GIS Map Projections and Datums 3 — Questions and Answers
Question 1: What is the primary advantage of using a secant case projection over a tangent case projection?
- It eliminates all distortion across the map
- It spreads distortion more evenly by reducing error between two standard lines (Correct answer)
- It produces an equal-area representation
- It is always conformal
Correct answer: It spreads distortion more evenly by reducing error between two standard lines
A secant projection intersects the ellipsoid at two standard lines, balancing distortion on either side and reducing overall error compared to the tangent case.
Question 2: The Robinson projection is best described as:
- A conformal projection for navigation charts
- A compromise projection designed to minimize overall visual distortion for world maps (Correct answer)
- An equal-area projection for thematic mapping
- An azimuthal projection centered on the poles
Correct answer: A compromise projection designed to minimize overall visual distortion for world maps
Robinson is a compromise (neither conformal nor equal-area) designed to make world maps look visually balanced with moderate distortion everywhere.
Question 3: Which of the following is a parameter required to fully define a geographic coordinate system (GCS)?
- False easting and false northing
- Central meridian and scale factor
- Datum, prime meridian, and angular unit (Correct answer)
- Zone number and hemisphere
Correct answer: Datum, prime meridian, and angular unit
A GCS is defined by a datum (which includes the ellipsoid), a prime meridian, and the angular unit (degrees, radians, etc.).
Question 4: Tissot's indicatrix is used in cartography to:
- Calculate the area of map features
- Visualize the type and magnitude of distortion at points across a projection (Correct answer)
- Define the central meridian of a projection
- Convert between geographic and projected coordinates
Correct answer: Visualize the type and magnitude of distortion at points across a projection
Tissot's indicatrix places infinitely small circles on the globe; their transformed shapes on the projection reveal distortion in area, shape, and direction.
Question 5: The State Plane Coordinate System (SPCS) zones use which two projection types?
- Mercator and Sinusoidal
- Transverse Mercator and Lambert Conformal Conic (Correct answer)
- Albers Equal-Area and Azimuthal Equidistant
- Robinson and Mollweide
Correct answer: Transverse Mercator and Lambert Conformal Conic
SPCS uses Transverse Mercator for states with a north-south orientation and Lambert Conformal Conic for states with an east-west orientation.
Question 6: An oblate spheroid (ellipsoid) used as a geodetic reference differs from a sphere in that it is:
- Larger at the poles than at the equator
- Flattened at the poles and bulging at the equator (Correct answer)
- A perfect mathematical sphere scaled to Earth's size
- Elongated along a single meridian
Correct answer: Flattened at the poles and bulging at the equator
Earth is an oblate spheroid — slightly flattened at the poles and wider at the equator due to its rotation, with a flattening ratio of about 1/298.257.
Question 7: In the UTM system, what is the width of each zone in degrees of longitude?
- 3 degrees
- 6 degrees (Correct answer)
- 9 degrees
- 12 degrees
Correct answer: 6 degrees
The world is divided into 60 UTM zones, each spanning 6 degrees of longitude, numbered 1 through 60 starting from 180°W.
GIS Map Projections and Datums 3
What is the primary advantage of using a secant case projection over a tangent case projection?