3.4 GIS Data Models, Coordinate Systems, and Attribute Quality

GIS Data Models, Coordinate Systems, and Attribute Quality

A geographic information system stores, analyzes, and displays spatial data. In surveying, GIS may support parcel indexing, utility mapping, asset management, environmental review, transportation planning, floodplain analysis, and field collection. The FS exam includes GIS because surveyors must understand both the power and the limits of spatial databases.

GIS data commonly appear as vector features or raster cells. Vector data represent points, lines, and polygons, such as monuments, water mains, parcel polygons, road centerlines, or easement areas. Raster data represent a grid of cells, such as imagery, elevation grids, land cover, or heat maps. A vector parcel layer may be efficient for ownership display, while a raster elevation model may be better for terrain analysis. The format should match the question being answered.

Coordinate reference information is critical. GIS layers are often combined from many agencies and dates. If one layer uses a projected coordinate system, another uses geographic latitude and longitude, and another lacks a defined coordinate system, the software may display them together but not necessarily correctly. Transformation, projection, datum, and unit choices can create shifts. For survey work, a layer's metadata must be checked before relying on it.

GIS quality checklist

Topology describes spatial relationships, such as parcels not overlapping, utility lines connecting at nodes, or polygons closing without gaps. A visually acceptable GIS layer can contain topology errors that affect area totals, network tracing, or ownership display. Attribute errors can be just as serious. A hydrant point in the correct place but tagged with the wrong size, status, or owner can cause a bad planning decision.

Surveyors often use GIS as a starting point, not as final evidence. A county parcel GIS can help locate records, identify adjoining owners, and plan field work. It usually should not be treated as a boundary survey by itself. A utility GIS can guide markout requests and field reconnaissance, but buried utility location still depends on records, detection, excavation, and project standards.

FS questions may ask whether a GIS layer is suitable for design, staking, boundary determination, or planning. The correct answer often turns on metadata and purpose. A layer with unknown source, unknown datum, and approximate compilation may be useful for general planning but not for setting monuments or computing construction quantities.

When evaluating GIS data, separate three issues: where the feature is, what the feature is, and how reliable both claims are. Geometry, attributes, and metadata work together. A good map product makes those limits visible rather than hiding them behind a polished display.