The indices currently used to quantify the structure of urban
forests are not appropriate for evaluating the visual effects of urban
forests because they view urban forests from a different perspective.
Urban forests have been simplified, by researchers, to biomass
(Nowak, 1996; Peper and McPherson, 1998) or as consistent layers
of leaves and twigs cover a certain area (Scott et al., 1998).
The amount of biomass or canopy cover is usually derived from
aerial photographs or satellite images, and from field surveys in
some cases. This simplification process is necessary for studying
the instrumental functions of an urban forest. However, as pointed
out by Rowntree (1998), canopy-cover numbers averaged for entire
cities are of limited value because they do not convey the important
spatial variation in canopy cover within the city