Key Points

Introduction to Data Collection

  • Start every project by asking whether suitable data already exists
  • Vector data stores discrete features; raster data stores values in a grid of cells
  • Check who made a dataset, when, how, at what detail, with what accuracy, and under what license
  • Make the data you collect FAIR: findable, accessible, interoperable and reusable

Finding Data in Repositories

  • Search existing repositories before creating data from scratch
  • Different repositories serve different needs: official statistics, local data, research collections, global layers
  • Read the metadata and the license before you use a dataset
  • QGIS can open data directly from a web address, so you do not need to download files

Data from the U.S. Census Bureau

  • Census boundary files provide the shapes; the Decennial Census and the ACS provide the numbers
  • GEOIDs link tables to shapes; keep them as text
  • The Decennial Census is a full count with few questions; the ACS is a yearly survey with many topics and margins of error
  • Use 5-year ACS estimates for small areas, and always check margins of error
  • The Census API needs a free key and lets you request exactly the variables and places you need

Geocoding and Adding Spatial Context

  • Geocoding converts text addresses into coordinates
  • The U.S. Census Geocoder is free and works best with complete U.S. street addresses
  • Always check match rates and inspect failed addresses
  • OpenStreetMap tags (key=value) let you download context layers such as stores or schools
  • Reproject to a meter based CRS before buffering or measuring distance

Acquiring Imagery: Scanned Maps and Satellite Data

  • Raster data includes scanned maps (which need georeferencing) and satellite images (already georeferenced)
  • Consider spatial, spectral and temporal resolution when choosing imagery
  • Declassified CORONA images (1960 to 1972) are a free historical record; CAST offers georeferenced copies
  • Sentinel-2 provides free 10 m multispectral imagery about every 5 days, from the Copernicus Browser

Collecting Data in the Field

  • Collect your own data when what you need does not exist or cannot be trusted
  • Design the schema first: clear fields, dropdowns, an “Unknown” option and a placement rule
  • Phone GPS is typically accurate to a few meters under open sky, and much worse near buildings and trees; record accuracy
  • Plan for consent, privacy and safety before collecting data about people and places
  • Check data the same day it is collected