Week 10 Hands-On — Cities Near Rivers¶

Hands-On Practice · Day 19 · Thursday of Week 10 — Geoprocessing

Topics¶

Cities near rivers analysis in QGIS
Chaining tools into a workflow

Materials¶

United States data for the cities-near-rivers analysis (posted on Learning Suite)

Feeds Lab 9.

At a glance¶


Goal	Students run a real geoprocessing chain, buffer, then select by location or clip, to find every U.S. city within 10 km of a major river, and see the same chain as a workflow diagram.
Why this week	Tuesday introduced geoprocessing and the Yellowstone Disaster scenario. Lab 9 is three chained analyses (floods, gas plume, search and rescue) using buffer, clip, intersect, and select by location. Today is one such chain, end to end.
Students bring	Laptop with QGIS 3.44 and United States.zip from the Week 10 Thursday entry on Learning Suite, unzipped (cities, rivers, states).
Graded item	In Class Activity: Cities Near Rivers (5 points). Upload a screenshot showing all U.S. cities within 10 km of a major river.
Feeds	Lab 9: The Yellowstone Disaster. Due Saturday. Quiz 8 closes Saturday.

Practice run before class¶

Tip

Twenty minutes. This is the cleanest geoprocessing session of the semester, and the whole class hangs on getting the buffer units right.

What you need. QGIS 3.44 LTR and United States.zip from Learning Suite, holding the states, the cities and the major rivers. Unzip it and set the project CRS to EPSG:5070, NAD83 CONUS Albers, before you load anything.

Do this, in order.

Sketch the workflow on paper first: rivers into Buffer gives river zones; river zones plus cities into Select by Location gives cities near rivers. Draw it before you click anything, because that habit is what Lab 11 and the final project are graded on.
Vector > Geoprocessing Tools > Buffer. Input rivers, distance 10 kilometers chosen from the unit dropdown rather than typed as 10000, segments 16, Dissolve result ticked, output saved into week10.gpkg as river_zones rather than left temporary. Run.
Style the buffer semi-transparent blue and zoom to the Mississippi to see the corridor.
Vector > Research Tools > Select by Location. Select from cities, where features intersect, comparing to river_zones. Read the count in the status bar; that number is the answer to the question.
Right-click cities > Export > Save Selected Features As... into week10.gpkg as cities_near_rivers. Style them red against gray.
Get the same answer two more ways: Clip cities by river_zones, and Join Attributes by Location so every city carries the zone it falls in.
Open Processing > History, find one of those runs, and copy it as a Python command.

You are ready when the buffer is a believable 10 km corridor rather than a blob or a hairline, and your three methods agree on the count.

The one that bites. A buffer distance entered while the project is in degrees produces either a continent-sized blob or an invisible hairline. Set EPSG:5070 first and use the unit dropdown.

Before class¶

[ ] United States.zip unzipped and the three layers loaded on the projector machine, project CRS set to EPSG:5070 (NAD83 CONUS Albers) so buffers are in meters and honest across the country.
[ ] The Processing Toolbox panel open (Processing > Toolbox).
[ ] The workflow diagram from the Day 18 deck (the annotated buffer, select, output chain) on a slide.
[ ] Learning Suite open to the Cities Near Rivers activity.

Plan (50 minutes)¶

Time	Segment
0:00	Mini-devotional
0:03	The question, then the diagram: rivers, buffer, cities, select, count
0:08	Buffer the rivers 10 km, dissolved
0:14	Select by Location: cities within the buffer; count; save selection as a layer
0:21	The same answer two other ways: Clip, and Join Attributes by Location; the Processing history
0:28	Students: run it, style it, screenshot, upload
0:42	Lab 9 pointer: three chains, one invalid-geometry trap

Walkthrough¶

1. Draw it before you click it¶

Put the question on the screen: which cities are within 10 km of a major river? Then the diagram: rivers (line) into Buffer 10 km gives river zones (polygon); river zones and cities (point) into Select by Location gives cities near rivers (point). Every geoprocessing problem in this course, including the final project, is a diagram like this first. Lab 11 formalizes it.

Check the project CRS badge: EPSG:5070. Say why: a 10 km buffer in a degrees CRS is not 10 km.

2. Buffer¶

Vector > Geoprocessing Tools > Buffer. Input: rivers. Distance 10 kilometers (change the unit dropdown; do not type 10000 in degrees). Segments 16. Tick Dissolve result so overlapping buffers merge into one polygon. Output: save to week10.gpkg as river_zones rather than a temporary layer. Run.
Style it as a semi-transparent blue. Zoom to the Mississippi to show the corridor.

3. Select by Location¶

Vector > Research Tools > Select by Location. Select features from: cities. Where the features: intersect (or are within). By comparing to: river_zones. Run.
The status bar shows the count selected. Say the number; it is the answer.
Right-click cities > Export > Save Selected Features As... to week10.gpkg as cities_near_rivers. Style them red, the rest gray. That is the screenshot.

4. Same answer, other tools¶

Vector > Geoprocessing Tools > Clip: input cities, overlay river_zones. The output is a new layer of just the cities inside, geometry unchanged for points.
Processing Toolbox > Join Attributes by Location: cities joined to river_zones, so every city gets a field saying which river zone it is in, and cities with no match keep null. Filter on that field.
Processing > History shows every run with its parameters. Copy one as a Python command to show that the click was really a function call. Model Designer, in two weeks, chains these boxes for you.

Student activity¶

Students run the buffer and the select on their own laptops, save the selected cities as a layer, style near-river cities distinctly from the rest, and take a screenshot of the whole country with the buffer visible. Upload it to In Class Activity: Cities Near Rivers. Full credit for a map that shows the buffer and the selected cities.

Lab 9 pointer¶

Lab 9 is the Yellowstone Disaster: floods (buffer rivers, clip, intersect with towns), a noxious gas plume (buffer a point, select by location), and search and rescue (a chain of three). Part 3 throws an "invalid geometry" error on purpose; the fix is Vector > Geometry Tools > Fix Geometries before the overlay. Say that now so nobody spends Friday night on it.

Common snags¶

Buffer is a giant blob or a hairline. The distance unit was degrees. Set the project CRS to 5070 and use the unit dropdown.
Select by Location selects zero. Cities and buffer are in different CRSs and one has no CRS assigned, or they picked "are within" against an undissolved buffer with holes. Use intersect.
The output layer is temporary and vanishes. They left the output as a scratch layer. Save to the GeoPackage.
Invalid geometry error on Clip. Run Fix Geometries on the input first, exactly as in Lab 9.
Everything is slow. Statewide-detail rivers are heavy. Use the major rivers layer, not the full hydrography.