Basics of Coordinate Systems and Map Projections

Which of the following decimal degree values is equivalent to the degrees/minutes/seconds location 32°19'11"?

32.3197

Which of the following decimal degree values is equivalent to the degrees/minutes/seconds location 45°30'18"?

45.5050

Horizontal coordinate system types

Geographic, projected, local

Tangents and secants are free of spatial distortion.

True

The earth's spherical coordinate system is an angular coordinate system.

True

The term "map projection" comes from the idea of projecting a globe onto a flat surface, but most map projections are actually created using mathematical formulas.

True

You can use a sphere instead of a spheroid when your map's scale is less than 1:5,000,000.

True

The term "equidistant" describes projections that have which property?

True scale along all straight lines passing through a specified point

Which term describes the one or two lines along which a map projection intersects the earth's surface?

Secants

The amount of flattening for an ellipse or ellipsoid is usually expressed as a decimal value between which of the following whole numbers?

0 and 1

Which of the following degrees/minutes/seconds values is equivalent to the decimal degrees location -106.76528?

106°45'55" W

The distance corresponding to a degree of latitude is almost constant. (It is not quite constant because of the earth's equatorial bulge.) What is the approximate distance of a degree of latitude?

68 miles to 70 miles (110 kilometers to 112 kilometers)

If a cone or cylinder is secant to a globe, where does it intersect with the globe?

At a pair of lines

Which of the following terms describes a sphere that has the same surface area as the earth's spheroid?

Authalic sphere

Which term applies to a map projection that preserves true direction from one point to all others?

Azimuthal

Defining coordinate systems

Coordinate systems can be horizontal, defining where features are across the globe, or vertical, defining how high or deep features are relative to a surface.

Projected coordinate system (PCS)

Coordinates are expressed using linear measurements rather than angular degrees.

Which property for all straight lines passing through a single, specified point does an azimuthal projection preserve?

Direction

In a conformal map, all shapes are accurately represented, regardless of their size.

False

The distance between meridians is a constant value.

False

The only logical choice for a prime meridian was the meridian that runs through Greenwich, England.

False

There is no spatial distortion in the area between the secant lines of a projection.

False

Which of the following spheroids is now regarded as the best model of the earth for the region of North America?

GRS80

Purpose of a coordinate system

Give a frame of reference for data, to locate features on the surface of earth, to align data relative to other data, to perform spatially accurate analysis, and to make maps.

Vertical coordinate systems

Gravity-based or ellipsoidal.

Local coordinate system

Have a false origin (0,0 or other values) in an arbitrary location that can be anywhere on earth. Local coordinate systems are often used for large-scale (small area) mapping. The false origin may be aligned to a known real-world coordinate or not, but for the purposes of data capture, bearings and distances may be measured using the local coordinate system rather than global coordinates. Local coordinate systems are usually expressed in feet or meters.

Geographic coordinate system (GCS)

Have units in decimal degrees measuring degrees of longitude (x-coordinates) and degrees of latitude (y-coordinates). Location is expressed as positive or negative numbers: positive x- and y-values for north of the equator and east of the prime meridian, and negative values for south of the equator and west of the prime meridian.

Default coordinate system

In a new, empty map or scene, the default horizontal coordinate system is WGS 1984 Web Mercator.

Which of the following is the difference between the earth's semiminor and semimajor axes?

Less than 0.34 percent

Which lines on the graticule are parallel, run east-west, and mark angular distance north and south of the equator?

Lines of latitude

Which lines on the graticule run north-south, converge at the poles, and mark angular distance east and west of the prime meridian?

Lines of longitude

On the earth, the shortest distance from A to B is along a great circle. However, navigation is easier along a line of constant compass bearing, or a rhumb line. In which map projection does every straight line represent a rhumb line?

Mercator

Which of the following is another name for a line of longitude?

Meridian

Ellipsoidal coordinate systems

Reference a mathematically derived spheroidal or ellipsoidal volumetric surface. Since they are calculated on a mathematical model, they are simpler than gravity-based vertical coordinate systems, but they may lack significant accuracy, especially in large-scale applications

Gravity-based vertical coordinate systems

Reference a mean sea level calculation (or, in some cases, it is derived from the level of a single point.)

You are making a cell phone coverage map for a mountainous region. This involves calculating the viewshed (i.e., what is visible) from the cell phone tower based on the terrain. In this scenario, which spatial property is most important to preserve?

Shape

Data vs Spatial data

Spatial date carries numerical information that allows you to position it somewhere on earth. These numbers are part of a coordinate system.

Which term describes the line or point of contact between a map projection and the earth's surface?

Tangent

After converting from DMS to decimal degrees, the point location for a spot in Southern California ended up over in China. What happened?

The coordinate for longitude needs to be negative.

Projection

The means by which you display the coordinate system and your data on a flat surface, such as a piece of paper or a digital screen. Since there is no perfect way to transpose a curved surface to a flat surface without some distortion, many different map projections exist that provide different properties. Some preserve shape, while some preserve distance. Others preserve area or direction.

What tool uses ellipses to visually compare and mathematically analyze the distortion inherent in a map projection?

Tissot's indiratrix

Transformations

Transformations are required to convert data between different geographic coordinate systems or between different vertical coordinate systems

A map projection's ability to minimize distortion becomes less important as the map scale becomes larger (i.e., approaches 1:1).

True

An ellipsoid that approximates the shape of a sphere is often called a spheroid.

True

Decimal degrees are generally used to store digital coordinate information because they make digital storage of coordinates easier and computations faster.

True

When choosing a map projection, which of the following questions should you ask? (Choose three.)

What is the map's theme or subject matter? What scale is being mapped? What region of the world is being mapped?