Chapter 15

Vector Spatial Analysis

Geometric intelligence. Learn how to combine layers through overlays and simulate proximity using buffers to reveal hidden geographic patterns.

At a Glance

Prereqs: Chapters 05, 13 Time: 30 min read + 30 min practice Deliverable: Vector analysis result layer

Learning outcomes

  • Choose the correct vector operation for a question (buffer, overlay, join).
  • Explain how projection/units affect vector measurements.
  • Validate results by checking counts, areas, and obvious edge cases.

Key terms

buffer, dissolve, intersect, union, spatial join, slivers

Stop & check

  1. Why can buffering in degrees produce nonsense results?

    Answer: Degrees are angular units and vary in ground distance by latitude.

    Why: Buffer distance needs linear units like meters.

    Common misconception: A buffer of 0.01 degrees is about the same everywhere.

  2. What are slivers after overlay and why do they matter?

    Answer: Tiny polygons created by boundary mismatch.

    Why: They inflate feature counts and can bias area summaries.

    Common misconception: They are harmless; they can dominate statistics when many exist.

Try it (5 minutes)

  1. Pick one question and name the operation: within 500m, overlaps, or closest.
  2. Write one sentence explaining your tool choice.

Lab (Two Tracks)

Both tracks produce the same deliverable: one derived vector layer plus a short QA checklist.

Desktop GIS Track (ArcGIS Pro / QGIS)

Run one buffer + one overlay (intersect) and report feature counts before/after.

Remote Sensing Track (Google Earth Engine)

Create buffers around points/lines (where applicable) and summarize an image statistic inside the buffer. Report results.

Common mistakes

  • Forgetting to dissolve buffers when you need a single influence zone.
  • Interpreting overlay without checking attributes and geometry validity.
  • Assuming spatial joins are reversible; direction matters.

Further reading: https://www.ucgis.org/site/gis-t-body-of-knowledge

📐 The Vector Toolkit

Unlike raster analysis which works with pixels, Vector Analysis works with discrete objects: points, lines, and polygons. By analyzing the geometric relationships between these objects, we can answer questions like: "Which houses are in the flood zone?" or "How many customers live within 10 miles of my store?"

⚡ Proximity Analysis: Buffering

Buffering creates a new polygon at a specified distance around an existing feature. It is used to define "impact zones," "safety buffers," or "service areas."

Example: A 100-meter buffer around a river defines a "riparian protection zone" where new building permits are prohibited.
Critical GIS: Digital Redlining

In the 20th century, physical maps were used to segregate communities. Today, we can inadvertently recreate these zones with buffers. When a delivery app draws a "Service Area" buffer that stops just short of a low-income neighborhood, it is a modern, algorithmic form of redlining. The boundary seems technical, but the impact is social exclusion.

🧪 Overlay Operations

Vector overlay is often described as "cookie-cutting" with maps. When we overlay two layers, we create new geometry and combine their attribute tables into one.

Input A
Input B

Summary of Big Ideas

  • Buffering creates zones of influence around features.
  • Clip extracts data based on a boundary (doesn't combine tables).
  • Intersect keeps only common areas and combines all data tables.
  • Union keeps all features from both layers and combines tables.

Chapter 15 Checkpoint

1. To find the area where Forest (Layer A) and Private Land (Layer B) meet, you should use:

Intersect
Clip

2. What is the First Law of Geography (according to Waldo Tobler)?

The shortest distance between two points is a line.
Near things are more related than distant things.

📚 Chapter Glossary

Geometric Overlay The process of superimposing two or more vector datasets to create a new output dataset that contains the geometry and attributes of the inputs (e.g., Intersect, Union).
Buffer A zone around a map feature measured in units of distance or time. A buffer is useful for proximity analysis (e.g., finding everything within 1 mile of a school).
MAUP Modifiable Areal Unit Problem—a source of statistical bias that can significantly impact the results of spatial hypothesis tests when point-based measures are aggregated into districts.
← Chapter 14: Raster Analysis Next: Chapter 16: Spatial Modeling →

BoK Alignment

Topics in the UCGIS GIS&T Body of Knowledge that support this chapter.