GIS Map Projections and Datums 4 — Questions and Answers
Question 1: Which projection property is preserved by the Azimuthal Equidistant projection from its center point?
- Equal area
- True scale (distance) along all radii from the center (Correct answer)
- Conformality everywhere
- Equal angles between meridians and parallels
Correct answer: True scale (distance) along all radii from the center
The Azimuthal Equidistant projection preserves true distances and directions from the central point to all other points on the map.
Question 2: A GIS analyst notices a 30-meter discrepancy between GPS points and legacy survey data. The most likely cause is:
- GPS multipath error in urban canyons
- A datum mismatch between WGS84 and NAD27 (Correct answer)
- Incorrect projection zone assignment
- Atmospheric refraction of GPS signals
Correct answer: A datum mismatch between WGS84 and NAD27
Horizontal offsets of 20–100 meters between WGS84/NAD83 and NAD27 are common in the contiguous US, the most frequent source of such discrepancies.
Question 3: The Albers Equal-Area Conic projection is most appropriate for mapping:
- Polar regions
- East-west extents in mid-latitudes such as the conterminous United States (Correct answer)
- Equatorial regions
- Navigation routes across oceans
Correct answer: East-west extents in mid-latitudes such as the conterminous United States
Albers is optimized for regions with greater east-west than north-south extent at mid-latitudes, making it ideal for CONUS maps.
Question 4: What is the geoid, and how does it relate to a geodetic datum?
- The geoid is a perfect ellipsoid; datums ignore it
- The geoid is an equipotential surface approximating mean sea level; datums use an ellipsoid that best fits it (Correct answer)
- The geoid and datum are identical for all practical purposes
- The geoid defines horizontal control; datums define only vertical control
Correct answer: The geoid is an equipotential surface approximating mean sea level; datums use an ellipsoid that best fits it
The geoid is an irregular equipotential surface (mean sea level); geodetic datums use a mathematical ellipsoid fitted to approximate the geoid for horizontal positioning.
Question 5: On a Mercator projection, rhumb lines appear as:
- Curves that arc toward the poles
- Straight lines (Correct answer)
- Great circles
- Ellipses
Correct answer: Straight lines
Mercator preserves angles (conformal), so a course of constant compass bearing (rhumb line) plots as a straight line, which is why it is valued for nautical navigation.
Question 6: Which parameter defines the orientation of a projected coordinate system's grid relative to the ellipsoid?
- False northing
- Central meridian (Correct answer)
- Scale factor at origin
- Latitude of origin
Correct answer: Central meridian
The central meridian defines where the projection's grid north aligns with true north, setting the east-west axis of symmetry for the projection.
Question 7: The PROJ (projection) library parameter '+ellps=GRS80' specifies:
- A vertical datum based on mean sea level
- The GRS80 reference ellipsoid for defining Earth's shape (Correct answer)
- A projection type for equal-area mapping
- A geographic transformation to WGS84
Correct answer: The GRS80 reference ellipsoid for defining Earth's shape
The '+ellps' parameter in PROJ specifies the reference ellipsoid; GRS80 is the ellipsoid adopted for NAD83 and most modern geodetic reference systems.
GIS Map Projections and Datums 4
Which projection property is preserved by the Azimuthal Equidistant projection from its center point?