islands (or donut holes)

[Virginia Jackson]

Is there a better way (than r.poly) to convert raster data to vector data 
that will preserve information about donut polygons?

We are using data in GRASS (4.1) which contain some polygons that look like 
donuts.  As long as we are in raster format, GRASS seems to handle these donuts gracefully.  After we process the data in raster format, we need to output the 
results in vector format for another system to use.  We create the vector poly-
gons from the raster polygons using r.poly.  The resulting vector polygons do 
not follow a logical pattern of what a donut would look like in vector format.  
(ex: list the outer edge of the donut, then mark the inner hole of the donut as an exception).  Instead, the vector file created by r.poly lists the outer edge of the donut as a series of lines, which when put together will make up the 
donut, but by just looking at the x,y vector data, there's no way to know it 
represents part of a donut.  So is there a better way (than r.poly) to convert 
raster data to vector data that will preserve information about these types of 
donut polygons?

Virginia Jackson
Engineering Research Associates
jackson at era.com