[Qgis-user] Making Maps of differences between two maps

[HPETER784 at aol.com]

Version 2.0 on Windows, just loaded normally.
Here is my problems.
I am studying the growth of a village in Somerset over the past few  
centuries.
 
I have an esri file of the roads in 2006.  I have a paper map of the  roads 
in 1969. First, I make up an layer for the roads in 1969 by deleting  
features from the 2006 layer and saving the result in the 1969 roads layerTo get  
a difference map, that is (say) the 1969 roads in black and the new (1969 
to  2006) roads in red, I map the 2006 map in red, then overlay the 1969 in  
black.  But the 'black' is not black, but some combination of red and  
black.  I have tried the options in Layer blending mode (page 73-74)  without 
success.  Question One is  - how do I overlay one colour over  another and 
still retain that colour?
 
Another way I tried was this.  From the attribute table for 2006 road  map, 
I selected those features corresponding to the new roads (page 105-106  in 
the User Guide) and saved them as a new layer , the difference layer.   Then 
just mapped the 1969 map and this new layer.  This worked.   Unfortunately, 
I missed a few of the new roads, so I want to select more  features from 
the 2006 layer and add these to the already existing difference  layer.  I 
cannot find out how to do this, that is add features to an  already existing 
layer.  Question Two is; how can one add features to  an already existing 
attribute table?
 
 I suppose all this is quite simple, but I am a beginner. 
 
Peter,
15  Woodbarton,
MILVERTON,
Taunton TA4 1LU
01823 400  272
hpeter784 at aol.com
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