Geospatial Data: GIS, Remote Sensing, GPS, and Field Work

Three technologies, three different questions

GPS (part of the wider GNSS family) uses a constellation of satellites to fix an absolute location — a coordinate pair — for a receiver on the ground. It answers "where exactly is this point?" and it is what lets a delivery driver, a census enumerator or a farmer tag data with a position.

Remote sensing collects information about the Earth's surface from satellites or aircraft without physical contact. Its strength is change over time and over inaccessible areas: deforestation, urban sprawl at the fringe, flood extent, crop health, ice loss. It gives imagery, not explanations.

GIS — a geographic information system — is software that stores spatial data in layers and lets an analyst overlay them and query the relationships. GIS is the only one of the three that can answer "which flood-prone blocks also have high proportions of residents over 65 and no bus route within 800 m?" That question requires three layers to be compared, and comparison is exactly what GIS does.

Where the data comes from

Quantitative spatial data (census counts, satellite pixel values, tax records) supports mapping of rates and totals. Qualitative data — field observation, interviews, photographs, landscape analysis, personal narratives — supplies the why behind a pattern, and the AP exam explicitly rewards knowing that a map showing what is happening rarely shows why.

Every dataset also carries limits: census categories change between counts, national statistics are aggregated to units that hide internal variation, and a satellite image cannot tell you who owns the land it shows.