4: Map Projections Flashcards
EXPLAIN WHY WE NEED LOCATIONAL REFERENCE SYSTEMS
We need a locational reference system so we can accurately describe where something is located.
Might be x y coordinates, a grid reference on a map, or latitude and longitude.
LIST TYPES OF LRS
Planar - assumes a 2D surface
Spherical - assumes a curved 2D surface
Most well know: latitude and longitude (spherical example of a geographic coordinate system)
EXPLAIN WHY MAP PROJECTIONS MATTER FOR GIS
Maps are flat, the planet is not, projections are used to project the globe to a 2D plane. However, this introduces distortions!
Data layers can only be aligned in GIS given the same locational reference system AND projection!
It can also cause distances and areas to be wrong if you use a projection that distorts either of these properties.
GIVE EXAMPLES OF MAP PROJECTION TYPES
Planar (azimuthal), Cylindrical, Conic
LIST DISTORTION CHARACTERISTICS OF MAP PROJECTIONS
Three important properties:
Conformal (shape)
Equal Area (area)
Equal Distance/Direction (angle/direction and distance)
Longitude
meridians cut through the poles always divide globe in half each is equal in length Prime Meridian (0) is Greenwich widest apart at equator closest at poles measured in degrees relative to the prime meridian (-180 west to +180 east)
Longitude
also called meridians cut through the poles always divide globe in half each is equal in length Prime Meridian (0) is Greenwich widest apart at equator closest at poles measured in degrees relative to the prime meridian (-180 west to +180 east)
Latitude
also called parallels
lie at right angles with longitude
concentric circles
each circle has a different circumference
0 at equator
are represented at the poles as a point
measured relative to equator -90 at south pole, +90 at north pole
How is a projection different than an LRS?
A map projection uses mathematical formulas to relate spherical coordinates (x, y) on the globe to planar 2D Cartesian coordinates
What is a projected coordinate system?
A geographical (spherical) coordinate system (LRS) projected onto a flat 2D plane.
How is a map projection different than an LRS?
A map projection uses mathematical formulas to relate spherical coordinates (x, y) on the globe to planar 2D Cartesian coordinates
Peters vs Mercator projections
Mercator - used for navigation, any course is a straight line
Peters - equal area
Peters vs Mercator projections
Mercator - used for navigation, any course is a straight line, distorts massively at the poles
Peters - equal area, most accurate at equator
3 types of map projections
Plane (azimuthal or planar)
resulting map is a circle, often used for mapping poles, distortion biggest at extremes and accurate in the middle
Cylinder (cylindrical)
Cone (conic)
wrap around one end of the globe, most accurate at the latitude where it touches
in all examples… most accurate where the ‘paper’ touches the globe (tangent) is the line of minimum distortion, centre map on this location
Conformal
Distortion characteristics in which shapes of small features are preserved - anywhere on the projection the distortion is the same in all directions
(navigation, straight line has a constant bearing)
Equal Area
Distortion characteristics in which shapes are distorted but preserve areal relationships (calculation of property areas, thematic mapping)
Equal Distance
Distortion characteristics in which shapes are distorted but great circle distances/directions are true from one to all other points
(calc travel distance)
Cylindrical projection
Wrap a cylinder around the globe
Mercator is one of the best known (cylinder wrapped around an equator)
Conformal distortion property
-at any point scale is the same in both directions
-shape of small features is preserved
-features in high latitudes are significantly enlarged
Planar (azimuthal) projection
Touch paper with the globe
Azimuthal equidistant projection is one of the best known planar projections
Tangent location
-at a point on the sphere (standard point)
-or cut through the sphere (standard line)
Tangency can be varied: projection aspect
-polar aspect (tangent at pole)
-oblique aspect (at mid-latitude)
-equatorial aspect (at equator)
Distance/direction distortion property
-true from centre point along great circles
Conic projection
Wrap a cone around the globe
Lambert Conformal Conic projection commonly used to map North America
-standard parallels occur where cone intersects the earth
-lines of latitude appear as arcs of circles
-lines of longitude are straight lines radiating from the north pole