3.4 GIS Data Models, Coordinate Systems, and Attribute Quality
Key Takeaways
- GIS stores geometry plus attributes, topology, and metadata; vector models points/lines/polygons, raster models a grid of cells.
- Topology rules (no gaps, no overlaps, no dangles) enforce spatial integrity in vector data such as parcels.
- A GIS layer can be fit for planning yet unsuitable for boundary, design, or stakeout because positional accuracy is too coarse.
- Metadata (datum, projection, source, accuracy, date) and attribute accuracy are as important as geometric position.
Vector vs. Raster Data Models
A geographic information system (GIS) stores spatial features together with their attributes, topology, and metadata. Two fundamental data models appear on the FS exam:
- Vector represents discrete features as points (wells, monuments), lines/polylines (roads, utilities, streams), and polygons (parcels, zoning, land cover). Vector geometry is defined by coordinates and is precise, compact, and good for boundaries and networks.
- Raster represents space as a regular grid of cells (pixels), each holding one value (elevation, land cover class, reflectance). Raster is ideal for continuous phenomena, imagery, and surface analysis; resolution is the cell size.
| Aspect | Vector | Raster |
|---|---|---|
| Structure | Points, lines, polygons | Grid of cells |
| Best for | Boundaries, networks, discrete features | Continuous surfaces, imagery |
| Position | Exact coordinates | Limited by cell size |
| Topology | Explicit (adjacency, connectivity) | Implicit (cell neighbors) |
Choosing the wrong model degrades the answer: a parcel boundary belongs in vector; a slope or land-cover surface belongs in raster. Each cell of a raster also imposes a resolution limit — features smaller than a cell cannot be resolved.
Topology, Attributes, and Coordinate Reference Systems
Topology is the explicit encoding of spatial relationships — adjacency, connectivity, and containment — independent of exact coordinates. Topology rules enforce integrity: polygons must not overlap or leave gaps (slivers), parcels must share coincident edges, and lines must connect without dangles or overshoots/undershoots. A parcel fabric that violates these rules cannot be trusted for area or adjacency analysis.
Attributes are the descriptive data joined to each feature in a table (owner, zoning, pipe diameter, install date). A feature can be geometrically correct but attributively wrong: a water main placed in the right location but labeled the wrong diameter or material will mislead every downstream user. Attribute accuracy and completeness are therefore first-class quality concerns.
Every GIS layer carries a coordinate reference system (CRS): a horizontal datum (NAD 83), a projection (State Plane Coordinate System or UTM), and possibly a vertical datum (NAVD 88). UTM divides the globe into 60 zones each 6 degrees wide; SPCS divides states into zones designed to keep distortion within about 1 part in 10,000. Layers in different CRSs must be reprojected to a common one before overlay, or features will appear shifted (mixing NAD 27 and NAD 83 can offset data by ~100+ m).
Metadata, Accuracy, and Fitness for Purpose
Metadata documents a dataset's lineage so users can judge it: source, datum and projection, capture date/currency, positional accuracy, attribute definitions, and processing history (commonly to an FGDC/ISO standard). A layer without metadata cannot be safely overlaid or trusted.
The central FS lesson is fitness for purpose. A municipal GIS parcel layer is excellent for tax administration, planning, and notification, but its positional accuracy (often several feet to tens of feet) makes it unsuitable for boundary determination, design, or stakeout. GIS represents the best available record geometry, not a survey. Using a planning-grade layer for a survey-grade decision is the precise error these questions test.
Key GIS analysis operations you should recognize:
- Buffer — a zone within a set distance of a feature (e.g., 100-ft wetland setback).
- Overlay/intersect/union — combining layers to find where conditions coincide.
- Spatial join — transferring attributes based on location.
- Geocoding/network analysis — addresses to coordinates; routing along a network.
These answer planning and analysis questions; none of them upgrade coarse positional accuracy to survey grade.
Spatial Accuracy vs. Attribute Accuracy
GIS quality has two independent dimensions the FS exam keeps distinct. Positional (spatial) accuracy is how close a feature's recorded location is to its true location. Attribute (thematic) accuracy is whether the descriptive data are correct. A feature can be perfect in one dimension and wrong in the other: a manhole digitized at exactly the right coordinates but tagged as a storm structure when it is sanitary will misroute an entire flow analysis even though its position is flawless. Both must be evaluated; neither implies the other.
Data also age. Currency (temporal accuracy) asks whether the layer still reflects the world — a parcel layer that predates a recent subdivision is positionally and attributively obsolete. Completeness asks whether all features that should exist are present. Together, accuracy, currency, and completeness define whether a layer is trustworthy for a given decision.
| Quality dimension | Question it answers |
|---|---|
| Positional accuracy | Is the feature in the right place? |
| Attribute accuracy | Is the descriptive data correct? |
| Currency | Is it up to date? |
| Completeness | Are all features present? |
How GIS and the Survey Relate
The correct mental model for the FS exam is that survey data feed GIS, and GIS organizes and serves them — but GIS does not replace the survey of record. A surveyor may deliver monumented, accurate geometry that a municipality loads into its GIS for administration; the authoritative boundary remains the recorded survey, while the GIS layer is a generalized index to it. When a GIS value and a recorded survey disagree, the recorded survey controls.
Confusing the convenient digital layer with the legal record is the conceptual error these questions are built to expose, and recognizing the proper hierarchy — record survey first, GIS as derived index — is the takeaway.
A county GIS parcel layer is geometrically tidy but positions are accurate only to about 10 ft. A surveyor is asked to set a property corner from it. What is the correct conclusion?
Two GIS layers overlay with a consistent ~100-meter shift between them. What is the most likely cause?
Which topology error describes a line that extends past the intersection it should have stopped at?