Developing and accuracy analysis of portable device for automatically measuring diameter at breast height and tree height

Abstract: Diameter at breast height and tree height are the foundations of evaluating site quality and tree's growing situation. A portable mini multifunctional smart station has been developed based on the image automatic recognition theory, the photogrammetry theory, the similar triangles theory and the triangular function theory, which can measure the diameter at breast height and tree height. Simultaneously, this device integrates charge coupled device (CCD) image sensor with inclinometer, central processing unit, memory, liquid crystal screen, power supply and laser ranging sensor. After assembling the components, the station can be used to automatically measure the diameter and tree height accurately. It can be used to observe the plot after being mounted on the holder. During the measuring process, light spot needs to be aimed at the position of the diameter at breast height that is to be measured. The CCD sensor can automatically transform the light into electric charge signal and then into digital signal by the inner analog digital converter (ADC). After the hue correction and white balance processing by the digital signal processor, the digital signal is finally transformed into the visual image on the screen. When measuring, it can acquire 2 values, namely the inclination and the distance from the measuring point to the point needing measuring. Moreover, it can record and save the image information measured. The measurements of forest stand mean height, volume of wood and tree number density can be realized on the above basis. This device realizes the automatic measurement of the diameter at breast height, tree height and diameter at any part, and the basic measurement, a total of 4 functions through its imbedded running program. The distance measuring range for this device is between 0.05 and 120 m, and its effective reflection distance is between 0.05 and 100 m. The minimum display value is 0.001 m and its measuring precision is ±1.5 mm. The measuring range of the micro-electromechanical systems (MEMS) inclination sensor is between -90° and +90° and its effective measurement range is between -75° and +75°. The measuring precision for the inclination is 0.1°. The diameter at breast height precision that the CCD calculates and stores is 1 mm. The temperature when this device works is between -10℃ and +50℃. The size of this device is 136 mm×52 mm×28 mm. Its length is 136 mm and its width is 52 mm. Its height is 28 mm. The continuous measurement time of this device is 5 000 s and its regular work time is 5 h. Testing data show the conditions of the mini smart station and the work mode of measuring the diameter at breast height and tree height. The best measurement distance for the diameter at breast height is between 5 and 20 m based on the analysis of testing data. And the effective measurement range for the tree height is between zero and 52 m. The absolute average error of the diameter at breast height is 0.147 cm, and the relative average error of the diameter at breast height is 1.26% according to 60 tests in the experiment. The average relative error for the tree height is 1.13%. These measurement precisions conform to the requirement on diameter at breast height and tree height measurement of the national forestry continuous investigation.