Specifically,
we firstly generated the road intersection density surfaces using road intersections
extracted from the street center line data.
Note that the road data for 2000 and 2010 were derived from the ARC 2007 street center line
data which were adjusted with the reference of the satellite
images acquired at the two years.
Secondly, we determined the threshold values iteratively that can mostly single out the urban
built-up area from the rural part for each road intersection density surface.
Any area with density values greater than the thresholds
was then grouped into the urban portion.
Finally, we combined the
generated urban portion with street centerline buffers with varying
distance values by road levels to create the complete urban masks.
The area outside the urban mask boundary was defined as the rural
subset.