Generally qualitative, but can be quite precise if implemented according to a rigorous protocol
Boundary mapping involves defining (i.e., mapping) the boundary of something in the field (e.g., a patch or population of some species). This method is typically implemented as part of an assessment or monitoring of rare or invasive species to document their extent and track how it changes over time. Boundary mapping works best when patches or populations can be easily defined.
A number of techniques exist for mapping population or patch boundaries in the field. Before the advent of GPS technology, boundaries were usually estimated on a topographic map or aerial photograph. A common approach now is to use a GPS to record the vertices of a polygon defining the patch/population. In some cases, patches can be defined from aerial photographs.
The main advantages to boundary mapping are that it is relatively quick and simple to implement, and it can provide direct information on the change in distribution of the population/patch over time.
This method, though, has some significant limitations which must be considered. Boundary mapping can be very challenging to implement if the species of interest is very widespread over a large area. In attempting to map the distribution of yellow starthistle (Centaurea solstitialis) on The Nature Conservancy's Garden Creek preserve in Hells Canyon, Idaho using a boundary-mapping technique, we found that it was easier to map where starthistle wasn't than to define boundaries for where the populations were.
For many species, it is often not clear where the edge of the population or patch is. A species may be present at high densities in the center of a population but the density gradually decreases from the center with no sharp edge to the population/patch. In this case, decisions need to be made as to where the edge of the patch should be located to ensure repeatability. Cryptic species or those at very low densities also pose a problem for defining boundaries because they are easy to miss.
A second issue with boundary mapping that must be considered it what differentiates one population or patch from another. Is the entire contiguous area where a species occurs considered a population/patch? How big of a separation must there be between populations/patches? Is there a maximum size that will be allowed for a population/patch. For invasive species assessment and monitoring, a common rule of thumb is that a patch
Boundary mapping is one of the only field methods that seeks to measure and use the spatial distribution of a species for assessment and monitoring. From a remote-sensing perspective, though, Boundary mapping is similar in objective to several remote sensing techniques like Target Detection/Extraction and Change Detection.
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