Digitizing Geographic Data

Digitizing Geographic Data
An Introduction

Prof. David Bernstein
James Madison University

Computer Science Department

bernstdh@jmu.edu

Overview

Digitization:
- The process of converting (usually analog) information into a (usually digital) representation that can be processed easily using a computer
Some Examples:
- Scanning images using a a camera
- Creating 3D-models using a laser scanner
- Sampling and quantizing analog audio

Our Interest

The Source:
- A map or globe
The Result:
- A vector representation (i.e., a description of the geographic features in the map) typically consisting of points and line segments

Common Geographic Features

Points:
- At a low resolution, cities and towns
- At a high resolution, intersections
Line Segments:
- Streets and street segments
- Rivers
- Communications lines
Polygons:
- Town/city boundaries
- County/state/regional boundaries
- Country boundaries

Approaches

Manual:
- Draw a grid on the map to be digitized and interpolate within cells
Computer Assisted:
- Use a pointing device (e.g., a mouse or puck) with a known resolution and collect "clicks"
Automated:
- Use an algorithm to perform a raster-to-vector conversion
- Drive/walk the geographic features with appropriate sensors (e.g., GPS)

A Common Process

The Steps:
1. Give each point an identifier and then collect the coordinates of all points (e.g., intersections when digitizing a street map)
2. Give each line segment an identifier and define it using the identifiers of its end points (e.g., define street segments in terms of the intersections at each end)
3. Give each polygon an identifier and define it using the identifiers of its line segments (e.g., define a county in terms of the street segments on its boundary) using a consistent winding rule
4. Add shape points to line segments as needed
The Advantages:
- Reduces errors/inconsistencies (e.g., the coordinates of the same point need not be measured more than once)
- Provides topological (loosely, the relationships between the features) information in addition to geographic information

An Example Street Map