GIS Applications & Remote Sensing — All Questions Solved

Question 1: GNSS Definition & GIS Database for Water Supply Infrastructure [5 + 25 Marks]

Part (a): Define GNSS [5 Marks]

GNSS (Global Navigation Satellite System) is a general term describing any satellite constellation that provides positioning, navigation, and timing (PNT) services on a global or regional basis. Unlike "GPS," which refers only to the U.S. system, GNSS is an umbrella term covering all such systems worldwide.

Major GNSS Constellations

How GNSS Works

Three Segments of GNSS

Advantages of Multi-Constellation GNSS

• More satellites visible = better accuracy, especially in urban canyons and hilly terrain
• Faster fix — time-to-first-fix (TTFF) is reduced
• Redundancy — if one system degrades, others maintain positioning
• Better coverage in high-latitude and obstructed environments

Part (b): Preparing a GIS Database of Water Supply Infrastructure Using GNSS [25 Marks]

Scenario: You are a GIS expert tasked with creating a complete GIS vector database of water supply infrastructure (source, pipelines, household benefited) in a municipality. You have manpower for field work.

Step 1: Planning & Preparation

Step 2: Database Schema Design

Step 3: Field Data Collection Using GNSS & Mobile GIS Apps

Step 4: Detailed Field Procedures

Collecting Water Sources (Points): Stand at the source with GNSS receiver, wait for satellite lock (4+ satellites, PDOP < 3), record the point, fill attributes (type, capacity, condition), take geotagged photographs.

Tracing Pipelines (Lines): Start at a known point, enable continuous GNSS tracking, walk along the pipe route, pause at junctions to add vertices with attributes (diameter, material, burial depth).

Recording Benefited Households (Points): Visit each household, record GNSS point at the tap/connection, fill in owner name, ward number, connection type, household members, and status.

Step 5: Data Processing in GIS

Step 6: Map Production & Outputs

Question 2: Interpolation & IDW (Inverse Distance Weighted) [10 Marks]

1. What is Interpolation?

Interpolation is the process of estimating unknown values at unsampled locations based on known values at surrounding sampled points. In GIS, it is the primary technique for creating continuous surfaces (rasters) from discrete point observations.

Types of Interpolation

2. Inverse Distance Weighted (IDW) Method

IDW is a deterministic interpolation technique based on Tobler's First Law of Geography: "Everything is related to everything else, but near things are more related than distant things."

The value at an unknown point is estimated as a weighted average of nearby known values, where the weights decrease as the distance increases.

IDW Formula

How IDW Works — Visual Explanation

Numerical Example (p = 2)

Effect of the Power Parameter (p)

Advantages & Limitations

IDW vs. Other Methods

Question 3: Seasonal Land-Use Variation Analysis (10 Years) [30 Marks]

Scenario: Analyze seasonal variation of land-use before and after monsoon in a municipality for the last 10 years using remote sensing and GIS tools. Software-agnostic answer expected.

1. Choice of Remote Sensing Data

Image Selection Criteria

Total images needed: 20 scenes (10 pre-monsoon + 10 post-monsoon)

2. Complete Workflow

Pre-processing Details

Classification Scheme

Change Detection: Post-Classification Comparison

Reason: Directly produces a "from–to" change matrix, works across different sensors/dates, and provides clear interpretable change classes.

Transition Matrix Example

10-Year Trend (Illustrative)

Summary of Key Decisions

Question 4: Parameters Extracted from Elevation Data [10 Marks]

Elevation data (DEM/DTM) is one of the most powerful GIS datasets. By analyzing how elevation values change across a surface, we can derive numerous terrain parameters.

1. Slope

Slope measures the steepness or gradient of a terrain surface — the rate of change of elevation over horizontal distance, expressed in degrees (0°–90°) or percent.

2. Aspect

Aspect is the compass direction a slope faces — measured 0°–360° (0°/360° = North, 90° = East, 180° = South, 270° = West).

3. Contour Lines

Contour lines are lines connecting points of equal elevation — generated from a DEM for topographic mapping, engineering design, and volume estimation.

4. Hillshade

Hillshade simulates how terrain looks illuminated from a specific sun angle — creating realistic 3D shaded relief useful for visualization and map backgrounds.

Summary Table

Question 5: Flow Accumulation in River Channel Analysis [10 Marks]

1. What is Flow Accumulation?

Flow accumulation calculates, for each cell in a DEM, the total number of upstream cells whose water would drain through it. It answers: "How much water collects at each point on the landscape?"

2. Hydrological Workflow

3. How Flow Accumulation Works (D8 Example)

4. Stream Extraction via Threshold

5. Applications

Question 6: Low Pass & High Pass Filters in Remote Sensing [5 + 5 Marks]

Spatial filters are convolution-based image processing techniques that replace each pixel's value with a weighted combination of its neighbors using a moving kernel window.

Part (a): Low Pass Filter [5 Marks]

A low pass filter (smoothing/averaging filter) allows low spatial frequencies to pass through while suppressing high frequencies. It replaces each pixel with the average of its neighbors.

• Smooths the image, reduces sharp variations
• Reduces noise (salt-and-pepper)
• Blurs edges — boundaries become less distinct
• Generalizes — reveals broad patterns

Part (b): High Pass Filter [5 Marks]

A high pass filter (edge enhancement/sharpening filter) allows high spatial frequencies to pass through while suppressing low frequencies. It highlights edges, boundaries, and fine details.

• Sharpens the image — enhances edges and boundaries
• Highlights rapid changes — roads, field edges, lineaments
• Removes gradual trends — suppresses smooth areas
• May amplify noise (side effect)

Comparison: Low Pass vs. High Pass