Computer Cartography Final Review
Computer Cartography Final Review GPY 200
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This 5 page Study Guide was uploaded by Maddy Moldenhauer on Tuesday April 19, 2016. The Study Guide belongs to GPY 200 at Grand Valley State University taught by Sun in Winter 2016. Since its upload, it has received 23 views. For similar materials see Computer Cartography in Geography at Grand Valley State University.
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Date Created: 04/19/16
Final Exam Review of GPY 200 Computer Cartography I. Mapping methods Graduated color maps – Use hue value to represent quantities Most often used for area features “The greater the attribute value, the darker the hue value” Choropleth maps – Type of graduated color map that exhibits attribute values that are best for mapping relative data values (densities, ratios) Use hue value to display areas (lightness or darkness), dark = more Monochromatic (single color) = depict ratio Dichromatic (use of two colors) = shows areas above or below an average Normalization – Used in graduated colors (choropleth) or graduated symbols mapping to: Convert raw counts/amounts to density data Express an attribute as a ratio of something Graduated symbol maps – Use symbol size to express quantities Typically used for point features, use of line thickness for lines Area features have a marker symbol placed in the center of the polygon Quantitative values grouped into classes to show rank or progression of values YOU CAN’T TELL THE VALUE OF INDIVIDUAL FEATURES Proportional symbol maps • Compare and contrast graduated symbol maps and proportional symbol maps Can be used to map point, line, & area features Size of a symbol reflects the actual data value of a phenomenon Represent data values MORE PRECISELY Difficulty arises when there are too many values where differences become (high values can be so large to obscure other symbols) Dot density maps Used for area features to show quantity by adjusting the # of dots (density) Each dot represents a certain # of units Dot size is identical Limitations: dots are RANDOMLY distributed throughout area, so the exact location cannot be discerned 1 II. Basic Geodesy What is geodesy? Study of the size and shape of the earth that provides basic reference systems to determine exact positions of points on Earth’s surface What are the three earth models? Spherical, ellipsoidal, and geoidal models Spherical model for SMALL SCALE mapping Authalic sphere based on the WGS84 ellipsoid Spherical Model: Represents Earth as a perfect sphere for easier math calculations Authalic sphere: same surface area as the ellipsoid Earth Ellipsoidal model combined with geodetic datum for LARGE SCALE mapping Semimajor (equatorial radius) f = (ab)/a Flattening (f) relationship between equatorial & polar radii 3 commonly used reference ellipsoids in the U.S. Clarke 1866 GRS (Geodetic Reference System) 80 WGS (World Geodetic System) 84 Geoid model for ground surveyed positions (true zero surface for elevation) What is the Geoid? Earthlike or Earthshaped reference surface for surveying, used as a true zero surface for measuring elevations What is mean sea level (MSL) used for? Geophysical model What is datum? Frame of reference used as a basis for calculating and measuring (Lat & Long values) • 3 commonly used datums in the U.S. NAD 27 NAD 83 WGS 84 III. Geographical Coordinates & Properties of the Graticule What are the two major types of coordinate systems? Geographical (Lat & Long) and Plane (X,Y) Coordinate Systems Longitude & latitude 2 Parallels Lines of equal latitude Meridians – Lines of equal longitude What is the Graticule? Imaginary network of parallels & meridians on Earth (Properties: Distance, Direction, Area) What is a great circle? Circle formed that passes through the center of the sphere How many directions and what directions are usually presented on a map? 1) True or geographic directions directions determined by the orientation of the graticule NS meridians and EW parallel 2) Magnetic directions 3) Grid directions IV. Major Map Projections What is map projection? • A map projection is a way to flatten the earth • A map projection uses mathematical formulas to transform the geographic coordinates of features on the curved (3D) earth surface to plane coordinates (x,y) on a flat (2D) surface Major types of map projection based on geometric property preserved 1) Conformal projections • Shapes true; area distorted • Ex: Mercator, Transverse Mercator, Lambert conformal conic 2) Equalarea (equivalent) projections • Area true; shapes distorted • Ex: Peters, Mollweide, Albers equalarea, Lambert equalarea Conformal or equalarea projection? LG. SCALE = CONFORMAL For largescale (below the state level) mapping, use conformal projections Ex 1: USGS uses a conformal projection (e.g., Lambert conformal conic, Transverse Mercator) for many of its topographic maps. Ex 2: Transverse Mercator (conformal) is the base for the UTM plane coordinate system 3 For smallscale (country, continents, world) mapping, use equalarea projections SM. SCALE = EQUAL AREA Ex: Many U.S. government agencies (e.g. the Census Bureau) use an equal area projection (Albers equalarea conic) for standard base maps. V. Plane Coordinate Systems Typically used in largescale mapping such as at a scale of 1:24,000 or larger Designed for detailed calculations and positioning False easting (X) and false northing (Y) Plane coordinate systems used on maps in the U.S. • Universal transverse Mercator (UTM) • grid syste m – Based on the Transverse Mercator Projection (conformal) Worldwide coordinate system, divides Earth into 60 zones (6°) Michigan in UTM Zone 16N • State plane coordinate (SPC) • sys • te • m • • Defined for each of the 50 U.S. States, may have 2 or more SPC Zones to minimize distortion • Uni • versa • l • polar stereographic (UPS) grid system • Covers polar areas using grid zones of 100 km squares • Public land survey system (PLSS) System is defined on the ground, not on a grid superimposed map Basic unit = acre Townships 6by6mile squares with NS and EW lines 4 VI. Projected coordinate systems commonly used on Michigan maps MiGeoref – Covers the entire state of Michigan in ONE zone Uses an oblique Mercator projection 4 parts in 10,000 UTM Below the state level: zones 15, 16, 17 Transverse Mercator projection 4 parts in 10,000 Disadvantage: zone boundaries are meridians that cut though state, county, & local gov’t boundaries SPC(S) Below the state level: North, Central, South More Accurate to one part in 10,000 Zone boundaries to follow county boundaries 5
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