Map Accuracy: Function of Map Projection

Map Projections  Map Projections: A method of portraying the 2-D curved surface of the Earth on a flat planar surfaceA method of portraying the 2-D curved surface of the Earth on a flat planar surface Purpose is to preserve one or several measurements of the following qualities: area, shape (conformity), direction, bearing, distance, scalePurpose is to preserve one or several measurements of the following qualities: area, shape (conformity), direction, bearing, distance, scale The reason we do this is because the Earth is an undevelopable shape. No matter how the Earth is divided up, it cannot be unrolled or unfoldedThe reason we do this is because the Earth is an undevelopable shape. No matter how the Earth is divided up, it cannot be unrolled or unfolded  Simple projections use geometric shapes that can be flattened without stretching their surfaces shapes like cylinder, cones and planes to shapes like cylinder, cones and planes to

Aspects in Map Making  Aspects to consider when creating a map: Conformity / ShapeConformity / Shape  Scale of a map anywhere is the same in any direction  Essentially, the shapes of the places are accurate Conformal: accurate shape and scale of a small area smallConformal: accurate shape and scale of a small area small Distance / ScaleDistance / Scale  measured distances are accurate Area/EquivalenceArea/Equivalence  the areas represented on the map are proportional to their area on the earth Direction / BearingDirection / Bearing  the angles of direction are portrayed accurately  Remember, every flat map misrepresents the surface of the Earth in some way

Cartographers Role  Cartographer Attempts to eliminate distortion in the aspects of the mapAttempts to eliminate distortion in the aspects of the map  Reality: distortion will occur (remember, a sphere will not lay flat) Determines which aspects are of least importanceDetermines which aspects are of least importance The purpose of the map is of primary importance when a cartographer chooses a projection.The purpose of the map is of primary importance when a cartographer chooses a projection. Different projections = different distortionsDifferent projections = different distortions

Classes of Projections  Cylindrical Projection Results from projecting a spherical surface onto a cylinderResults from projecting a spherical surface onto a cylinder Earth is projected on a secant cylinder that is cut lengthwise and laid flatEarth is projected on a secant cylinder that is cut lengthwise and laid flat  Conic Projections Results from projecting a spherical surface onto a coneResults from projecting a spherical surface onto a cone  Azimuthal Projections Results from projecting a spherical surface onto a planeResults from projecting a spherical surface onto a plane  Miscellaneous Projections Projections that do not fall into the other three categories.Projections that do not fall into the other three categories.

Cylindrical Projections  Cylindrical Mercator Projection  Most common systems in use today  Problem: Distances are only true along the equator / distorts greatly as you move north or southDistances are only true along the equator / distorts greatly as you move north or south Area and shape of large areas are distorted (increases away from the equator)Area and shape of large areas are distorted (increases away from the equator)  It is conformal in that angles and shape in small area are accurate

Cylindrical Projections  Robinson (Pseudocylindrical) Projection  Goal is to give better balance of size and shape of high- latitude lands compared to Mercator  Problem All points have some distortionAll points have some distortion  Low at equator; greatest at poles Not conformalNot conformal

Conical Projections  Lambert Conformal Conic Projection  One of the most widely used map in the US  Problem: Distortion of shape and area minimal but increases away from standard parallelsDistortion of shape and area minimal but increases away from standard parallels Map is conformal but not perspective, equal area or equidistantMap is conformal but not perspective, equal area or equidistant

Conical Projections  Equidistant Conic (Simple Conic) Used in atlases to show areas in the middle latitudes (areas on one side of the Equator)Used in atlases to show areas in the middle latitudes (areas on one side of the Equator)  Problem: Distortion increases away from standard parallels; shapes and areas distortedDistortion increases away from standard parallels; shapes and areas distorted Not conformal, perspective or equal areaNot conformal, perspective or equal area

Azimuthal Projections  Orthographic Projection Gives a perspective view of the Earth, Moon and other planetsGives a perspective view of the Earth, Moon and other planets Problem:Problem:  Directions true from center point  Scale decrease along lines radiating from center of projection  Map is perspective but not conformal or equal area  Distances are true on along the equator

Azimuthal Projections  Stereographic Projection Used to map large continent-sized areasUsed to map large continent-sized areas

Miscellaneous  Unprojected Maps Maps that are formed by looking at longitude and latitude as a rectangular systemMaps that are formed by looking at longitude and latitude as a rectangular system  Problem: Distortion of scale, distance, area and shape increase towards polesDistortion of scale, distance, area and shape increase towards poles

Map Projection in NB  NB Stereographic Double Projection Conformal projectionConformal projection  Provides slightly higher accuracy  Reasonable distribution of error  Single zone solution  Secant Azimuthal

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