ROAD 
  
  
  
  
  
  
  
  
  
  
  
  
  
HOUSE / 
SPOT 
ET : 
PARCEL LANDSAT TM 
  
  
  
  
  
L di i J 
0 10 20 30 
METRES 
FIGURE 2: Dimensions of residential features (Sydney) compared 
with Landsat TM and SPOT pixel sizes. 
(b) traditional clustering techniques for surface classification may also 
become more appropriate because within pixel homogeneity will increase, 
(c) textural type variables will have an increased potential in surface cover 
prediction and housing density studies. 
This latter point will be expanded further. For a homogeneous extended surface 
the response at a sensor will show little variation. Homogeneity however is 
dependent on both pixel size and the size of the elements being viewed. 
Variance over a pixel neighbourhood (say 3 x 3) will increase as the pixel 
size approaches the size of the elements and will then reduce as the size of 
the pixel continues to decrease. This suggests that the resolution of Landsat 
D TM and SPOT (in particular) is approaching the optimum for maximum textural 
discrimination of surface features typically found in low density cities. It 
necessarily follows then, for a constant pixel size, that as housing density 
varies from very low to very high, variance will pass through a maximum, as 
shown schematically in Figure 3. 
The increased number of spectral bands available on Landsat D TM, will also have 
advantages in urban monitoring, because of the increased numbers of predictive 
surface equations or cluster dimensions that can be formed. Their better 
placement and increased radiometric resolution will allow better vegetation 
discrimination but in addition should have a number of advantages for other 
urban surface materials. While further research is required to determine urban 
Spectral response in the new bands, the following tentative conclusions are put 
orward. 
(a) Concrete surfaces should remain high in the 1.55-1.7 um band (TM 5) (in 
a similar way to hydrothermally altered rock) and give enhanced differen- 
tiation with vegetation that has a lowered (but local maximum) response. 
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