earthengine.google.com
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Google Earth Engine is a cloud-based platform for planetary-scale geospatial analysis. It provides a vast data catalog of satellite imagery and geospatial datasets, plus a code editor, API, tutorials, and sample scripts to visualize, analyze, and model environmental changes.
More resources on GIS & Remote Sensing
qgis.org
QGIS.org is the official homepage for QGIS, the free open-source desktop GIS. It provides software downloads, comprehensive documentation, tutorials, a plugin repository, and community resources for learning and applying GIS and surveying/geomatics workflows.
USGS EarthExplorer
USGS portal for searching and downloading Landsat, Sentinel, aerial photography, and elevation archives by date, cloud cover, and map extent. Provides the free imagery needed to practice change detection and classification on real scenes.
Esri Learn ArcGIS
Esri's library of guided lessons and learn paths built around real datasets, each walking through a mapping or analysis task in ArcGIS Pro or ArcGIS Online. Builds working software fluency rather than GIS theory.
Introduction to R and Geographic Information Systems (GIS) (MIT RES.1-002)
Ten short video sessions, about 2.6 hours in total, on working in R: navigating the interface, importing different data formats, fitting and interpreting linear models, and handling spatial data with GIS tools in R. Includes programming assignments for hands-on practice.
Getting to Know ArcGIS
Esri Press workbook teaching ArcGIS for Desktop through sequential hands-on exercises with supplied data, covering map creation, geoprocessing, geodatabases, georeferencing, and sharing. Finishing it leaves you able to run a standard desktop GIS workflow unaided.
gisgeography.com
GISGeography is an online learning resource that provides practical tutorials, how-to guides, and reference articles on GIS and remote sensing, covering topics from QGIS and ArcGIS workflows to cartography, data sources, and spatial analysis.