Domain 1: Geospatial Data Foundations, Coordinate Systems, and Data Quality
Domain 1 examines how location is represented, stored, and trusted. Expect questions on coordinate reference systems (CRS), datums, projections, vector and raster models, topology, metadata, and data-quality reporting. The exam tests whether you can choose the right representation and defend the accuracy of a result.
CRS, datums, and projections
A CRS pairs a datum-the mathematical model of Earth and its origin-with a coordinate system. Geographic coordinates are angular (longitude, latitude); projected coordinates are linear (metres or feet). A datum is not a projection: WGS84 is a datum; UTM Zone 15N is a projected CRS. NAD83 and WGS84 are close but not identical, and NAD27 can differ from NAD83 by tens to hundreds of metres in the conterminous United States. Record the EPSG code and datum, and apply an explicit datum transformation when mixing sources.
Vector, raster, and topology
Vector data store points, lines, and polygons as coordinates; raster data store a grid with cell size, origin, and nodata value. Topology encodes adjacency, connectivity, and containment, helping prevent gaps, slivers, and dangling lines. In a geodatabase, feature datasets can enforce a shared spatial reference and topology rules.
Metadata and quality
Metadata documents source, currency, lineage, positional and attribute accuracy, completeness, and licensing. Positional accuracy is often reported as RMSE; when RMSE is the 2D radial value, the 95% circular error is roughly 1.73 × RMSE.
Worked example
A 30 m land-cover raster has 12,000 cells classified as forest inside a park. Area = 12,000 × (30 m × 30 m) = 10,800,000 m² = 1,080 hectares. Reprojecting to a CRS in US survey feet changes the cell dimensions numerically but not the ground area. If a NAD27 road layer is overlaid on a NAD83 parcel layer without transformation, offsets of 10–100 m can appear. Correct practice: project both layers to a common CRS, apply the appropriate datum transformation, and document it in metadata.
Traps
- Measuring area or distance in a geographic CRS in degrees.
- Treating on-the-fly reprojection as a substitute for a datum transformation.
- Using bilinear resampling for categorical land-cover classes.
- Assuming a smaller cell size always means higher accuracy.