GIS - Geographic Information System Map Projections and Datums Questions and Answers

Question 1

A GIS analyst in North America overlays two vector datasets of a county boundary. One dataset is referenced to the North American Datum of 1927 (NAD27) and the other is referenced to the North American Datum of 1983 (NAD83). The analyst observes a consistent spatial offset of approximately 40 meters between the two layers. What is the most appropriate step to resolve this discrepancy?

Accepted Answer

Perform a datum transformation on one of the datasets.

Answer

The observed consistent offset is a classic example of a 'datum shift'. NAD27 is a local datum with its origin at Meades Ranch, Kansas, while NAD83 is a geocentric datum centered on the Earth's center of mass. Due to these different reference frames, the same latitude and longitude coordinates will plot in different locations, often by tens of meters. The correct procedure is to use a specific datum transformation algorithm to convert the coordinates from one datum to the other, which correctly aligns the data. Changing the projection alone does not resolve the underlying datum difference.

Question 2

A demographer is creating a world map to visually compare the population density of different countries. To ensure that the graphical size of each country on the map is directly proportional to its actual land area, which type of map projection property is most critical?

Accepted Answer

Equal-area

Answer

Equal-area projections, also known as equivalent projections, preserve the property of area. This means that any two areas on the map have the same proportional relationship as their real-world counterparts. This is essential for thematic maps where visual comparison of size is important, such as maps of population density or land use. Conformal projections preserve shape, equidistant projections preserve distance from certain points, and azimuthal projections preserve direction from a central point, but all of these will distort area.

Question 3

Which of the following statements most accurately describes the core difference between a Geographic Coordinate System (GCS) and a Projected Coordinate System (PCS)?

Accepted Answer

A GCS defines locations on a 3D, curved model of the Earth using angular units (latitude/longitude), while a PCS represents locations on a flat, 2D surface using linear units (meters/feet).

Answer

The fundamental distinction is the model of the Earth's surface and the units used. A Geographic Coordinate System (GCS) uses a three-dimensional spherical or ellipsoidal model to define locations with angular units, specifically latitude and longitude. A Projected Coordinate System (PCS) is created by applying a mathematical projection to a GCS, transforming the 3D coordinates onto a 2D planar surface with Cartesian (x,y) coordinates measured in linear units like meters or feet.

Question 4

An environmental agency is mapping a long, narrow river corridor that runs primarily north-to-south through a state. To minimize distortion for distance and shape measurements within this corridor, which map projection would be most suitable for their State Plane Coordinate System zone?

Accepted Answer

Transverse Mercator

Answer

The Transverse Mercator projection is created by orienting a cylinder tangent along a meridian of longitude, rather than the equator. This makes it ideal for mapping areas that have a greater north-south extent than east-west. For this reason, it is the projection commonly used in the State Plane Coordinate System for states or zones that are taller than they are wide. In contrast, the Lambert Conformal Conic is used for zones with a greater east-west extent.

Question 5

Which of the following best defines a horizontal datum?

Accepted Answer

A reference framework consisting of an ellipsoid, its orientation, and an origin point, which defines the size and shape of the Earth for mapping purposes.

Answer

A horizontal datum is a fundamental reference framework for defining horizontal positions (latitude and longitude). It consists of three main components: a mathematical model of the Earth's shape (an ellipsoid or spheroid), the orientation of that model relative to the Earth (its position and rotation), and an origin or initial point that ties the model to a physical location on the Earth. This framework is the basis upon which all coordinate measurements are made.

GIS - Geographic Information System Practice Test

GIS - Geographic Information System Practice Test

GIS - Geographic Information System Map Projections and Datums Questions and Answers