Cartographic Principles: Map Generalization and Scale Calculation

Understanding Map Generalization

What is Cartographic Generalization?

Reality has far too much information to be conveyed on a single two-dimensional map. Map generalization is the process of selecting and simplifying map features appropriate to the map's scale and purpose.

Key Techniques in Map Generalization

• Selection: Choosing only relevant line, point, and area features to be mapped. This decision is based on the map's intended use.
• Simplification: Reduction of unnecessary detail, often involving reducing the number of vertices (corners) in a polygon shape or line.
• Smoothing: Similar to simplification, this technique smooths out abruptly joined lines, improving aesthetic quality.
• Displacement: Shifting overlapping features slightly so that all important elements remain visible and legible.
• Enhancement: Giving map symbols more realistic appearances or emphasizing important features.
• Aggregation: Combining separate areas of a phenomenon into one unified area.
• Dissolution and Segmentation: Eliminating one area feature in favor of another category of area, often used when features merge or boundaries are removed.
• Classification: Recognizing similar features and finding a common symbol to represent these similar features effectively.

Types of Map Symbols

Map symbols are fundamental to representing real-life objects on a map:

• Line Symbols

  Represent real-life objects that follow a linear path, typically very long with insignificant width (e.g., roads, rivers).

• Point Symbols

  Represent an object with a dot. These objects occur at a single, specific point on the Earth’s surface (e.g., buildings, landmarks).

• Area (Polygon) Symbols

  Represent real-life objects spread over the Earth’s surface. These are often depicted using geometric shapes (circular, triangular, or rectangular) or shading/texture.

Understanding Map Scale

Scale is a ratio comparing the relationship of features on a map to their actual size in reality.

Representations of Map Scale

Map scales can be expressed in several ways:

• Verbal or Statement Scale: Written out (e.g., "One inch equals 24,000 inches").
• Representative Fraction (RF): Written as a ratio (e.g., 1:24,000) or a fraction.
• Graphic, Linear, or Bar Scale: A visual bar used for measurement.

Example: Calculating Real-World Distance

Let’s say that we have a map with a scale of 1:24,000:

1. This means 1 inch on the map equals 24,000 inches in reality.
2. If an airport runway measures 3.5 inches long on the map:
3. Calculation: 3.5 inches × 24,000 inches = 84,000 inches in reality.
4. Conversion to Feet: 84,000 inches ÷ 12 inches/foot = 7,000 feet long runway.

Distance and Direction Concepts

Defining Distance

• Absolute Distance

  The spatial separation between two places on the Earth’s surface as measured by an accepted standard unit (e.g., miles, kilometers, feet).

• Relative Distance

  Does not necessarily use accepted standard units, but converts linear measurements into something meaningful to the individual. Example: “I live 30 minutes from work.”

Types of North Reference

Accurate direction finding relies on understanding different definitions of North:

• True North (Geographical North)

  The northerly direction along a line of longitude (meridian), pointing directly to the geographic North Pole.

• Magnetic North

  The direction the north arrow of a compass aligns with, following the Earth’s magnetic lines of force. This point is not the geographic North Pole and shifts over time.

• Grid North

  The direction of north-trending grid lines used for special coordinate systems (e.g., State Plane or Universal Transverse Mercator (UTM) grids).