Interactive Demo: Analysis Workflow — Buller River Catchment
~10 min activity
Vector Analysis Workflow
Apply buffer, clip, dissolve, and spatial join to a real river network
Streams: 3,241
Catchments: 3,241
Visible: 3,241
Result: —
Analysis Operation
Select an operation to see it applied to the Buller River stream network and sub-catchments.
Riparian Buffer Distance
100 m
50m100m200m500m1 km
Riparian buffer zones protect waterways from adjacent land use.
The orange stroke on the map is schematic (screen pixels, not ground metres).
This exceeds the entire catchment (≈6,500 km²) — why is that impossible? (Hint: overlapping buffers are never merged in this naive calculation.)
Click an order-3 catchment on the map to use as a clip boundary. Only streams within it will be shown.
Demo simplification: whole streams are selected by catchment attribute — a true clip would cut each stream exactly at the boundary.
Watch order-2 sub-catchments (3,241) merge into order-3 catchments (1,458). Each colour is one order-3 group — dissolve removes the white boundaries inside a colour, never between colours.
Zoom in to see individual boundaries — at full catchment extent the polygons are only a few pixels across.
Each order-3 catchment is coloured by how many order-2 stream reaches it contains. This transfers a count attribute from the stream layer to the catchment layer via spatial join.
Vector analysis transforms raw layers into answers. Buffer creates proximity zones around features. Clip trims features to a boundary. Dissolve merges features by a shared attribute. Spatial join transfers attributes between layers based on their spatial relationship.
Guided Questions
Q1
Set the riparian buffer to 100m, then 500m. How does the estimated buffer area change? Is the relationship linear with buffer distance? Why or why not?
Q2
Use Clip to select a catchment. What is the difference between clipping streams to a catchment boundary vs. doing a spatial join? When would you use each?
Q3
Switch to Dissolve and toggle between Order 2 and Order 3. What happens to the sub-catchment boundaries? What aggregate statistics would be useful to compute during the dissolve (e.g., total stream length per merged catchment)?
Q4
In Spatial Join mode, some order-3 catchments contain many order-2 streams while others contain few. What might explain this variation? What is the "one-to-many" problem in spatial joins?
Q5
Design a GIS workflow to answer: "What is the total area of riparian buffer zones within the 10 largest sub-catchments?" List each step and the operation used.