Introduce st_join() before or within current Chapter 14

[mpjashby]

Rationale

The downloadable report associated with current Chapter 14 uses st_join() to attach neighbourhood attributes to homicide points, but students have not previously been taught what a spatial join does. In the proposed chapter order, the next substantive use is not until current Chapter 15, so the report contains a forward dependency that students may be unable to understand or debug.

Suggested location

The preferred location is current Chapter 9, shortly after its explanation of left_join(). Students are already learning how tabular joins match rows using column values, so this provides a natural contrast: st_join() matches spatial features according to their locations.

If adding it to current Chapter 9 would overload that chapter, add a concise just-in-time explanation in current Chapter 14 before the example report.

Suggested content

• Explain that st_join(x, y) retains the geometry and rows of x and adds attributes from spatial features in y that match spatially.
• Contrast matching by location with left_join(), which matches values in columns.
• Use a small point-in-polygon example relevant to crime locations and area boundaries.
• Note that both objects need compatible coordinate reference systems.
• Explain that unmatched features produce missing values and that multiple spatial matches can duplicate rows.
• Show a simple validation step, such as counting unmatched rows and checking the resulting row count.
• Add a cross-reference from the Chapter 14 report code to the explanation.

Completion criteria

• st_join() is explained before it appears in the downloadable Chapter 14 report.
• The explanation distinguishes spatial joins from attribute joins and covers common validation checks.
• The report can be followed without relying on material from current Chapter 15.