Vector Data: Points, Lines & Polygons Explained
Vector data is one of the two fundamental data models in GIS (the other being raster). It represents real-world features using precise geometric shapes: **points, lines, and polygons**, each paired with an attribute table of information. ---
📋 Prerequisites
- QGIS Desktop 3.22+ installed on your computer.
- Basic understanding of GIS data concepts.
Step-by-Step Instructions
Understand the Point Feature
A **Point** represents a single location with X, Y (and sometimes Z) coordinates. It has no length or area. **Used for:** Schools, hospitals, wells, GPS locations, cities on a small-scale map **Example attribute table for points:** | ID | Name | Type | |---|---|---| | 1 | City Hospital | Hospital | | 2 | Central School | School | ---
Understand the Line Feature
A **Line** (or polyline) is a connected series of points, representing features with length but no area. **Used for:** Roads, rivers, pipelines, electricity lines **Key properties stored:** Length, direction, connectivity (important for network analysis like routing) ---
Understand the Polygon Feature
A **Polygon** is a closed shape made of connected lines, representing features with both perimeter and area. **Used for:** Administrative boundaries, lakes, land parcels, forest cover **Key properties stored:** Area, perimeter, centroid ---
Vector File Formats You'll Commonly Use
| Format | Extension | Notes | |---|---|---| | **Shapefile** | .shp (+.dbf, .shx, .prj) | Most widely used, but limited to 2GB and one geometry type | | **GeoJSON** | .geojson | Lightweight, web-friendly, human-readable | | **GeoPackage** | .gpkg | Modern, single-file, supports multiple layers | | **KML** | .kml | Used by Google Earth | ---
Create a Vector Layer in QGIS
1. Go to **Layer → Create Layer → New Shapefile Layer** 2. Choose geometry type: Point, Line, or Polygon 3. Define the CRS (usually match your project CRS) 4. Add attribute fields (e.g., Name — Text, Population — Integer) 5. Click **OK** — a new empty layer appears, ready for digitizing ---
Add Features to Your Layer
1. Select your new layer in the Layers Panel 2. Click **Toggle Editing** (pencil icon) 3. Select the **Add Point/Line/Polygon Feature** tool 4. Click on the map to draw your feature 5. Fill in the attribute form that pops up 6. Click **Save Edits** when done ---
🧠 Quick Recap & Practice Questions
## Quick Recap
- Point = location only (no length/area)
- Line = length, no area (roads, rivers)
- Polygon = area + perimeter (boundaries, land parcels)
- Common formats: Shapefile, GeoJSON, GeoPackage, KML
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## Practice Questions
1. Which geometry type would you use to represent a river? A city boundary? A well?
2. What are the three companion files that come with a Shapefile?
3. Why might GeoPackage be a better choice than Shapefile for a modern project?
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*Next Tutorial: Raster Data & Digital Elevation Models (DEM)*