📘 Unit 4: Searching for Geographic Objects

[Total: 12 Hours]

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🔹 Definition: Geographic Objects

Geographic objects are features that exist in space and can be represented in GIS. These include:

• Points (e.g., trees, bus stops)

• Lines (e.g., roads, rivers)

• Polygons (e.g., land parcels, buildings)

• Rasters (e.g., elevation, land use maps)

They are defined by:

• Geometry (location and shape)

• Attributes (information or data about the feature)

🔹 4.1 Finding Information in Raster Systems

Raster systems store data in grid cells. Searching in raster systems involves:

• Querying cell values (e.g., pixels with temperature > 30°C)

• Using coordinate-based selection

• Performing classification-based extraction (e.g., finding forest areas in a land cover raster)

Tools in QGIS:

• Raster Calculator

• Identify Tool

• Zonal Statistics

🔹 4.2 Finding Features in Vector Systems

Vector systems store features as geometries and attribute tables.

Searching in vector systems can be done by:

• Attribute Queries (e.g., Select all cities with population > 100,000)

• Spatial Queries (e.g., Select all buildings within 1 km of a river)

Tools in QGIS:

• Select by Expression

• Select by Location

• Query Builder

🔹 4.3 Searching Polygons in a GIS

Polygons represent area features (e.g., countries, lakes).

Searching polygons involves:

• Using polygon attributes (e.g., land use = “residential”)

• Selecting polygons that intersect with another layer (e.g., buffer zones)

• Performing topological queries (e.g., find touching polygons)

🔹 4.4 Locating 2D Map Objects

Locating 2D objects refers to:

• Finding features by coordinates (latitude/longitude or X/Y)

• Using the “Zoom to Feature” or “Locate” tools

• Utilizing OpenStreetMap or geocoding services for addresses

🔹 4.5 Defining Groups for Searching

Grouping helps in searching or analyzing similar geographic objects.

Examples:

• Group all cities with a population > 1 million

• Group all parks in a specific administrative region

Techniques:

• Use attribute-based grouping (e.g., by category)

• Use spatial clustering (e.g., cluster by proximity)

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🧪 Practical Work Using QGIS

The practical sessions for Unit 4 include advanced operations in GIS using QGIS, PostgreSQL/PostGIS, and pgRouting.

🔸 1. Acquiring Data for Geospatial Applications

• Download from open sources (OpenStreetMap, Natural Earth)

• Import into QGIS using Add Vector/Raster Layer

🔸 2. Visualizing GIS Data

• Style vector and raster layers

• Use symbology, labels, legends, and classification techniques

🔸 3. Vector Data – Extract, Transform, and Load (ETL)

ETL involves:

• Extract: Select relevant data (e.g., filter buildings)

• Transform: Reproject, clean, or modify data

• Load: Import data into GIS database (e.g., PostGIS)

Tools: Processing Toolbox → Vector Tools → Extract by Attribute / Location

🔸 4. Raster Analysis

• Perform terrain analysis (slope, aspect)

• Conduct raster classification and zonal analysis

• Use raster calculator for mathematical expressions

🔸 5. Publishing Results as Web Application

• Use plugins like qgis2web to export layers

• Create web maps in Leaflet or OpenLayers

• Host using platforms like GitHub Pages or QGIS Cloud

🔸 6. Postgres with PostGIS and pgRouting

• PostGIS enables spatial databases in PostgreSQL

• Import vector data into PostGIS using DB Manager or command line

• pgRouting allows network analysis, such as:

  • Shortest path

  • Service area

  • Isochrone (travel-time zones)

🔸 7. Using OpenStreetMap (OSM) for Topology

• Download OSM data using plugins (e.g., QuickOSM)

• Analyze road networks, buildings, and land use

• Check and correct topological errors

🔸 8. Database Importing and Topological Relationships

• Import shapefiles or GeoJSON to PostGIS

• Use Topology Checker in QGIS to validate:

  • Overlaps

  • Gaps

  • Dangles

• Perform spatial joins and relational queries

🔸 9. Creating Travel Time Isochrone Polygons

Isochrone = area that can be reached within a certain time.

Steps:

1. Load a road network with travel times

2. Use pgRouting to calculate reachable areas

3. Generate polygons showing time-based zones (e.g., 5, 10, 15 minutes)

4. Visualize results on map

📌 Summary of Key Concepts

Topic	Description
Geographic Objects	Spatial features like points, lines, polygons
Raster Search	Cell-based queries
Vector Search	Attribute and spatial queries
Polygon Search	Area-based selections
2D Object Location	Coordinate-based feature location
Grouping for Search	Grouping by attributes or spatial clusters
PostGIS & pgRouting	Spatial database and network analysis
Travel Time Isochrone	Time-based reachability mapping