Introduction to Geographic Information Systems (GIS)
Fundamental Concepts in GIS
a. Characterizing Geographic Concepts
In the discrete object view, the world is seen as empty except where occupied by objects with well-defined boundaries. Examples include lakes in Minnesota or mountains in Scotland.
In the continuous field view, the world is described by variables measurable at any point on Earth’s surface, with values changing across the surface.
b. Digital Representation
Discrete objects are represented using vector data, while continuous fields are represented using raster data.
c. Continuous Field with Nominal/Ordinal Data
A choropleth map is used to represent a continuous field containing non-overlapping areas measured at a nominal or ordinal scale.
Map-Making and Projections
a. Geometrically Different Projection Models
- Planar
- Cylindrical
- Conic
b. Projection Aspect
Projection aspect refers to the direction of the projection model. For example, a cylindrical projection with equatorial aspect is represented with a cylinder wrapped around the Equator.
c. Secant Projection Advantages
Secant projections offer smoother scaling because the scale factor is 1 along two lines of secant intersection, compared to one line for tangential projections. This results in a scale factor less than 1 between secant intersections and greater than 1 elsewhere, unlike tangential projections where the scale factor is always greater than 1.
Geographic Data Capture
a. Primary Raster Data Capture Example
Remote sensing
b. Primary Vector Data Capture Example
- GPS
- Ground surveying using techniques like triangulation
c. Secondary Raster Data Capture Example
Scanning
d. Secondary Vector Data Capture Example
- Table-top digitization
- On-screen digitization
Map Layout Elements
Fundamental elements of a map layout include:
- Title
- Legend
- Scale bar (and/or ratio)
- Projection (and geographic datum)
- Data source (copyright)
- North arrow (if needed)
- Map grid (when using geographic coordinates)
- Author and date
- Inset map
Cartography and Map Design
a. Multivariate Symbolisation Example
- Pie diagrams combining size with color coding
- Arrow symbolization combining direction with color
b. Choropleth Class Definition Methods
- Natural breaks
- Standard deviation
- Equal interval
- Quintile/percentile
c. Cartogram Transformation Example
- Underground routes
- Areas distorted to reflect population size, transportation costs, or greenhouse gas emissions
Geographic Analysis
a. Great Circle Metric
The great circle metric describes the effect of Earth’s curvature on straight-line directions projected onto a plane (like a map).
b. Measuring Shape
Shape is measured by the degree of contortion, expressed as the ratio of polygon perimeter to the perimeter of a circle with the same area: S = P / 2√(πA), where S is the shape index (S ≥ 1).
c. Alternative Slope Definitions
- Ratio of change in elevation to the actual distance traveled (range: 0 to 1)
- Ratio of change in elevation to the horizontal distance traveled (range: 0 to infinity)
- Angle between the surface and the horizontal (range: 0 to π/2)
Pattern Analysis
a. Archetypal Pattern Types
- Random pattern: Points are located independently, and all locations are equally likely.
- Clustered pattern: Some locations are more likely than others, and the presence of one point may attract others to its vicinity.
- Dispersed pattern: The presence of one point may make others less likely in its vicinity.
b. Generating Processes
- First-order processes: Points are located independently but may still result in clusters due to varying point density.
- Second-order processes: Involve interaction between points, leading to clusters when interactions are attractive and dispersion when they are competitive or repulsive.
Overlays in ArcGIS
a. Union Overlay
A union overlay adds the overlapping areas to the input features.
b. Intersect Overlay
An intersect overlay replaces the input features with the overlapping areas.
Symbolization in ArcGIS
a. Creating a 3D Illusion
Make the top layer (elevation) semi-transparent.
b. Enhancing the 3D Illusion
Replace the elevation layer with a non-equidistant representation of altitude, such as a triangulated irregular network (TIN).