Grade 12

Advanced

8 min

Lesson 5 of 12

Urban Network Analysis

Not Started

Using network datasets to model transportation, logistics, and accessibility within urban environments.

When an ambulance is dispatched, the system doesn't just look for the 'closest' hospital—it calculates the fastest route through a complex web of one-way streets, speed limits, and traffic. How can we model this invisible logic to build better cities?

Learning Objectives

1.Model the shortest and most efficient paths using network datasets and impedance factors.
2.Define service areas (isochrones) for public facilities to evaluate spatial equity.
3.Analyze urban walkability using connectivity metrics like intersection density and the Gamma Index.

The Anatomy of a Network

In GIS, a Network Dataset is more than just lines on a map; it is a system of topological connectivity. Unlike simple polylines, networks consist of Edges (links like roads or pipes) and Junctions (nodes like intersections). The most critical component is Impedance, which is the 'cost' required to traverse an edge. This cost isn't always distance; it is often time, fuel, or even 'scenic value.' We calculate the total cost

Z

of a path as the sum of individual edge costs:

Z = \sum_{i=1}^{n} c_i

where

c_i

represents the cost of segment

i

. To model reality, we must also include Global Constraints like one-way streets and turn restrictions at junctions.

Quick Check

In network analysis, why might a 1-mile path be 'longer' than a 2-mile path?

Think about the difference between distance and time.

Answer

Because the 1-mile path may have a higher impedance (cost), such as a lower speed limit or heavy traffic, resulting in a longer travel time.

Service Areas and Isochrones

A Service Area is a region that encompasses all accessible network locations within a specified impedance cutoff. Unlike a simple 'as-the-crow-flies' circular buffer, a network-based service area follows the actual street geometry. These are often visualized as Isochrones (lines of equal time). For example, a 10-minute fire station response zone will look like an irregular 'star' shape, stretching further along highways and shrinking in dense, winding alleys. This is vital for Sustainable Development Goal 11, ensuring all citizens have equitable access to essential services.

1. Identify the 'Facility' (e.g., a Fire Station). 2. Set the 'Impedance' to Time ( $T$ ). 3. Calculate $T$ for each road segment using $T = \frac{d}{v}$ , where $d$ is distance and $v$ is the speed limit. 4. Generate a polygon covering all road segments where $\sum T \le 5$ minutes.

Measuring Connectivity and Walkability

To analyze how 'walkable' a neighborhood is, we look at Network Connectivity. A highly connected network offers many alternative routes, reducing travel distance. We can quantify this using the Gamma Index (

\gamma

), which measures the ratio of actual edges to the maximum possible edges:

\gamma = \frac{e}{3(v-2)}

where

e

is the number of edges and

v

is the number of vertices. A

\gamma

value closer to 1 indicates a highly connected grid, while a value closer to 0 indicates a disconnected 'lollipop' suburb with many cul-de-sacs.

Evaluate the accessibility of a new transit hub: 1. Calculate the Intersection Density (nodes per $km^2$ ) to determine route choices. 2. Model a 400m (5-minute walk) service area around the hub. 3. Calculate the Ratio of Service Area to a 400m circular buffer. A ratio closer to 1.0 suggests a perfect grid; a lower ratio suggests barriers like highways or gated communities.

Quick Check

What does a Gamma Index of 0.3 suggest about an urban street network?

Recall that 1.0 is a perfectly connected grid.

Answer

It suggests a poorly connected network with few alternative routes and likely many dead-ends or cul-de-sacs.

Key Takeaways

1Network analysis relies on topology (connectivity) rather than just spatial proximity.
2Impedance ( $Z$ ) is the cost of travel, which can be measured in time, distance, or effort.
3Isochrones provide a realistic measure of accessibility compared to simple Euclidean buffers.
4The Gamma Index ( $\gamma$ ) is a mathematical way to compare the efficiency of different urban layouts.

Quick Check

Multiple Choice

Which of the following represents a 'Junction' in an urban network dataset?

Multiple Choice

If a city wants to improve 'spatial equity,' which analysis should they prioritize?

True/False

A circular buffer and a network service area will always cover the same geographic extent.

What's Next?

Review Tomorrow

In 24 hours, try to sketch the difference between a 5-minute buffer and a 5-minute isochrone. Can you remember the formula for the Gamma Index?

Practice Activity

Open a mapping tool (like Google Maps) and compare the 'walking' vs 'driving' time between two points in a dense city. Identify which 'impedance' factors (one-way streets, pedestrian paths) changed the result.

Browse

Browse

Urban Network Analysis

Learning Objectives

The Anatomy of a Network

Service Areas and Isochrones

Measuring Connectivity and Walkability

Key Takeaways

Quick Check

What's Next?

Urban Network Analysis

Learning Objectives

The Anatomy of a Network

Service Areas and Isochrones

Measuring Connectivity and Walkability

Key Takeaways

Quick Check

What's Next?