Geographic Information Systems
This domain tests GIS data models, spatial reference, accuracy, and cadastral integration. Expect questions on vector and raster data, topology, metadata, positional accuracy standards, and the relationship between GIS parcels and survey records.
Core rules
- Vector data uses points, lines, and polygons with attributes; raster data uses cells. Topology enforces rules such as no gaps or overlaps in a parcel fabric.
- Every layer needs a coordinate reference system and datum. Transformations between datums require published parameters and, where relevant, an epoch.
- Metadata follows FGDC or ISO 19115 standards and documents source, accuracy, and processing.
- NSSDA expresses horizontal accuracy at 95 percent confidence as 1.7308 x radial RMSE. Vertical accuracy is 1.9600 x RMSE z.
- COGO tools build parcels from record bearings and distances; GIS parcel lines are representations, not monumented boundaries.
- QA/QC checks include topology validation, attribute domains, and comparison against independent sources.
Worked example
A LiDAR-derived surface has a vertical RMSE of 0.15 m against check points. NSSDA vertical accuracy at 95 percent confidence is 1.9600 x 0.15 = 0.294 m. Reporting 0.15 m as the 95 percent figure understates the error and is a common mistake.
Traps
- Reporting RMSE as if it were 95 percent accuracy.
- Treating GIS parcel lines as survey-grade boundaries.
- Ignoring datum and epoch in transformations.
- Leaving topology errors such as slivers and overlaps.
- Using the same points for adjustment and accuracy testing.