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Geographic Database Design in GIS


Geographic database design means planning how spatial data (maps + attributes) will be stored in a GIS system.

It is done in three main phases:

  1. Conceptual Design → What data is needed?

  2. Logical Design → How should data be structured?

  3. Physical Design → How will it be implemented in software?

Conceptual Database Design (The "WHAT" Phase)

🔹 Meaning

This is the high-level planning stage.
It focuses on understanding real-world geographic features and their relationships.

It is independent of any software (not linked to PostgreSQL, ArcGIS, etc.).

🔹 Key Terminologies

  • Entity → A real-world object
    Example: River, Road, Building, Village

  • Attribute → Information about an entity
    Example:

    • River → Name, Length

    • Road → Type, Width

  • Relationship → How entities are connected
    Example:

    • Road crosses River

    • Village located near River

  • ER Diagram (Entity-Relationship Diagram)
    A diagram that shows entities, attributes, and relationships.

🔹 Object-based vs Field-based Model

TypeMeaningExample
Object-based modelDiscrete featuresRoad, School, Lake
Field-based modelContinuous surfaceTemperature, Elevation, Rainfall

✔ In your rainfall or TWI analysis work, rainfall is a field model (continuous surface).
✔ In urban footprint extraction (Palakkad project), buildings are object model.

🎯 Goal of Conceptual Design

Define:

  • What data is needed?

  • What features exist?

  • How are they related?

Logical Database Design (The "HOW" – Abstract Phase)

🔹 Meaning

Now we convert the conceptual idea into a structured data model.

Still independent of specific software, but more technical.

🔹 Key Terminologies

1. Spatial Data Types (Geometry Types)

Geometry TypeExample
PointBorewell location
LineRoad, River
PolygonVillage boundary
RasterElevation map, NDVI map

2. Table Structure

Entities become tables

Example:

Table: Road

Road_IDNameTypeGeometry

3. Primary Key

A unique ID for each feature.

Example:
Road_ID → uniquely identifies each road.

4. Foreign Key

Links one table to another.

Example:
Village table contains District_ID to connect with District table.

5. Normalization

Organizing tables to:

  • Avoid duplication

  • Reduce redundancy

  • Improve data integrity

Example:
Instead of repeating district name in every village record → create a separate district table.

6. Topology (Spatial Relationships)

Defines spatial rules like:

  • Connected to

  • Adjacent to

  • Within

  • Contains

  • Intersects

Example:

  • Road must be connected at junctions

  • Building must be inside municipal boundary

In your GIS work, topology helps avoid:

  • Gaps

  • Overlaps

  • Duplicate boundaries

🎯 Goal of Logical Design

Create:

  • Tables

  • Fields

  • Keys

  • Spatial relationships

  • Clean data structure

Physical Database Design (The "HOW" – Technical Phase)

🔹 Meaning

Now the database is implemented in a real GIS-enabled DBMS.

Examples:

  • PostgreSQL + PostGIS

  • Oracle Spatial

  • ArcGIS Geodatabase

  • SpatiaLite

🔹 Key Terminologies

1. Data Types

Example in PostGIS:

geometry(Point, 4326)  geometry(Polygon, 32643)

  • GEOMETRY → planar coordinates

  • GEOGRAPHY → earth-based spherical coordinates

2. Spatial Index

To make spatial queries fast.

Example:

  • R-Tree Index

  • GiST Index (PostGIS)

Used for:

  • Finding nearest road

  • Intersect queries

  • Buffer analysis

3. SQL Implementation

Example:

CREATE TABLE roads (    road_id SERIAL PRIMARY KEY,    name VARCHAR(50),    type VARCHAR(20),    geom GEOMETRY(LineString, 4326)  );

4. Optimization

Includes:

  • Indexing

  • Clustering

  • Storage tuning

Improves:

  • Query speed

  • Performance

  • Large dataset handling

🎯 Goal of Physical Design

Create:

  • Real tables

  • Spatial columns

  • Indexes

  • Efficient storage

Summary 

PhaseFocusQuestion AnsweredOutput
ConceptualReal-world understandingWhat data is needed?ER Diagram
LogicalData structureHow should data be organized?Tables & schema
PhysicalImplementationHow to implement in DBMS?SQL tables & indexes

Simple Real Example (Village Mapping Project)

Step 1 – Conceptual

Identify:

  • Village

  • Road

  • River

  • Relationships

Step 2 – Logical

Create tables:

  • Village table

  • Road table

  • River table

Define:

  • Primary keys

  • Geometry types

  • Topology rules

Step 3 – Physical

Implement in:

  • QGIS Geopackage

  • PostGIS database

Create:

  • Spatial index

  • Constraints

  • SQL structure

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