Geographic Information Systems Flashcards

In raster GIS, what does 'resampling' refer to when changing the cell size of a bathymetric grid?

Answer: Recalculating cell values when converting between grid resolutions using methods like nearest neighbor or bilinear interpolation

Resampling recalculates cell values when a raster is reprojected or rescaled, using algorithms like nearest neighbor, bilinear, or cubic convolution to estimate new cell values.

What is the purpose of the S-57 Object Catalog in hydrographic GIS?

Answer: It defines all permissible object classes, attributes, and their relationships for ENC data

The S-57 Object Catalog (IHO Publication S-57 Appendix A) defines the complete dictionary of feature classes and attributes allowed in Electronic Navigational Charts.

Which attribute of a GIS point feature is most critical for validating the absolute horizontal accuracy of a sounding position?

Answer: The horizontal uncertainty value associated with each position

The horizontal uncertainty (or positional accuracy) attribute quantifies the confidence in each sounding's geographic position, which is essential for IHO Order compliance.

When overlaying two polygon layers in GIS, which operation retains only the area that exists in BOTH input layers?

Answer: Intersect

The Intersect operation outputs only the geographic area and attributes common to both input polygon layers, discarding everything outside the overlap.

A GIS layer for charted obstructions uses the IHO object class 'OBSTRN'. Which attribute would indicate whether the obstruction is a wreck, rock, or submerged pile?

Answer: CATOBS (Category of Obstruction)

CATOBS is the S-57 attribute that categorizes the type of obstruction (e.g., wreck, rock, submerged pile), distinguishing between different hazard types under the OBSTRN object class.

In a hydrographic survey GIS, what is a 'least depth' surface used for?

Answer: Preserving the shallowest depth in each grid cell to ensure the most conservative safe navigation depth

A least depth surface retains the minimum (shallowest) depth in each cell, ensuring that submerged hazards are not masked by deeper adjacent soundings — critical for navigational safety.

What does 'feature generalization' accomplish when preparing hydrographic data for smaller-scale chart production?

Answer: It simplifies or aggregates complex geometries to reduce detail appropriate for smaller map scales

Generalization reduces geometric complexity (by simplifying lines, merging small polygons, etc.) so that charts at smaller scales remain readable without overcrowding.