Vegetation mapping units (the individual patches of vegetation depicted on a map) and vegetation classification units (the vegetation type described at a given scale or hierarchy level) are not necessarily the same. A vegetation map is a symbolic representation of visually distinct groupings of plants. A vegetation classification can afford much more detail and description. Classifications can involve more floristic and structural details than those perceptible in aerial photographs, presented in digital signatures, or depicted on maps.
Another important difference between a classification and a map is the issue of scale. A map is limited by the resolution of its base imagery and by its interpretation at a certain scale, but a classification need not be. The development of a hierarchical, quantitative classification system allows the user to benefit from the knowledge that all the pieces of vegetation are arranged from the smallest identified units to the largest ones. In a system that starts with described samples of vegetation, all categories, or units, at every level in the hierarchy represent the samples according to their similarities. This approach affords a consistent interpretation at different scales driven by a consistent set of rules. Conversely, in a vegetation map, the matter of scale will always be compromised by interpretation. The cartographer cannot represent all vegetation units smaller than those of a minimum mapping unit. Some vegetation units become included within others, often in unrelated, more extensive vegetation units. In addition, different species that appear similar in aerial photographs or digital signatures merge into a single map unit, obliterating important ecological differences.
All maps present a classification in the legend, but the best maps provide a legend that summarizes an accompanying classification. The cartographer can present the mapping units elegantly as a set of increasingly generalized classes, and associated text describes the classification in detail. However, GIS or geodatabases have usurped much of a legend’s value. Software and databases can store multiple attributes for each vegetation patch, or polygon, mapped. This process enables a multitude of uses and is a main reason why recent vegetation maps are of significant value to many users from land use planners to developers and scientists.
Not all vegetation maps are of equal value. We advocate for the most detailed and most accurate data-driven maps. Those that have been poorly ground checked and are based on an unverified classification are of low value. Those developed specifically from a systematic, field-based vegetation survey and data analyses, augmenting any existing data, are usually of high value. In these integrated, data-driven approaches, new and existing vegetation information for the region is compiled in a standardized set of keys and descriptions that refine our classification knowledge at the state and national level.
In addition to vegetation information, modern GIS-based vegetation maps can provide important structural and human impact information that extends well beyond a simple map with vegetation type labels. A good vegetation map also is checked for accuracy based on a standardized and statistically valid set of independent field samples, or, if the area is small, an expert can visit each polygon on the ground, obviating the need for any further validation. For good examples of vegetation map products, see the online presentations by the CNPS and the California Department of Fish and Wildlife (CDFW) vegetation program websites (http://www.cnps.org/cnps/vegetation/; http://www.dfg.ca.gov/biogeodata/vegcamp/).