Case study — GIS for integrated roadway asset management
The hook
GIS = spatial database. Every record has a location AND attributes; every map is a query result. Surveyors feed GIS the high-accuracy ground truth; planners and analysts use the result. Two fundamental data models: vector (points / lines / polygons with sharp edges) and raster (a grid of cells, like a satellite image).
Memorize these
Concepts that show up on the exam
Vector data
Points, lines, polygons defined by vertex coordinates. Precise edges, small file size, easy to edit. Used for parcels, roads, utilities, contour lines.
Raster data
A grid of cells, each holding a value. Used for elevation models, satellite/aerial imagery, classified land cover. Each cell has a fixed resolution (e.g., 30 m for Landsat).
Attribute
Non-spatial data attached to a feature. A parcel polygon has attributes like owner, zoning, tax ID, area. Stored as a database table linked to the geometry.
Layer
A single thematic dataset (parcels, zoning, soils, elevation). Layers stack on top of each other; visibility and order are controlled by the user.
Topology
Spatial relationships: which polygons are adjacent, which lines connect at nodes, which points are inside which polygon. Critical for routing, network analysis, and editing.
Geocoding
Converting a street address to a coordinate. Reverse geocoding goes the other way. Both require an address-locator dataset (typically maintained by USPS or a vendor).
Spatial join
Combining features from two layers based on a spatial relationship (within, intersects, nearest). E.g., "for every parcel, what zoning category does it fall in?"
Datum / CRS in GIS
Every layer has a Coordinate Reference System. Mixing layers in different CRS without transformation causes mis-registration. Always check + standardize.
Don't fall for these
What trips people up
Treating GIS data as survey-grade
Most GIS parcel layers are MAINTAINED for tax purposes, not boundary surveys. Positional accuracy is typically 5-50 ft. Never use a GIS parcel layer as a boundary source — go to the recorded plat.
Mixing CRS without checking
One layer in NAD83 state plane, another in WGS84 lat/lon, a third in NAD27 — overlay them and they'll be off by meters. Always verify all layers project to the same CRS before analysis.
Raster resampling artifacts
Resampling raster data to a different cell size or projection introduces error. The classic blunder: clipping a 30 m DEM to a 1 m parcel boundary and quoting elevation to the cm.
Test yourself
How well did it stick?
A quick 5-question check on GIS. See where you stand and what to review.