Geographic Information Systems
What the exam tests
GIS items test data models, topology, attribute operations, projections, metadata, and accuracy. Expect questions on selecting features, joining tables, and judging whether GIS data can support a survey decision.
Data models and topology
- Vector data uses points, lines, and polygons; raster data uses cells with a value per cell.
- Topology rules prevent gaps, overlaps, and dangling lines in parcel and utility layers.
- Attribute joins require a common key such as a parcel ID; a one-to-many join can inflate counts.
Projections, metadata, and accuracy
- On-the-fly projection changes display, not the stored coordinates.
- Metadata records source, date, datum, projection, units, and accuracy.
- NSSDA horizontal accuracy at 95% confidence is approximately 1.7308 times the radial RMSE. A GIS layer that meets a mapping standard may still be far less accurate than a boundary survey.
Worked example
A parcel layer is stored in NAD83 state plane feet and joined to a tax table by parcel ID. To select parcels zoned R1 larger than 0.5 acre, first confirm the area field units and projection, then query zoning = R1 and area > 21,780 square feet. If the area field were square meters, the threshold would be about 2,023 square meters.
Traps
- Assuming a GIS parcel line is a surveyed boundary.
- Mixing datums or units between layers.
- Ignoring the difference between stored and displayed coordinates.
- Trusting an area field without checking units.
- Using a one-to-many join that duplicates parcel records.