Geographic Information Systems Flashcards

In a GIS wetland project, you discover that two polygon layers from different agencies have a 15-meter spatial offset at the same site boundary. What is the most likely cause?

Answer: The layers use different datums or projections that were not properly reconciled

A consistent spatial offset between layers typically indicates a datum or projection mismatch, where coordinates were interpreted under different geographic reference systems.

Which remote sensing product is most useful for detecting seasonal inundation in forested wetlands where the tree canopy obscures the ground surface?

Answer: Synthetic Aperture Radar (SAR) imagery

SAR can penetrate forest canopy and detect surface water by measuring backscatter differences, making it the most effective tool for mapping inundation beneath tree cover.

A GIS layer of hydric soils is overlain with a wetland vegetation layer. Which overlay operation produces a new layer containing only areas where both hydric soils AND wetland plants are present?

Answer: Intersect

The Intersect operation retains only the geographic areas where both input layers overlap, identifying locations that meet both the soil and vegetation criteria simultaneously.

When preparing a wetland delineation map for a Corps jurisdictional determination, the minimum map scale typically required is:

Answer: 1:24,000 or larger (more detailed)

Corps guidance requires delineation maps to be at 1:24,000 scale or larger to clearly show wetland boundaries and their relationship to project features with adequate spatial detail.

Which attribute in the National Wetlands Inventory (NWI) geodatabase encodes the wetland classification according to the Cowardin system?

Answer: ATTRIBUTE

The ATTRIBUTE field in the NWI geodatabase contains the Cowardin alphanumeric code that classifies each wetland by system, subsystem, class, subclass, and modifiers.

A wetland delineator uses GIS to calculate that a proposed fill will impact 0.38 acres of wetland. For Section 404 permitting purposes, why is the calculation method important to document?

Answer: Because acreage thresholds determine permit type and mitigation requirements, so the method must be reproducible and defensible

Documented, reproducible area calculations are critical because acreage determines whether a Nationwide Permit or Individual Permit is required and drives mitigation ratios under Section 404.

What is the purpose of using the 'Select by Location' tool in GIS during a wetland delineation project?

Answer: To identify features in one layer that have a specified spatial relationship to features in another layer

Select by Location allows the analyst to query features based on spatial relationships such as intersects, within, or within a distance, enabling targeted analysis of wetland features relative to project boundaries.