Lecture 2

Question 1

Which of the following is NOT a data source for spatial data in cartography?

a) Terrestrial surveys

b) Photogrammetric surveys

c) Weather forecasts

d) Satellite imagery

Answer 1

c) Weather forecasts
Explanation: Weather forecasts are not a primary data source for spatial data in cartography. They may be derived from spatial data but do not directly contribute to mapmaking processes.

Question 2

What is the primary advantage of using vector data over raster data?

a) It is easier to process large datasets.

b) It provides more detailed representations of spatial features.

c) It requires less storage space.

d) It eliminates the need for topology.

Answer 2

b) It provides more detailed representations of spatial features.

Explanation: Vector data defines spatial features precisely using points, lines, and polygons, making it suitable for detailed spatial analysis and representation.

Question 3

Explain the difference between digitizing and scanning in the context of data input.

Answer 3

Digitizing involves manually tracing features from non-digital sources to create vector data, while scanning uses hardware to convert non-digital data into raster data.

Explanation: Digitizing is used for precise feature extraction, while scanning quickly converts paper maps into digital formats but often requires further processing.

Question 4

Why is topology important in cartographic data structures?

Answer 4

Topology ensures spatial data integrity and supports spatial operations like adjacency, connectivity, and containment.

Explanation: Topological data structures maintain relationships between spatial objects, facilitating accurate spatial analysis and preventing errors.

Question 5

Given a scanned raster map of a city, describe the process to convert it into a vector format suitable for GIS.

Answer 5

1. Use a flatbed or drum scanner to digitize the map into a raster image.
2. Apply thinning algorithms to reduce lines to single-cell width.
3. Use the Douglas-Peuker algorithm to reduce data points along the lines.
4. Manually trace features or use automated tools to create vector polygons and lines.
5. Validate and refine the topology for accuracy.

Explanation: These steps ensure the raster data is effectively transformed into a usable vector format with proper topology.

Question 6

Using the “spaghetti” model as input, outline the steps to create a polygon topology.

Answer 6

1. Digitize boundaries and center points.
2. Sort arcs by extents and examine intersections.
3. Split intersecting arcs and build node tables.
4. Create polygon tables with identifiers and pointers.
5. Check for closure and resolve islands using a point-in-polygon algorithm.

Explanation: This systematic process transforms the basic “spaghetti” model into a structured topological format.

Question 7

A map of a rural area shows significant data inaccuracies when analyzed in GIS. The source data was input using the “spaghetti” model. Discuss possible reasons and how to address them.

Answer 7

• Reasons include lack of topological relationships, digitization errors, and misaligned arcs.
• Address by creating a topological data structure, validating node connections, and ensuring polygons are correctly closed.

Explanation: The “spaghetti” model’s lack of inherent topology can lead to inaccuracies, which must be corrected through systematic topology building.

Question 8

You are tasked with creating a GIS database for a city using satellite imagery and census data. What steps would you take to ensure data accuracy and integration?

Answer 8

1. Process satellite imagery for geometric and radiometric corrections.
2. Convert raster data to vector format as needed.
3. Georeference census data to match spatial coordinates.
4. Validate data integration using topology checks and field verification.

Explanation: Careful processing and validation ensure the data is accurate and compatible for GIS analysis.

Question 9

Compare and contrast raster and vector data structures in terms of storage, processing, and applications.

Answer 9

• Raster: Grid-based, large storage, efficient for continuous data (e.g., elevation).
• Vector: Point/line-based, smaller storage, precise for discrete data (e.g., boundaries).

Explanation: Raster excels in continuous data representation, while vector is better for discrete, precise mapping.

Question 10

Describe the role of the Douglas-Peuker algorithm in cartographic data processing.

Answer 10

It reduces the number of points in a line while maintaining its shape within a tolerance.

Explanation: This algorithm simplifies line data, making it efficient for storage and analysis without significant loss of detail.