Sentinel-2 and Landsat 8 and 9 are the two free satellite programmes most GIS users reach for first, and choosing between them is really a trade-off among resolution, revisit time, history and spectral depth. Sentinel-2, operated by the European Space Agency under Copernicus, images the Earth at 10 metres in visible and near-infrared bands, 20 metres in red-edge and shortwave infrared, and 60 metres in atmospheric bands, with two satellites together revisiting most places every five days. Landsat, run by NASA and the USGS, offers 30-metre multispectral imagery, a 15-metre panchromatic band, and a thermal band, with a 16-day revisit per satellite and a continuous archive stretching back to 1972.

Resolution decides what you can actually see. At 10 metres, Sentinel-2 can separate individual farm fields, large buildings and river channels, while a 30-metre Landsat pixel blends a small house, its garden and part of the road into one mixed value. That matters for field-scale work, urban change mapping and delineating small water bodies, where Landsat may show a blur where Sentinel-2 shows a boundary. Conversely, Landsat's coarser pixel is perfectly adequate for regional land-cover mapping, deforestation monitoring across a province, or tracking a reservoir over decades, and its larger pixels also mean smaller files and faster processing for continental projects.

A common follow-up question is whether to harmonise the two sensors into a single time series, and the answer depends on how strict your change detection must be. Harmonised Landsat-Sentinel products adjust for band differences and use a common grid, which makes trend charts smoother, but they also hide sensor-specific quirks that a careful analyst may want to see. For most local projects, a simpler approach works: plot both sensors on the same chart with different symbols, note the sensor in the legend, and only claim change when both sensors agree or when a ground visit confirms it. That discipline prevents a sensor switch in 2015 from being mistaken for a sudden landscape change.