Spatially Balanced Sampling: Application to Environmental Surveys

Spatially balanced sampling is becoming a popular design for surveys in biological and environmental management. For large scale surveys, where the region of interest is too large to visit every site, a sample is taken from a selection of sites. The process used to select these sites is called the survey design. Spatially balanced designs ensure there is spatial coverage of the entire survey area. The resultant sample should be representative of the population of interest.One of the first and the most commonly used spatially balanced design is called GRTS (Generalized Random Tessellation Stratified sampling) where sample effort is spread evenly over the target region. The term “spread evenly” in this context means having coverage of survey effort over the region. The coverage from GRTS has a stochastic component rather than a fixed interval, regularly spaced coverage as in a systematic sampling design.We have extended the idea of GRTS to a new design called Balanced Acceptance Sampling (BAS). The BAS design allows surveys to be balanced in dimensions higher then two (n-dimensional space). Until now, most designs have considered balance in 2-D geographic space. With BAS we can achieve balance in 3-D space (e.g., water bodies in marine surveys), or in higher dimensions. In ecological surveys BAS can be used to ensure balanced coverage over both the geographic space and dimensions related to factors such as ecological threat, conservation status, species population structure, and the time interval of repeat site surveys