CH Geographic Information Systems 3 — Questions and Answers
Question 1: A hydrographic surveyor discovers a systematic offset between GPS positions and legacy chart features. What GIS process corrects this misalignment?
- Rubber-sheeting or georeferencing transformation (Correct answer)
- Raster reclassification
- Attribute join
- Topology repair
Correct answer: Rubber-sheeting or georeferencing transformation
Rubber-sheeting or affine georeferencing transforms shift, rotate, and scale legacy data to align with accurately positioned reference data, correcting systematic spatial offsets.
Question 2: In GIS metadata standards, which element specifically documents the horizontal positional accuracy of a hydrographic dataset?
- Lineage
- Positional accuracy report (Correct answer)
- Data quality completeness
- Distribution information
Correct answer: Positional accuracy report
The positional accuracy report element in metadata (per ISO 19115 or FGDC standards) quantifies the closeness of spatial coordinates to their true ground positions.
Question 3: Which type of GIS analysis would a hydrographer use to determine all water depths along a vessel's planned transit route?
- Spatial join between route and depth contours
- Raster calculator on the entire survey area
- Network analysis with turn restrictions
- Profile extraction along a linear path through a bathymetric surface (Correct answer)
Correct answer: Profile extraction along a linear path through a bathymetric surface
Profile extraction samples depth values along a linear transect through a bathymetric raster or TIN, yielding a cross-section of water depths along the planned route.
Question 4: What does the term 'datum shift' mean in the context of hydrographic GIS?
- A change in the vertical tidal reference level over time
- A coordinate offset that occurs when converting between different geodetic datums (Correct answer)
- The drift of GPS receivers during a survey
- A rotation of the magnetic declination grid
Correct answer: A coordinate offset that occurs when converting between different geodetic datums
A datum shift is the positional difference in coordinates that arises when the same point is expressed in two different geodetic datums, requiring transformation parameters to reconcile.
Question 5: In the S-100 framework (successor to S-57), what is the primary architectural improvement for hydrographic GIS?
- It eliminates the need for metadata
- It supports multiple data types including raster, vector, and gridded data through a universal feature model (Correct answer)
- It restricts data to 2D vector formats only
- It mandates the use of WGS84 for the first time
Correct answer: It supports multiple data types including raster, vector, and gridded data through a universal feature model
S-100 introduces a universal hydrographic data model that accommodates vector, raster, gridded, and point cloud data, enabling a much richer and interoperable suite of hydrographic products.
Question 6: Which GIS concept describes the minimum area or length a feature must have to be represented in a dataset at a given map scale?
- Spatial resolution
- Minimum mapping unit (MMU) (Correct answer)
- Feature generalization tolerance
- Attribute precision
Correct answer: Minimum mapping unit (MMU)
The minimum mapping unit defines the smallest feature size that is captured and represented at a specific map scale, directly affecting what detail is included in a hydrographic dataset.
Question 7: A hydrographer wants to identify all seabed features within 500 meters of a proposed pipeline route. Which GIS operation accomplishes this most directly?
- Clip the seabed layer to the survey boundary
- Buffer the pipeline route by 500 m, then intersect with the seabed features layer (Correct answer)
- Dissolve all seabed features by type
- Reproject the pipeline to a local datum
Correct answer: Buffer the pipeline route by 500 m, then intersect with the seabed features layer
Buffering the route creates a 500 m zone around it, and intersecting that buffer with the seabed features layer identifies all features within that corridor.
CH Geographic Information Systems 3
A hydrographic surveyor discovers a systematic offset between GPS positions and legacy chart features.
What GIS process corrects this misalignment?