Fish Site Snapping

Examination of the observed occupancy points has shown that some of the locations do not spatially align with the NHDHRDV2 stream network. Sample locations were found to fall into four classifications, involving three versions of flowlines. The processing steps outlined in this repository snap the locations to the stream network in ArcGIS. The snapping effort is specifically in support of the Network Analysis (Trout GRF) project, but may serve as a template for similar work in the future.

Flowline Versions

Three flowlines versions are used in the snapping process:

1. detailed - All inclusive high resolution flowlines

2. truncated - The high resolution flowlines trimmed to a 0.75 km² drainage area threshold
3. dem - The flowlines generated by setting the 0.75 km² threshold on the flow accumulation layer.

The "dem" flowlines are the threshold that is used to trim the "detailed" flowlines and create the "truncated" flowlines. This layer includes all cells that have 0.75 km² drainage area or greater, not just those that fall within the high resolution network. For practical purposes these can be considered undocumented streams, meaning they are not included in the original NHD high resolution data but could still exist. These streams were considered because of case #2 below.

Location Classification Types

Fish sample sites fall into one of four classifications outline below:

1. Detailed Network Only - The point is on the "detailed" network (red), but not on the "truncated" network (green).

Figure 1: A site only on the detailed network

2. Undocumented Stream - The point is on a "dem" network (blue), but not located on the "detailed" network (red).

Figure 2: A site on an undocumented stream

3. Outside Buffer - Points are not near any of the network versions. These are accounted for by setting a snap distance.

Figure 3: A site outside of the maximum snapping distance

4. Truncated Network - The points are on all of the network versions. These points are the ones kept for use with stream network analysis.
   

Figure 4: A site usable with the stream network analysis

Classification Summary

The current breakdown of where the sample locations fall in relation to the flowlines is listed in the table below. The maximum snapping distance was set to 100m. Any points outside of this distance were left unsnapped. The sites in the "Truncated Network" classification are the ones used in the network analysis project. The points used are the observed occupancy sites from December 2013. The whole process is scripted so it should be simple enough to update with new sites as needed.

Location Classification	Number of Sites
Detailed Network Only	1291
Outside 100 Meter Buffer	1935
Truncated Network	33011
Undocumented Stream	529
Total Sites	36766
Table 1: Classification of sites

Process Execution

The snapping processing is carried out in the mapOccupancySitesToNetwork.py script. Define the variables in the "Specify inputs" and execute the script in Arc Python.

• networkGrid_DEM - the filepath to the DEM flow grid
• networkGrid_Detailed - the filepath to the high resolution flow grid
• networkGrid_Truncated - the filepath to the truncated flow grid
• points - the filepath to the locations shapefile
• version - the version ID of the fish points (e.g. "VTFS")
• bufferInMeters - length in meters of the maximum snapping distance
• workingDirectory - the filepath to the file geodatabase where snapped points will be saved
  
  

Contact Info

Kyle O'Neil
koneil@usgs.gov