â–¶What is the difference between raster and vector data, and when should I use each?
Vector data represents discrete objects (points, lines, polygons) with defined boundaries and attributes—e.g., buildings, roads, parcel boundaries. Raster data divides space into a grid of cells, each with a value—e.g., satellite imagery, elevation grids, climate models. Vectors excel at representing discrete features with clear edges and allow efficient queries ('which buildings are within 500 meters of a school?'). Rasters excel at continuous phenomena (temperature, vegetation index) and are efficient for large-area analysis. Choose vectors for cadastral mapping, infrastructure networks, and precise boundaries; choose rasters for remote sensing, climate modeling, and large-scale continuous surfaces. Real analyses often blend both: start with raster satellite imagery, convert interesting regions to vectors for further analysis, and overlay vector infrastructure on raster environmental data.
â–¶What is spatial autocorrelation and why does it matter for analysis?
Spatial autocorrelation is the tendency for values at nearby locations to be similar to each other—violating the independence assumption of standard statistics. High temperature in one neighborhood predicts high temperature in adjacent neighborhoods; poverty rates cluster geographically. If your data exhibit spatial autocorrelation and you ignore it, regression standard errors are underestimated and significance tests are misleading. Methods to account for autocorrelation include: spatial lag models (include the average value of neighboring units), spatial error models (error terms are correlated across space), or geographically weighted regression (estimates vary smoothly across space). Test for spatial autocorrelation using Moran's I or similar statistics. Ignoring autocorrelation is a common mistake that leads to false confidence in results.
â–¶What are map projections and why can't I just use latitude/longitude for analysis?
The Earth is a sphere; maps are flat. Projections convert geographic coordinates (lat/lon, measured in degrees) to projected coordinates (x/y, measured in meters or feet) suitable for mapping and analysis. Different projections preserve different properties: Mercator preserves angles (useful for navigation) but distorts area (Greenland looks huge); equal-area projections preserve area but distort shapes. For analysis, you must work in a projected coordinate system: distances and areas are only meaningful in projected coordinates, not in degrees. Choose a projection appropriate for your region (e.g., UTM zones for small areas, Albers for country-scale). Always check your data's coordinate system and reproject if needed before measuring distance or area. Mixing coordinate systems causes errors; use tools to verify and standardize your data's CRS.
â–¶How do I detect spatial clusters or hot spots in my data?
Spatial clustering reveals where values are unusually high or low relative to surrounding areas—useful for identifying disease outbreaks, crime hotspots, or resource concentration. Methods include: local Moran's I (identifies clusters of high/high and low/low values), Gi* (Getis-Ord) statistic (identifies hot spots and cold spots), spatial scan statistics (finds circles of unusual concentration), and DBSCAN clustering (groups nearby points). Most GIS software includes hot-spot analysis tools; R and Python have spatial statistics libraries. Visualize results as maps (color-coding hot-spot classifications) and report statistics (cluster significance, size, characteristics). Interpretation requires domain knowledge: a crime hot spot might reflect policing patterns as much as actual crime; disease clusters might reflect population density or healthcare access. Always combine statistical clustering with qualitative understanding.
â–¶What is remote sensing and what data sources are available?
Remote sensing is observation from a distance, typically from aircraft or satellites. Data types include multispectral imagery (multiple wavelengths, e.g., Landsat with 8 bands), hyperspectral (hundreds of bands), and SAR (Synthetic Aperture Radar, penetrates clouds and darkness). Free/public sources include: Landsat 8 (30m resolution, 16-day revisit, USGS), Sentinel-2 (10m resolution, 5-day revisit, Copernicus), Modis (1km resolution, daily, NASA). Commercial high-resolution imagery (1m or finer) from Planet, Maxar, Airbus is available at cost. Google Earth Engine provides free access to major archives with cloud computing for analysis. Applications: land-cover classification, vegetation monitoring (NDVI), change detection, disaster mapping. Remote sensing requires understanding spectral signatures (what bands reveal what features) and validation with ground truth data (field visits confirming what the image shows).
â–¶How do I prepare and validate spatial data for analysis, and what quality issues should I watch for?
Spatial data quality issues include: positional error (points/boundaries off their true location, e.g., from GPS error), attribute error (values assigned to wrong features), completeness (missing features or attributes), and timeliness (outdated). Validation steps: check coordinate system and projection (reproject if needed), remove or flag duplicate geometries, verify attribute values (e.g., population can't be negative), overlay with reference data (satellite imagery, prior surveys) to spot visual errors, and conduct spot-checks in the field. Document metadata: data source, collection date, accuracy specs, and known issues. Before spatial analysis, validate by overlaying your point data on satellite imagery—if points are visibly off the map, investigate whether it's a projection mismatch, systematic error, or missing reference data. Good data preparation prevents misleading analyses.
â–¶What are the advantages and limitations of cloud-based GIS platforms like Google Earth Engine?
Cloud platforms (Google Earth Engine, Sentinel Hub, AWS) eliminate the need to download massive satellite datasets; they provide pre-processed imagery archives and computational resources at no cost or low cost. Advantages: access to decades of satellite data globally (Landsat, Sentinel, MODIS), server-side processing power for large-scale analysis, no installation/maintenance burden, and rapid prototyping. Limitations: less familiar interface if you're trained on desktop GIS, limited ability to handle proprietary/local data, potential vendor lock-in, and less control over the full pipeline. For local, fine-scale analysis or proprietary data, desktop GIS (ArcGIS, QGIS) remains essential. Most workflows combine both: use cloud platforms for initial exploration and large-scale raster work, then bring results into desktop GIS for finalization and advanced vector operations.