59 
<nown, but 
ine process- 
this re- 
is the land 
se polygon, 
which might 
, such as 
Darks -- 
Dts may be 
sa types 
h enough 
d-use map 
sistently 
nt accuracy 
ownership 
eneral 
11y con- 
equency. 
potential 
use data 
In this, 
by census 
ation zones, 
ed following 
f the classi- 
puter-aided 
functions 
within the 
ered. 
East and 
as gained in 
nnecticut. 
d West affect 
ati ve cover, 
s, is a pro 
weathering 
On the positive side, certain surfaces which are a product of their 
environment form highly reliable spectral signatures. A good example is 
illustrated in figure 2 of Terre Haute, Indiana where the old Central Business 
District (C.B.D.), with its weathered (and soot encrusted) roof surfaces form 
a cluster quite unlike anything else in the scene, either rural or urban. The 
semi-circular adjacent ring of older residential land is also a distinctive 
signature thanks to weathered surfaces, a uniform density of houses, and a 
similar degree of landscaping, all of about the same age. The fact that hous 
ing density is high enough for individual picture elements to have a pre 
dominance of man-made surfaces is a further advantage. 
The character of the natural environment may also be employed to advantage 
in arid areas. In Phoenix, Arizona the scanner/computer combination correctly 
identified single picture element size (.46 hectare) parcels of desert land 
within generally built-up areas. Somewhat startling was the "discovery" of 
numerous non-vegetated vacant lots within what was generally thought to be a 
continuously built-up string commercial street development. Subsequent 
ground checking proved the identifications correct. 
On the negative side, the principal problem faced by the spectral inter 
preter is how to distinguish between a man-made land use or land cover and the 
natural cover to which it is most closely related in the natural environment 
in which it is found. In an eastern city, for example, problems are met in 
low density residential areas which are heavily wooded. During full foliage 
they are nearly indistinguishable from uninhabited forest, and even in winter 
the exposed surfaces of roof tops and driveways is either relatively too small 
in area or too similar spectrally to surfaces found in nature; commonly, a 
forest floor covered with highly reflective fallen leaves from broadleaf 
deciduous trees behaves spectrally like bright man-made surfaces. In Western 
desert areas, a flat factory or warehouse roof and its parking lot constructed 
from local building materials may have nearly exactly the same spectral re 
sponse as those materials in the natural environment. 
ACKNOWLEDGEMENTS 
The work reported on was undertaken by the United States Geological 
Survey under a contract with Purdue University. Publication of this paper 
is authorized by the Director, U.S. Geological Survey. 
mces in 
ze on advan- 
and apprecia- 
;hey behave in