Criterion RD 1000 instrument setup options for the diameter measurement bars (Laser Technology 2006).

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Fig. 3. Bland-Altman plots for diameter at breast height (dbh) to visualise the standard deviation of the differences. Differences between measurement instruments are compared to the mean value of all measurements and are visualised for the different tree species. View larger in new window/tab.

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Fig. 4. Bland-Altman plots for upper stem diameter (d6) to visualise the standard deviation of the differences. Differences between measurement instruments are compared to the mean value of all measurements and are visualised for the different tree species. View larger in new window/tab.

Table 2. Standard deviation for diameter at breast height (dbh) and upper stem diameter (d6) for Criterion with reference to earlier research with a steel calliper. Number of paired observations is n, maximum standard deviation for Criterion is Supper, referenced standard deviation for Calliper is Sref and Criterion standard deviation is SCRI. Measurement unit is shown in millimetres.
Species variable n n (outliers) Supper Sref SCRI
All dbh 553 3 19.96 6.90 18.73
Scots pine dbh 215 0 16.40 6.90 14.87
Norway spruce dbh 231 0 21.50 6.90 20.36
Birch and other dbh 107 3 22.82 6.90 21.75
All d6 519 4 16.88 7.00 15.36
Scots pine d6 205 0 11.39 6.00 9.68
Norway spruce d6 214 0 16.51 7.00 14.95
Birch and other d6 100 4 20.05 7.00 18.79
Table 3. Standard deviation and bias for Criterion with reference to the mean measurement value. Diameter at breast height is dbh, upper stem diameter d6, number of paired observations is n, standard deviation is SCRI and measurement unit is shown in millimetres.
Species variable n SCRI SCRI (%) bias bias (%)
All dbh 556 12.21 4.99 –3.31 –1.35
Scots pine dbh 215 10.67 4.40 –0.96 –0.40
Norway spruce dbh 231 12.29 4.76 –4.11 –1.59
Birch and other dbh 110 14.64 6.62 –6.20 –2.81
All d6 519 8.88 4.47 –2.44 –1.23
Scots pine d6 205 7.73 4.01 –1.73 –0.90
Norway spruce d6 214 9.45 4.33 –3.27 –1.50
Birch and other d6 100 9.78 5.79 –2.12 –1.25
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Fig. 5. Boxplots visualizing the data distribution of diameter at breast height (dbh) measurements and for different tree species. O1 = Observer 1 and O2 = Observer 2. View larger in new window/tab.

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Fig. 6. Boxplots visualising the data distribution for the upper stem diameter (d6) measurements and for different species. O1 = Observer 1 and O2 = Observer 2. View larger in new window/tab.