Lecture0004 - Lecture 4 Map projections A map projection is...

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1 Lecture 4. Map projections A map projection is used to convert data from a geographic coordinate system (latitude and longitude) to a projected (planar) coordinate system (x, y coordinates). Whether you treat the earth as a sphere or a spheroid , you must transform its curved surface to create a flat map sheet. This mathematical transformation is commonly referred to as a map projection. Just like there are many geographic coordinate systems, there are many different map projections as well —each preserves the spatial properties of data (shape, area, distance, and direction) differently. Project in order to perform analysis (measure distances, calculate areas and perimeters, determine the shortest route between two points) or if you need to show a particular spatial property for features on a map.
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2 Map projections A flat map showing a geographic coordinate system (latitude and longitude, set data frame to WGS_1984 before adding layer). A flat map showing a projected (planar) coordinate system (x, y coordinates).
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3 Map projections Imagine the earth's surface is clear with the graticule drawn on it. Wrap a piece of paper around the earth. A light at the center of the earth will cast the shadows of the graticule onto the piece of paper called the developable or projection surface. The shape of the graticule on the flat paper is different from that on the earth. The map projection has distorted the graticule.The shape of the graticule on the flat paper is different from that on the earth. Without further distortion, however, you can unwrap the paper and lay it flat.
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4 Map projections Developable surfaces are surfaces that can be projected onto: a cylinder, a cone, or a plane. Distortion increases with distance away from lines or points of contact between the developable surface and the globe.
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5 Cylinder Cylindrical projections produce maps with straight, evenly-spaced meridians and straight unevenly space parallels that intersect meridians at right angles.
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6 Cone Conic projections produce maps with straight converging longitude lines and concentric circular arcs for latitude lines.
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7 Plane
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