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an interactive mapping system there is no conflict between the two 
claims for detailed discernability and realistic overall impression. In 
fact, it can always neglect the former: Detailed information recovery, 
if needed, can be offered to the user in the form of an interactive le 
gend instead (see next section). 
6. INTERACTIVE LEGEND PROBLEMS AND THEIR SOLUTION 
Interactive computer-assisted cartography differs from traditional carto 
graphy by the fact that map producer and map reader are one and the same 
person. As a consequence, MORRISON's (1978) distinction between a map 
creation transformation and in inverse map reading transformation (at 
least as far as it goes back to the data base) becomes blurred. The user 
knows the data he started from or else he can recover them by means of 
an interactive legend at any time. 
Let us look at some desirable components of an interactive legend facili 
ty, again in the context of a CRT display system (compare with STEINER 
198lb). We utilize the relational model once more to demonstrate pos 
sible conceptual solutions. What are the questions a user may possibly 
want to ask? We may distinguish two basic groups, one associated with 
analog-to-digital and one with digital-to-analog problems. 
1) Analog-to-digital: 
Given the displayed map M(m,s) there are two kinds of elements, name 
ly m and s, for which the user may want to recover some associated 
digital information. However, since each s on the map is always rela 
ted to at least one location m, he can use m as a starting point un 
der any circumstance and work forward through the model as is desi 
rable. Two questions which make sense are: 
a) Given a particular map location m, what is the geographical unit 
u of which m is a part? (in other words: The user looks at a par 
ticular geographicl unit shown on the map and wishes to know what 
its number is.) This is the problem of geographical unit identifi 
cation. 
b) Given a particular symbol (color tone) s occurring at some map lo 
cation m, what is the attribute class c it represents? (The user 
looks at a particular geographical unit on the map with a particu 
lar color tone which he cannot identify without doubt; he there 
fore wishes to establish the class associated with it.) This is 
the problem of attribute class identification. (A related similar 
problem would be to find the original attribute value x.) 
2) Digital-to-analog: 
Given some digital information associated with elements of the model 
interior, the user wishes to know where on the map the corresponding 
analog information appears. Problems of this kind can be solved by 
entering the model at the desired element position and working 
through to the end. Thereby one makes use of the VLT facility in the 
following way: At the address location(s) concerned the present sym 
bol code is replaced temporarily by some legend code which produces 
a visually striking symbol, for example, an intermittent white. Call 
the (partially) new contents of the VLT g' and the resulting symbols 
s'. We then have temporarily a new map M(m,s') which differs from the 
map under scrutiny M(m,s) locally. There are two specific questions 
of this kind which deserve attention: