Vector vs raster, and map projections

Geospatial data comes in two fundamental forms: vector and raster. Choosing the right model for a task is one of the highest-leverage decisions in a GIS workflow because almost every operation downstream (storage, querying, analysis, rendering) is asymmetric between the two.

Vector data: points, lines, polygons

Vector data represents the world as discrete geometric objects with attributes. A spaceport is a point. A rocket's ground track is a line (or a multiline if it crosses the dateline). A range-safety exclusion zone is a polygon. Each feature has a geometry and an attribute table — like a spreadsheet where one column happens to be geometry.

Vector data is best for things that are precisely located and countable. The GeoJSON format is the lingua franca:

{
  "type": "FeatureCollection",
  "features": [
    {
      "type": "Feature",
      "geometry": {"type": "Point", "coordinates": [-80.6, 28.6]},
      "properties": {"name": "Kennedy Space Center", "operator": "NASA"}
    }
  ]
}

Raster data: gridded pixels

Raster data is a grid of cells, each holding a value. A satellite image is a raster. A digital elevation model is a raster. A thermal brightness-temperature map is a raster. The grid is defined by an extent, a resolution (cell size in real-world units), and a coordinate system; every cell has an implicit position derived from its (row, col) index.

Raster data is best for continuous fields — temperature, elevation, reflectance — and for anything sampled at regular intervals. The GeoTIFF format is the lingua franca. rasterio is the standard Python library:

import rasterio
with rasterio.open('goes18_band7.tif') as src:
    band7 = src.read(1)  # numpy array, shape (height, width)
    transform = src.transform  # maps (row, col) → (x, y)
    print(f"Shape: {band7.shape}, dtype: {band7.dtype}, CRS: {src.crs}")

Map projections: flattening the sphere

Earth is approximately an ellipsoid. Screens are flat. Every projection is a mathematical compromise: you can preserve shape (conformal), area (equal-area), or distance (equidistant), but never all three. The right projection depends on what you're trying to do.

Web Mercator (EPSG:3857) is conformal — it preserves angles, which makes it ideal for slippy web maps where the user pans and zooms freely. It catastrophically distorts area near the poles (Greenland looks the size of Africa). Never compute areas in Web Mercator.

Universal Transverse Mercator (UTM) divides Earth into 60 zones, each 6° wide. Within a single zone, UTM is conformal AND nearly equidistant. It's the right projection for any local analysis where you need accurate distance and area at city or regional scale. Cape Canaveral is in UTM zone 17N (EPSG:32617).

Equirectangular (plate carrée, EPSG:32662) just plots latitude vs longitude as x vs y. It's the cheapest projection — a one-to-one map of degrees to pixels — and it's how most full-globe satellite imagery is published (including GOES "geographic" products). It badly stretches near the poles but is fine near the equator.

Polar stereographic (EPSG:3413 north, 3031 south) is the right choice when you actually care about the poles: Antarctic sea-ice extent, Arctic shipping lanes, polar-orbiting satellite ground tracks at high latitudes.

Picking a projection for a launch trajectory

A SpaceX Falcon 9 from Cape Canaveral arcs east over the Atlantic, ascending through ~120 km. To display the trajectory on a globe, equirectangular or Web Mercator is fine. To measure the downrange distance, project to UTM 17N for the early ascent, then use a great-circle calculation for the longer arc. To compute the maritime exclusion polygon (overlapping shipping lanes), keep everything in WGS84 lat/lon and use geodesic operations.

The lab walks through reprojecting a real Falcon 9 trajectory through three projections and shows visually (and numerically) what each one does to your distances. The takeaway you'll keep for the rest of the course: choose a projection per task, not per workflow. A single pipeline often touches three or four CRSes.