The next upstream flowline is identified as “47” in the UPHYDROSEQ - (Upstream Mainstem Hydrologic Sequence Number) - field. Somewhere in the middle of that stream a particular flowline with the HYDROSEQ value of “45” has been identified. To illustrate how UPHYDROSEQ and DNHYDROSEQ work, consider a stream made up of many flowlines. UPHYDROSEQ and DNHYDROSEQ - (Aliases: UpstreamMainPathHydroSeq and DownstreamMainPathHydroSeq) Not every flowline with a higher number is upstream, and not every flowline with a lower number is downstream ![]() HYDROSEQ numbers decrease from upstream to downstream HYDROSEQ numbers increase from downstream to upstream It is generally a compact numbering system within an NHDPlus HR vector processing unit (VPU), meaning the numbers are in sequence without large gaps between themĪt any flowline, all upstream flowlines have higher hydrologic sequence numbers and all downstream flowlines have lower hydrologic sequence numbers It is a nationally unique sequence number that places each stream flowline in hydrologic order This is what you need to know about HYDROSEQ: This identifier, HYDROSEQ, is the foundation of the VAA system. ![]() A more “intelligent” approach is to organize the numbers so that all upstream flowlines have a higher number than the current flowline’s number and all downstream flowlines have a lower number than the current flowline’s number. One way is to start at the top of the list (table) and start counting off so that the first flowline is one, the second is two, and so on. There are several ways to identify network segments so that they can be managed.
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