CAJ | 학술논문

为定量研究照相法测量规则林地覆盖度中采样策略、相机非垂直拍摄等因素对覆盖度计算结果的影响，该文对河北怀来的一片梨树林进行了实地测量，通过对测量数据的统计计算，利用植被结构生成软件 Onyx tree professional重建梨树林场景，采用基于物理的光线追踪算法模拟相机拍摄的照片，提出了规则林地覆盖度照相法测量的采样方案，并通过计算机模拟检验了采样方案。通过精确控制模拟相机的拍摄条件，进一步分析了非垂直拍摄对测量精度的影响。结果表明，相机的非竖直拍摄对覆盖度计算结果的精度有较大的影响，误差最大可达27.9%。该文提出的测量方案为规则林地的覆盖度实地测量提供了参考。
위정량연구조상법측량규칙임지복개도중채양책략、상궤비수직박섭등인소대복개도계산결과적영향，해문대하북부래적일편리수림진행료실지측량，통과대측량수거적통계계산，이용식피결구생성연건 Onyx tree professional중건리수림장경，채용기우물리적광선추종산법모의상궤박섭적조편，제출료규칙임지복개도조상법측량적채양방안，병통과계산궤모의검험료채양방안。통과정학공제모의상궤적박섭조건，진일보분석료비수직박섭대측량정도적영향。결과표명，상궤적비수직박섭대복개도계산결과적정도유교대적영향，오차최대가체27.9%。해문제출적측량방안위규칙임지적복개도실지측량제공료삼고。
Digital photography is now the most widely used method to obtain the vegetation cover in field measurements. It uses a digital camera to shoot the research area straight downward, classify the digital images into plants and soil, and calculate the percentages of the pixel numbers of plants. When measuring the vegetation cover of regular planted woodland with digital photography, the accuracy of the calculated vegetation cover is affected by following factors such as the sampling strategy and non-vertical shooting of digital camera. However, it is difficult to analyze these factors mentioned above quantitatively in situ measurement because there are lots of uncertain errors involved. In this study, we used the computer simulation method to evaluate these factors because in a computer simulated scene, we can strictly control all these uncertainties. We measured a regular planted pyrus orchard to get the structural data in Huailai (40.373°N, 115.723°E), Heibei province. Through statistics of the measured structural data, a regular planted pyrus orchard scene was reconstructed with the plants structural generating software, onyx tree professional which is widely used in forest management, plants reconstruction, and remote sensing of plants. Then the physically based ray-tracing (PBRT) algorithm was used to simulate the illumination conditions of the reconstructed scene and a virtual camera was simulated to get the images taken by a digital camera. The virtual camera settings are just like the digital camera when we measure the vegetation cover in situ. By controlling the shooting conditions strictly within the computer model, we can obtain simulated images under different shooting conditions. The supervised classification method was used to classify the simulated images to obtain the vegetation cover values. Due to the obvious three different samples in the regular planted woodland, this study proposed an area-weighted sampling strategy to calculate the vegetation cover of regular planted woodland. The strategy was then validated in the reconstructed scene using a computer simulation method. It showed that the area-weighted sampling strategy can obtain the vegetation cover of the regular planted woodland accurately, and the relative error is about 7.3% in this simulated pyrus scene. Furthermore, the study analyzed the influence of the non-vertical shooting. We simulated different images taken by the digital camera of these three samples and the viewing zenith angle of digital camera was from-20°~20°. The results showed that the non-vertical shooting had a great impact on the accuracy of the calculated vegetation cover. The relative error of vegetation cover can reach 98.4% for one sample and 27.9% for the calculated vegetation cove of the regular planted pyrus scene. The computer simulation method presented in this paper was demonstrated to be feasible and objective for analyzing the accuracy of digital photography. An area-weighted sampling strategy to calculate the vegetation cover of regular planted woodland and the non-vertical shooting were evaluated in this paper. The results can provide guidance for obtaining the FVC in field measurements using digital photography. Besides, the computer simulation methods came up with in this paper can be used to evaluate the accuracy of other instruments such as LAI2000, or TRAC in following studies.