📘 Unit 5: Geographic Pattern Analysis

[Total: 18 Hours]

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🔹 5.1 Distance Measurement

Distance is a core element of spatial analysis in GIS. It helps in understanding spatial relationships between features.

✅ Types of Distance:

1. Absolute Distance

   • Straight-line (Euclidean) distance between two points.

   • Used in simple measurements like proximity or range buffers.

2. Relative Distance

   • Distance considering context or perception.

   • Example: two points may be close geographically but feel distant due to poor road connectivity.

3. Functional Distance

   • Considers travel time, cost, or effort between points.

   • Common in network analysis, such as finding the shortest path by time or fuel consumption.

QGIS Tool: Measure tool, Buffer analysis, Network analysis plugins

🔹 5.2 Statistical Surfaces

A statistical surface represents continuous phenomena (e.g., temperature, elevation).

✅ Characteristics:

• Vary continuously across space

• Represented as rasters or interpolated vector layers

✅ Applications:

• Climate mapping

• Soil quality assessment

• Pollution spread

✅ Predicting Values:

• Interpolation techniques are used to estimate unknown values:

  • IDW (Inverse Distance Weighting)

  • Kriging

  • Spline

QGIS Tool: Interpolation Plugin, Processing Toolbox → Raster Creation

🔹 5.3 Topological Surfaces

Topology in GIS defines spatial relationships between features, such as:

• Adjacency (which features touch each other)

• Connectivity (how features are linked)

• Containment (what is inside what)

✅ Examples:

• Checking if parcels share borders

• Ensuring roads connect properly at junctions

• Avoiding overlapping polygons

QGIS Tool: Topology Checker, Geometry Validation, Vector Analysis

🔹 5.4 Networks

Networks in GIS refer to a set of interconnected linear features, like roads, rivers, or utility lines.

✅ Key Concepts:

• Connectivity Measurement: Determines how well-connected the network is.

• Impedance Values: Represents the cost of traveling through a segment (e.g., time, length, slope).

• One-way Paths: Restrict movement to a single direction (common in traffic).

• Circuits & Turns: Closed routes and direction changes.

• Intersections: Points where two or more features meet.

• Directing Traffic: Establishing rules and restrictions for travel.

These are important for route optimization, emergency response planning, and urban infrastructure design.

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🧪 Practical Work Using QGIS

🔸 1. Creating a Simple Routing Network

• Load vector road data

• Convert lines to a network using Road Graph plugin

• Define speed and direction attributes

🔸 2. Automate Multiple Route Computations (Batch Processing)

• Prepare multiple origin-destination pairs

• Use QGIS Processing Modeler or scripts to automate route calculations

🔸 3. Calculating Shortest Paths with Road Graph Plugin

• Install and configure the Road Graph plugin

• Assign cost (distance/time)

• Visualize shortest paths directly on the map

🔸 4. Routing with One-Way Streets

• Add directionality in attribute table (e.g., “oneway” field)

• Adjust Road Graph settings to consider one-way streets

🔸 5. QGIS Network Analysis Library

• Use Shortest Path (point to point) tool under Processing Toolbox

• Inputs: road layer, start point, end point

🔸 6. Routing Point Sequences

• Calculate optimal travel path across multiple waypoints

• Use plugins or manual editing of path sequences

🔸 7. Matching Points to Nearest Line

• Use “Distance to nearest hub” tool

• Useful in finding the closest road from an address point

🔸 8. Creating Routing Network for pgRouting

• Use PostgreSQL with PostGIS and install pgRouting

• Import road data with pgRouting-compatible structure

• Build topology using pgr_createTopology

🔸 9. Visualizing pgRouting Results in QGIS

• Use DB Manager or pgRoutingLayer Plugin

• Query routes and display them as styled line features in QGIS

🔸 10. Using the pgRoutingLayer Plugin

• Simplifies route creation from pgRouting queries

• Provides GUI-based query builder

🔸 11. Getting Network Data from OpenStreetMap (OSM)

• Download road network using QuickOSM plugin

• Clean and prepare the dataset for routing

• Convert OSM tags to usable routing attributes (e.g., maxspeed, oneway)

📌 Summary of Key Concepts

Concept	Explanation
Distance Types	Absolute, Relative, Functional
Statistical Surface	Continuous data like elevation or rainfall
Topological Surface	Spatial relationships like connectivity or adjacency
Network Analysis	Finding shortest paths, routing, traffic simulation
Road Graph Plugin	For calculating shortest routes within QGIS
pgRouting	Advanced routing using PostgreSQL/PostGIS