Basin Customization - Adding Links Lab 3

Step 5 - Merge Phantom Streamlinks.

Objective: Merge small streamlinks that are not associated with their own basin.

Due to the way the streamlink grid is converted to a bifurcated line network, there may be some streamlinks that are not associated with a true basin. Instead, the grid to line function in ArcInfo may create a very short streamlink segment in places where more than 2 basins flow into the same point.
Phantom Streamlinks
The easiest way to handle this situation and to maintain the proper stream network topology, the phantom streamlinks should be merged with the downstream streamlink. The phantom streamlink will usually have the Pfafstetter ID of one of the upstream basins. This upstream basin then has 2 stream segments mapped to it.

The procedure is to make selections on the streamlink theme to select all streamlinks to be merged and then run the "Merge Basins" function.

Procedure:

  1. Start ArcView and open project basins/cust/cust3.apr.
  2. Open View1.
  3. Make the slmerged1.shp streamlink theme active by clicking on its legend.
  4. Click on the "Open Theme Table" button to open the attribute table for the streamlink theme.
  5. Return to the view and from the View menu, select "Add Event Theme..."
    1. For the table, select the attribute table for the streamlink theme.
    2. For the X and Y fields, choose "X_first" and "Y_first", then click the OK button. A point theme will be added to the view.
  6. Make the new point theme the active theme by clicking on its legend.
    1. Select "Properties" from the theme menu.
    2. Rename the theme to sl-pts (this is to avoid confusion with the streamlink theme's attribute table).
  7. Click on the "Open Theme Table" button to open the attribute table for the sl-pts theme.
  8. From the Bsn-Tools menu, select "Check for Duplicates" and select the [Pfaf_id] field.
  9. Convert the selection to a new theme:
    1. Return to the View and make the sl-pts theme active by clicking on its legend.
    2. From the Theme menu, select "Convert to Shapefile...". Use the default name and answer yes to add the new theme to the view.
    3. Delete the sl-pts theme from the view:
      1. Make the sl-pts theme active by clicking on its legend.
      2. From the Edit menu, select "Delete Themes".
  10. Make the new point theme visible and select all points. This theme will be used to track which streamlinks have been checked by starting with all records selected and unselecting them after they are checked:
    1. Make the point theme active by clicking on its legend. Also make sure the theme is visible.
    2. Click on the "Zoom to Active Themes" button.
    3. Click on the "Select Feature" tool.
    4. Drag a box on the view to select all points.
    5. Click on the "Save Project" button.
  11. Inspect all selected records using the points to guide where to look but examining the spatial relationship between the line segments and the basin polygons. There may be some records where the [Pfaf_id] needs to be changed or the line segment merged. For this step, focus only on the segments that need to be merged (ignore other needed corrections for now). You will primarily be looking for situations as in the figure above, however there may be other situations where the line segments should be merged. Repeat the following process until all points have been processed:
    1. Zoom in to a pair of selected points (pairs have the same [Pfaf_id]) then zoom further in to the downstream point.
    2. If the downstream segment is classified as a phantom segment:
      1. Make the streamlink theme the active theme by clicking on its legend.
      2. Click on the "Select Next Downstream" tool.
      3. Hold the shift key down to add to the selection and click on the phantom streamlink. This will select the phantom streamlink and the downstream basin to which it needs to be merged.
    3. Make the point theme the active theme by clicking on its legend.
    4. Use the "Zoom to Previous Extent" button to zoom out to the extent of the pair of points being examined.
    5. Deselect the points just checked:
      1. Click on the "Select Next Downstream" tool.
      2. Hold the control key down to subtract from the selection and click on the upstream point of the pair. This will deselect both points.
    6. Click on the "Save Project" button. This saves the current selection. If you accidentally lose the selection, close ArcView without saving and reopen the project.
    7. Click on the "Zoom to Selected" button to see the remaining points to check.
  12. Once all streamlinks to be merged are selected:
    1. Make the streamlink theme the active theme by clicking on its legend.
    2. From the Bsn-Tools menu, select "Merge Basins".
    3. Name the new shapefile slmerged2.shp.
    4. Answer no to recompute Area/Length.
  13. Delete the temporary point theme and clean up the project:
    1. Make the point theme the active theme by clicking on its legend.
    2. From the Edit menu, select "Delete Themes".
    3. Open the Project window and click on "Tables".
    4. Select all tables in the list.
    5. From the Project menu, select the Delete entry and delete all tables from the project.
    6. Click on the "Save Project" button.
    7. Return to the View window.
    8. From the File menu, select "Manage Data Sources" and delete the temporary point theme from disk.