Results
The most prevalent composite class in Philadelphia contains trees, grass, paved surfaces, and low rise buildings (‘tgpl’) (Table 1). The ‘tgpl’ class accounts for about 57% of total city area and can be found in all parts of the city and was largely homogenous in spatial distribution (Figure 1A). The second largest class, ‘tgplm’ at 8.5% of the area, which is similar to ‘tgpl’ except it includes midrise buildings, is concentrated in the center of the city and along a few main corridors to the North and West. STURLA classes were able to identify the role of urban structure influencing ST (Figure 1B). Classes generally hosted ST that were unique (Figure 1B) and significantly different (Table 2) compared to all other classes with the exception of ‘tgbp’ with similar ST values to ‘tgwp’ and ‘tgwpl’.
The prevalence and distribution of the STURLA classes in Philadelphia differs from what we found in previous studies of urban structure NYC and Berlin (Figure 2). In Berlin and NYC, ~1/3 of the landscape can be explained by one highly composite STURLA class. Another difference between the results in Philadelphia and previous studies is the number of classes that cumulatively explain 90% of the area of the city. Ten classes covered 90% of the area of Philadelphia while the same number of classes only covered 79% of the area of New York City and 68% of the area in Berlin. Despite these differences, pairwise comparison of each city revealed that STURLA class proportions were not significantly different between the cities (all p>0.05) Still, Berlin and NYC were highly correlated (r2=0.952, p<0.05) while Philadelphia remained unassociated to the other cities (both r2> 0.1, p>0.05).
Due to the compositional nature of a STURLA cell where the relative proportions of all elements sum to one Figure 2 shows provides an example compositional variability within the most common class in Philadelphia ‘tgpl’ using six grid cells taken from a larger city-wide random sample. The different grid cells and corresponding satellite imagery show the different types of buildings and proportion of each element of the class, trees, grass, paved surfaces, and low-rise buildings, can vary greatly from one another but still fall into the class. Most grid cells from the ‘tgpl’ class show row houses or single-family detached houses since they fall within the size parameters of low-rise buildings (1-3 stories).