Map design, spatial analysis, and Geographic Information Systems. Covers map projections and their distortion trade-offs, coordinate systems, thematic mapping techniques (choropleth, proportional symbol, dot density, isoline), remote sensing and satellite imagery, GIS data models (vector and raster), spatial analysis operations (overlay, buffer, interpolation, network analysis), and cartographic design principles. Use when creating maps, analyzing spatial data, selecting projections, interpreting satellite imagery, or reasoning about spatial relationships in any domain.
Cartography is the science and art of map-making. Geographic Information Systems (GIS) extend cartography into a computational framework for storing, analyzing, and visualizing spatial data. Together they provide the tools for every other branch of geography to represent and reason about spatial patterns.
Agent affinity: tobler (cartographic projection, spatial relationships, first law of geography), reclus (physical features), massey (social data mapping)
Concept IDs: geo-map-projections, geo-coordinates-scale, geo-thematic-mapping, geo-gis-remote-sensing
A sphere cannot be flattened onto a plane without distortion. Every map projection preserves some properties at the expense of others. The four properties that can be distorted are:
| Property | Definition | Projection type that preserves it |
|---|---|---|
| Area | Relative sizes of regions | Equal-area (equivalent) projections |
| Shape |
| Local angles and small shapes |
| Conformal projections |
| Distance | True distances from one or two points | Equidistant projections |
| Direction | True bearings from a central point | Azimuthal projections |
No projection preserves all four simultaneously. Choosing a projection means choosing which distortion is acceptable for the map's purpose.
Cylindrical projections wrap a cylinder around the globe:
Conic projections project onto a cone tangent or secant to the globe:
Azimuthal projections project onto a plane tangent to the globe:
Compromise projections minimize all distortions without eliminating any:
Waldo Tobler's first law of geography -- "everything is related to everything else, but near things are more related than distant things" -- has direct implications for projection choice. Maps must preserve the spatial relationships that matter for the analysis. A choropleth map of population density must use an equal-area projection (distorted areas produce distorted densities). A navigation chart must use a conformal projection (distorted angles produce wrong headings).
Latitude measures angular distance north or south of the equator (0 to 90 degrees N/S). Longitude measures angular distance east or west of the Prime Meridian (0 to 180 degrees E/W). Together they define a unique position on the globe.
Datum: A mathematical model of Earth's shape. WGS 84 (World Geodetic System 1984) is the standard for GPS and most global datasets. NAD 83 (North American Datum 1983) is the standard for US surveys. Coordinates in different datums can differ by tens of meters.
UTM (Universal Transverse Mercator): Divides Earth into 60 zones of 6 degrees longitude each. Within each zone, positions are given as easting (meters from a false origin) and northing (meters from the equator). Minimizes distortion within each zone. Standard for large-scale mapping and military grids.
State Plane Coordinate System (SPCS): US-specific system with zones designed to keep distortion under 1:10,000. Uses Lambert conformal conic for east-west-oriented states and transverse Mercator for north-south-oriented states.
Map scale = map distance / ground distance. Expressed as a representative fraction (1:24,000), a verbal statement ("1 inch = 2,000 feet"), or a graphic bar. Large scale (1:24,000) shows small areas in detail; small scale (1:1,000,000) shows large areas with less detail. The terminology is counterintuitive: "large scale" means the fraction 1/24,000 is larger than 1/1,000,000.
Thematic maps display the spatial distribution of a specific attribute or phenomenon.
| Type | Data type | Best for | Pitfall |
|---|---|---|---|
| Choropleth | Ratio/rate data by area | Population density, income per capita, election results | Must normalize by area or population; raw counts on choropleths are misleading |
| Proportional symbol | Count or magnitude at points | City populations, earthquake magnitudes, facility production | Overlapping symbols in dense areas; perception of circle area is non-linear |
| Dot density | Counts distributed across areas | Crop production, livestock, population distribution | Dot placement within polygons is arbitrary; choose dot value carefully |
| Isoline (contour) | Continuous phenomena | Elevation, temperature, atmospheric pressure, rainfall | Assumes smooth spatial variation; breaks down with abrupt boundaries |
| Flow | Movement between origins and destinations | Migration, trade, commuting | Line width must be proportional to flow volume; avoid spaghetti |
| Cartogram | Any variable, distorting area | Election results, GDP comparison | Distorted shapes can be disorienting; always provide reference map |
Visual hierarchy: The most important information should be most visually prominent. Background (base map) recedes; foreground (thematic data) advances.
Color: Use sequential palettes (light to dark) for ordered data, diverging palettes (two hues from a neutral midpoint) for data with a meaningful center, and qualitative palettes (distinct hues) for categorical data. Avoid rainbow color schemes -- they create false boundaries and are inaccessible to colorblind readers.
Classification: How continuous data is binned into classes affects the map's message. Equal interval, quantile, natural breaks (Jenks), and standard deviation methods produce different visual patterns from the same data. Report the method.
Remote sensing acquires information about Earth's surface from a distance, typically via satellite or aircraft sensors.
Electromagnetic spectrum: Sensors detect energy in visible (0.4--0.7 micrometers), near-infrared (0.7--1.3 micrometers), shortwave infrared (1.3--3 micrometers), thermal infrared (3--14 micrometers), and microwave (1 mm--1 m) wavelengths.
Key satellite platforms:
Spectral indices:
Vector model: Represents features as points, lines, and polygons with associated attribute tables. Points for wells, cities; lines for rivers, roads; polygons for countries, land parcels. Precise boundaries. Efficient for discrete features.
Raster model: Represents space as a grid of cells (pixels), each with a value. Elevation, temperature, satellite imagery, land cover classification. Efficient for continuous phenomena. Resolution = cell size.
| Operation | Description | Example |
|---|---|---|
| Overlay | Combine two or more layers to identify spatial relationships | Find areas that are both within a flood zone AND zoned residential |
| Buffer | Create a zone of specified distance around features | Identify all parcels within 500m of a highway |
| Interpolation | Estimate values at unsampled locations from known points | Create a temperature surface from weather station readings |
| Network analysis | Optimize routes, find shortest paths, define service areas | Find the fastest ambulance route to a hospital |
| Spatial join | Attach attributes from one layer to another based on location | Assign census tract demographics to school point locations |
| Reclassification | Reassign raster cell values to new categories | Combine slope, aspect, and soil type into a landslide risk index |