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Upper Stem Diameter and Volume Prediction Strategies in National Forest Inventory (NFI)

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Title: Upper Stem Diameter and Volume Prediction Strategies in National Forest Inventory (NFI)
Author(s): Stenman, Virpi
Contributor: University of Helsinki, Faculty of Agriculture and Forestry, Department of Forest Sciences
Discipline: Forest Resource Science and Technology
Language: English
Acceptance year: 2019
In forest inventories, the field data is needed as a reference data, calibration and for assessing the accuracy. Gathering the field data needs resources, such as work forces, equipment and data management operations. This means that time and budget as well as quality must be carefully considered when National Forest Inventory activities are to be planned. Therefore, the development of cost efficient, simpler, safer and more accurate and reliable field data measurement methods and tools are topics of great interest. One of the field measurement variables is the upper stem diameter, which has been part of the NFI field data set until the 11th NFI. The measured upper stem diameter helps to produce more reliable tree stem volumes with 3-parameter Laasasenaho volume model. However, only if the quality of the measurements is on an adequate level. The upper stem diameter has been traditionally measured with parabolic caliper assembled in the top of the 5-meter aluminium rod. This equipment combination takes time to assemble and disassemble, it is not very compact to be carried around as well as it can be a health and safety issue during the thunderstorm. Therefore Criterion, laser-based dendrometer performance was further examined in this study as an optional measurement equipment for future upper stem diameter field measurements. The Criterion upper stem as well as dbh measurement precision was analysed based on the 326 sample tree measurements with Sonar, Caliper and Criterion. The standard error for Criterion was 17,26 mm in dbh measurements for all species and 10,36 mm in d6 measurements for all species. The reference standard errors from earlier studies were 2,70 mm for dbh with Steel Caliper and 7,00 mm for d6 with Caliper. When analysing the Criterion performance with reference to mean of the measurements, the standard error in dbh measuements for all species was 9,72 mm and for d6 measurements 7,07 mm. Furthermore, the accuracy and precision were analysed with Bland-Altman technique, which is a suitable method for comparing two measurements of the same variable when both have some errors. The Bland-Altman results supported the earlier findings. Within and between observer analysis showed that the impact in measurement accuracy or precision is not caused by the observers. The comparison of the sample tree volumes produced the results that the relative standard error was increased by 2,13 % for all species when 2-parameter volume model was compared with 3-parameter model. Likewise, the relative bias was increased by 1,53 %. In efficiency experiment the scenario where only one sample tree with measured d6 was chosen from each of the sample plot and dbh percentile of p70 was providing most accurate and precise sample plot volumes with RMSE of 3,92 m3/ha and bias -0.78 m3/ha. The results show that there is a real challenge to achieve reliable and accurate upper stem diameter measurements and therefore new measurement methods need to be further studied and analysed.
Keyword(s): measurement errors forest inventory accuracy mittausvirheet metsän inventointi tarkkuus

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