Geographic Information Systems Flashcards

Which coordinate system is most appropriate for measuring accurate distances and areas in a regional hydrographic survey covering a US coastal zone?

Answer: UTM (Universal Transverse Mercator) in NAD83

UTM zones in NAD83 provide a conformal projection with low distortion over limited east-west extents, making them standard for accurate distance and area measurements in US coastal surveys.

In GIS-based hydrographic data quality assessment, what does 'completeness' refer to?

Answer: The degree to which all required features within the survey area are present and correctly attributed

Completeness in data quality assesses whether the dataset contains all features that should be present within the defined scope, including both commission (excess features) and omission (missing features) errors.

A hydrographer uses a kriging interpolation to create a bathymetric surface. What does the semivariogram model describe?

Answer: The spatial autocorrelation structure — how depth similarity decreases with distance between sample points

The semivariogram models the spatial autocorrelation of the data, quantifying how quickly depth values become uncorrelated as distance increases, which drives the kriging interpolation weights.

When integrating LiDAR topographic data with multibeam bathymetric data in a coastal GIS, what is the primary challenge at the land-sea interface?

Answer: Seamlessly merging surfaces that use different vertical datums and have different point densities in the intertidal zone

LiDAR uses orthometric or ellipsoidal heights while bathymetry references tidal datums, and both datasets have lower point density in the intertidal zone, making seamless integration challenging.

In S-57 ENC data, the meta-object 'M_COVR' (Coverage) with CATCOV=1 indicates what about the sea area?

Answer: The area is covered by ENC data and is suitable for navigation use

CATCOV=1 on the M_COVR meta-object indicates that ENC coverage exists for that area, signifying that the data has been compiled and is available for electronic chart display systems.

Which GIS operation would a port authority use to calculate the volume of sediment deposited in a channel between two bathymetric survey epochs?

Answer: A surface difference (cut/fill) analysis between the two bathymetric rasters

A cut/fill or raster difference analysis subtracts one bathymetric surface from another to compute depth changes, which when multiplied by cell area yields sediment deposition or erosion volumes.

What is the role of a 'conflation' process when merging two independently compiled hydrographic GIS datasets covering the same area?

Answer: Matching, aligning, and merging geometrically and semantically corresponding features from both datasets into a single consistent layer

Conflation involves identifying corresponding features in two datasets, reconciling their geometric and attribute differences, and producing a single unified layer that captures the best information from both sources.