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一、引言公有林业机构和私有企业为了评定现在和未来林地的木材状况需进行定期清查。自三十年代以来,美国林务局的森林清查与分析组织,主要从有林地区所做的大量抽样调查中收集了很多有关林木资源的外业材料。由于1974年森林和牧场再生资源条例的通过,清查的范围已经扩大;除野生动物、牧场和林木资源之外,还包括评定分散的游憩资源。量测非林木资源特征需要的有关技术与外业林木资源调查的技术是相同的。为了有效地进行外业调查,要求这些技术易于在培训和监管很少的情况下,技术人员容易掌握。估计应该精确。与即定精度水平和成本有关的最优样木单元数,是一个很重要的考虑因素。本文介绍了一种估计植被屏蔽的方法。屏蔽可定义为,被观测的周围地面的百分数,当平行于一般地表面进行观测时,就得到近似的观测地面的百分数。屏蔽是一个重要的森林特
I. Introduction Public-owned forestry agencies and private-owned companies need regular inventories to assess the current and future woodland status of their woodlands. Since the 1930s, the Forest Inventory and Analysis Organization of the United States Forest Service has collected a great deal of field materials on tree resources from a large sample of surveys conducted in the forested areas. As a result of the passage of the Forests and Range Reclamation Resources Regulations 1974, the scope of the inventory has been expanded; in addition to wildlife, rangeland and tree resources, decentralized sources of recreation have been assessed. The techniques used to measure non-forest resource characteristics require the same techniques as the field forest resources survey. In order to conduct field surveys effectively, it is required that these techniques be readily available to technicians in situations where there is little training and regulation. Estimates should be accurate. The optimal number of wood units associated with a given level of precision and cost is a very important consideration. This article describes a method of estimating vegetation shields. Shielding can be defined as the percentage of the observed surrounding ground, which gives an approximate percentage of the observed ground when viewed parallel to the general ground surface. Shielding is an important forest special