GIS For Engineering (Knowledge) 5 — Questions and Answers
Question 1: What is the primary use of Positional Dilution of Precision (PDOP) in GIS engineering GPS surveys?
- Measuring the vertical accuracy of a DEM
- Indicating the quality of GPS satellite geometry affecting positional accuracy (Correct answer)
- Calculating the distance between two GPS points
- Determining the datum used by a GPS receiver
Correct answer: Indicating the quality of GPS satellite geometry affecting positional accuracy
PDOP measures how satellite geometry affects GPS accuracy; a low PDOP (below 4) indicates well-spaced satellites and more reliable positional measurements.
Question 2: In GIS-based floodplain analysis for engineering design, which federal dataset defines base flood elevations in the US?
- National Land Cover Database (NLCD)
- FEMA Flood Insurance Rate Maps (FIRM) (Correct answer)
- USGS National Hydrography Dataset (NHD)
- Census TIGER/Line Files
Correct answer: FEMA Flood Insurance Rate Maps (FIRM)
FEMA's Flood Insurance Rate Maps define the 100-year floodplain boundaries and Base Flood Elevations used by engineers for site design and regulatory compliance.
Question 3: What is 'conflation' in the context of GIS engineering data management?
- Converting vector data to raster format
- Merging and aligning two datasets of the same area to create an improved combined dataset (Correct answer)
- Generating contour lines from a DEM
- Attaching attribute tables to feature classes
Correct answer: Merging and aligning two datasets of the same area to create an improved combined dataset
Conflation aligns features from two overlapping datasets and transfers attributes, used to update older engineering basemaps with more accurate survey data.
Question 4: Which map projection preserves area and is most appropriate when an engineering study requires accurate acreage calculations across a large region?
- Mercator
- Albers Equal-Area Conic (Correct answer)
- Robinson
- Stereographic
Correct answer: Albers Equal-Area Conic
The Albers Equal-Area Conic projection preserves area relationships across wide extents, making it the standard choice for US-wide area measurement studies.
Question 5: In an engineering GIS workflow, what is the purpose of 'edge matching' when working with adjacent map sheets?
- Assigning coordinate systems to new feature classes
- Ensuring that features align correctly and attributes match across tile boundaries (Correct answer)
- Merging raster images into a mosaic
- Calculating lengths of linear features
Correct answer: Ensuring that features align correctly and attributes match across tile boundaries
Edge matching aligns vector features (roads, pipes, parcels) that cross boundaries between adjacent map tiles or data sources to eliminate positional and attribute discrepancies.
Question 6: What is the function of a 'spatial index' in a GIS engineering database containing millions of features?
- It assigns coordinate values to unspatial records
- It accelerates spatial queries by organizing features into a searchable spatial structure (Correct answer)
- It reprojects features to a new coordinate system
- It validates topology rules across feature classes
Correct answer: It accelerates spatial queries by organizing features into a searchable spatial structure
Spatial indexes (such as an R-tree or grid index) allow the database engine to quickly locate features within a query extent without scanning every record.
Question 7: When integrating CAD drawings (.dwg) into an engineering GIS project, what is a common first step to ensure proper alignment with existing GIS data?
- Convert all polylines to raster format
- Define or assign the correct coordinate system to the CAD data and verify it aligns with GIS layers (Correct answer)
- Merge the CAD file with an aerial image
- Run a topology check on the CAD file
Correct answer: Define or assign the correct coordinate system to the CAD data and verify it aligns with GIS layers
CAD files often lack embedded coordinate system information or use local design coordinates, so defining the correct CRS and reprojecting as needed is essential before integration.
GIS For Engineering (Knowledge) 5
What is the primary use of Positional Dilution of Precision (PDOP) in GIS engineering GPS surveys?