Ch 3: Scales and Projections Flashcards
scales
refer to how map units relate to real world units
projections
deal with the methods and challenges around turning a 3D earth into a 2D map
Representative fraction
Representative fraction → the most commonly used measure of map scale, where map scale is shown as a ratio
Graphic scale
scale bars are graphical representations of distance on a map
Verbal description
→ e.g. “one inch represents one km”
extent of a map
The extent of a map describes the area visible on the map
Extent is like describing the region to which the map is zoomed
E.g. extent of a map is national
resolution
resolution describes the smallest unit that is mapped
-e.g. the state
coordinates
Locations on the earth’s surface are measured in terms of coordinates, a set of 2 or more numbers that specifies a location in relation to some reference system
Simplest system is a Cartesian coordinate system, a grid forms by putting together two measurement scales (x and y)
geographic coordinate system
Geographic coordinate system is designed specifically to define positions on the Earth’s roughly-spherical surface
Used an east-west scale (longitude) and the International Date Line
Opposite of IDL is the prime meridian (line of longitude defined as 0 degrees)
North-south scale (latitude) ranges from north to south
Longitude specifies positions East and West and latitude specifies positions
At higher latitudes, the length of parallels decreases to zero
Lines of longitude are not parallel but converge toward the poles
longitude
positions east and west
latitude
positions north and south
are lines of longitude parallel
Lines of longitude are not parallel but converge toward the poles
projection
Map projection refers to both the process and product of transforming spatial coordinates on a 3D sphere to a 2D plane
Most projections transform part of the globe to one of three “developable” surfaces, so called because they are flat or can be made flat: plane, cone, and cylinder
The resultant projections are called planar, conical, and cylindrical
We use developable surfaces because they eliminate tearing, although they will produce shearing and compression
tangent point/line
The place where the developable surface touches the globe is known as the tangent point/line
Maps will most accurately represent objects on the globe at these tangent points or line, with distortion increasing as you move farther away due to shearing and compression
It is for this reason that cylinders are often used for areas near the equator, cones used to map the mid-latitudes, and planes used for polar regions.
how do projections distort features?
Projections must distort features on the surface of the globe during the process of making them flat because projection involves shearing, tearing, and compression
Since no projection can preserve all properties, it is up to the map maker to know which properties are most important for their purpose and to choose an appropriate projection
conformal projections
Conformal projections preserve shape and angle, but distort area in the process
-For example, with the Mercator projection, the shapes of coastlines are accurate on all parts of the map, but countries near the poles appear much larger relative to countries near the equator than they actually are
when should conformal projections be used
if main purpose of map involves measuring angles or representing the shapes of features
Useful for navigation, topography(elevation) and weather maps
-used as basis for large scale surveying and mapping
equal area projection
the size of any area on the map is in true proportion to its size on Earth
Countries shapes many appear to be squished or stretched compared to what they look like on a globe but their land area will be accurate relative to other land masses
E.g. Gall-Peters projection, the shape of Greenland is significantly altered, but the size of its area is correct in comparison to Africa
what are equal area projections important for
This type of projection is important for quantitative thematic data, especially in mapping density (an attribute over an area)
-For example, it would be useful in comparing the density of Syrian refugees in the Middle East or the amount of cropland in production.
Equal-area projections are preferred for small-scale thematic mapping, especially when map users are expected to compare sizes of area features like countries and continents
what gets distorted on equal area projection
The ellipses maintain the correct proportions in the sizes of areas on the globe but that their shapes are distorted
equidistant projections
Equidistant projections preserve distance
Equidistant maps are able, however, to preserve distances along a few clearly specified lines
For example, on the Azimuthal Equidistant projection, all points are the proportionally correct distance and direction from the center point
what are equidistant projections used for
This type of projection would be useful visualizing airplane flight paths from one city to several other cities or in mapping an earthquake epicenter
what gets distorted on an equidistant projection
Azimuthal projections preserve distance at the cost of distorting shape and area to some extent
The flag of the United Nations contains an example of a polar azimuthal equidistant projection
Compromise, Interrupted and Artistic Projections (Robinson projections)
Don’t preserve a specific property but seek a compromise that minimizes distortion of all kinds, as with the Robinson projection, which is often used for small-scale thematic maps of the world
Interrupted projections such as the interrupted Goode Homolosine projection represent the earth in lobes, reducing the amount of shape and area distortion near the poles
