Parent Topic: <A href="http://www.pcigeomatics.com/cgi-bin/pcihlp/RTV">RTV</a><hr>
<H3>VECTOR DATA</H3>
Vector data consist of lines and points. A line consists of a list
of (X,Y) coordinates called vertices. Vertices are joined by
straight line segments called vectors (edges). Points consist of a
single (X,Y) coordinate.<p>
The vector data generated by RTV has the following characteristics:
<P>
<UL>
<LI> The (X,Y) coordinate system is the georeference system from
segment 1 on the input database. 
<LI> Bounding lines are along edges of pixels. This results in
90-degree corners and step effects along polygon boundaries. 
<LI> Boundaries between polygons are represented exactly once. 
<LI> Polygons are 4-connected. 
<LI> Lines always terminate at vertices where three or more vectors
terminate (sometimes referred to as nodes.) 
<LI> Lines are tagged with attribute values of 0. 
<LI> Exactly one interior point is generated for each polygon and the
grey level value for the polygon is used as an attribute. 
<LI> Interior points tend to be generated along the bottom left of
polygons rather than the polygon centre. This is a result of the
polygon identification algorithm used. 
</UL>
The following example shows the vector data generated for a 3x3
raster image with borders along the image edge.
<P>
<PRE>
                 +-------+---+    +-------+---+                
  1   1   3      |       |   |    |     X |   |          X      
                 |   +---+   |    |   +---+   |                
  1   9   3      |   |   |   |    |   | X | X |          X   X
                 +---+---+   |    +---+---+   |                 
  9   3   3      |   |       |    | X |       |      X          
                 +---+-------+    +---+-------+                  

 Raster data         Lines        Lines and Points      Points

  1,3,9         -&gt; raster grey level values
  +----+        -&gt; vectors
  +             -&gt; vertex
  X             -&gt; interior point
</PRE>
The following lines would be generated:
<P>
<PRE>
 Line 1:  (2,0),(2,1)
 Line 2:  (0,2),(0,0),(2,0) 
 Line 3:  (0,2),(1,2)
 Line 4:  (1,2),(1,1),(2,1) 
 Line 5:  (2,1),(2,2),(1,2) 
 Line 6:  (0,2),(0,3),(1,3) 
 Line 7:  (1,2),(1,3)
 Line 8:  (2,0),(3,0),(3,3),(1,3)
</PRE>
The following points would be generated:
<P>
<PRE>
 Point 1:  (1.5,0.5)    Attribute value: 1
 Point 2:  (1.5,1.5)    Attribute value: 9
 Point 3:  (0.5,2.5)    Attribute value: 9
 Point 4:  (2.5,1.5)    Attribute value: 3
</PRE>
Note that the two pixels with a raster value of 9 are considered as
two separate polygons even though they `touch' diagonally. This is
because RTV uses 4-connectedness for determining polygons (i.e.,
touching left-right or up-down but not diagonally).<p>
TOPOLOGY and GIS<p>
One of the major uses of RTV is to convert raster data to vector
form in preparation for the export of the image channel (coverage)
to a vector GIS system. Typically, the image channel contains the
final result of processing rather than raw imagery.<p>
To fully capture the information content of the image, it is
important that every polygon be completely enclosed by lines and
that each polygon have a single point in its interior tagged with
the grey level for that polygon. A collection of vector data of this
sort is often called a topology.<p>
If RTV is run with POLYINFO = BOTH and BORDER = ON, then the vector
data generated is guaranteed to be a topology completely describing
the input image. Note, however, that it is possible to destroy the
topology if the vector data is modified by another PACE program
(e.g., VED or VECMERG).
<P>
<hr>
Parent Topic: <A href="http://www.pcigeomatics.com/cgi-bin/pcihlp/RTV">RTV</a><br>
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