•Duplicate geometry is impossible to see and creates "stacked" lines and invalid topology. It can cause the stream ordering algorithms to fail and abort.


•Such duplication is different to double digitised or polylines within polylines as these types of errors are either partial overlaps or weave on and off each other.


•A brute force test for duplication would require RivEX to test every polyline to see if it is overlapped by another polyline. This becomes a computationally heavy task for large river networks (>500K polylines) and very long run times would be required (several hours on a fast computer). To overcome this RivEX does a first pass through the data sorting polylines by their number of vertices and length. This significantly reduces the number of overlap tests that RivEX would need to do. Run times are typically less than a minute.


•This first pass could be considered "sub-optimal" because you could potentially have what are visually identical lines with the same lengths but with different number of vertices. The benefits of very fast processing versus extremely long run times (hours crushed into minutes) outweigh this limitation.


•The error log file is called DuplicatePolylines.txt and will list the polyline ID's of any duplication found.