GIS Map Projections and Datums 5 — Questions and Answers
Question 1: A Stereographic projection is classified as:
- Cylindrical and equal-area
- Azimuthal and conformal (Correct answer)
- Conic and equidistant
- Pseudocylindrical and compromise
Correct answer: Azimuthal and conformal
The Stereographic projection is azimuthal (projected onto a plane) and conformal (preserving local shapes and angles), commonly used for polar regions.
Question 2: When a GIS project requires area measurements to be accurate across a large country, the best projection choice is:
- Conformal (e.g., Mercator)
- Equal-area (e.g., Albers Equal-Area) (Correct answer)
- Equidistant (e.g., Azimuthal Equidistant)
- Azimuthal (e.g., Gnomonic)
Correct answer: Equal-area (e.g., Albers Equal-Area)
Equal-area projections preserve relative areas across the map, making them the correct choice for any analysis involving area calculations.
Question 3: The difference between NAD83 (2011) and NAD83 (HARN) realizations of the same datum is due to:
- Use of different ellipsoids in each realization
- Updated survey network adjustments providing more accurate station coordinates (Correct answer)
- Changes to the prime meridian definition
- Different angular units (degrees vs. gradians)
Correct answer: Updated survey network adjustments providing more accurate station coordinates
HARN (High Accuracy Reference Network) was a 1990s re-observation of control stations; NAD83(2011) uses GPS-derived adjustments, both improving on the original NAD83 realization.
Question 4: In a Lambert Conformal Conic projection, which areas experience the least scale distortion?
- Near the poles
- Along the standard parallels (Correct answer)
- Along the central meridian
- At the corners of the map extent
Correct answer: Along the standard parallels
Scale distortion is zero at the one or two standard parallels where the cone intersects the ellipsoid, increasing with distance from those lines.
Question 5: The Gnomonic projection has the unique property that:
- All meridians appear as parallel vertical lines
- Great circles appear as straight lines (Correct answer)
- The entire Earth can be shown without interruption
- It is the only truly conformal azimuthal projection
Correct answer: Great circles appear as straight lines
On the Gnomonic projection, any straight line drawn on the map is a great circle, making it useful for planning the shortest flight paths.
Question 6: A scale factor of 0.9996 at the central meridian in a Transverse Mercator projection means:
- The projection overstates distances by 0.04% along the central meridian
- Distances along the central meridian are 0.04% shorter than true scale, reducing overall distortion across the zone (Correct answer)
- The ellipsoid is flattened by a factor of 0.9996
- Only 99.96% of the zone is covered by the projection
Correct answer: Distances along the central meridian are 0.04% shorter than true scale, reducing overall distortion across the zone
Reducing the central meridian scale to 0.9996 shifts the lines of true scale inward from the edges, balancing distortion across the 6-degree UTM zone.
Question 7: Which vertical datum is commonly used for orthometric (elevation above mean sea level) heights in the United States?
- WGS84
- NAVD88 (Correct answer)
- NAD83
- IGLD85
Correct answer: NAVD88
NAVD88 (North American Vertical Datum of 1988) is the standard vertical datum for the US, defining elevations relative to a tidal benchmark in Rimouski, Quebec.
GIS Map Projections and Datums 5
A Stereographic projection is classified as: