topic How does SnapOn = 1 work? in Water Resources Questions

How does SnapOn = 1 work?

LornaMurison — Fri, 07 Jan 2011 19:24:02 GMT

Hi,
I have delineated just over 7000 catchments (batch watershed delineation) with SnapOn set to 1. For all except roughly 5, the watershed point produced is snapped to the center of the flow direction grid cell in which the original batch point falls.
For the other ones, the watershed point has been snapped to another cell 5 or 6 cells over. This has resulted in my watershed being delineated to the main stream, and not to the tributary I am interested in.
Does anyone know how the SnapOn feature actually works, and why only 5 of my 7000 points would be moved?

Thank-you.

Re: How does SnapOn = 1 work?

LornaMurison — Mon, 10 Jan 2011 19:13:51 GMT

Apparently the points that snapped were the ones close to the synthetic stream network.
My follow up question is, does anyone know how to set the maximum snapping distance in ArcHydro?

Re: How does SnapOn = 1 work?

curtvprice — Wed, 21 Jan 2015 21:43:16 GMT

Yes, this is indeed what it does - it snaps to Input Snap Stream Grid (see the Snap Pour tool for more details).

If snapping is requested, all the points located within 6 cells (default) of a Snap Stream Grid cell (or Stream grid cell if Snap Stream Grid is left null) will be snapped onto the closest cell on the stream. If the snapped point is not located in the same catchment as the original point, the tool populates the field BatchDone with the value 2 to flag the input point.
Note
The default snapping tolerance is read from the configuration XML and is set to 6 cells. This value can be modified by editing the parameter HydroConfig/ProgParams/ApFunctions/ApFunction(BatchWatershedDelineation/SnapToleranceNumCells

For some work I'm currently doing, I'm running a select by location on seed points to find all that not within 150m of a synthetic flow line feature - I then visit them one by one and edit them by hand on to the cell I want - and set SnapOn to zero. These points are the ones whose drainage area is too small to land on a synthetic flow line - so I am moving them to a high-fac cell to delineate from.