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What is a map projection? Because the earth is spherical and maps are flat, GIS applications require that a mathematical formulation be applied to the earth to represent it on a flat surface. This causes some distortions of distance, area, shape, or direction. Map Projections - Distortion preserve shape - conformal (orthomorphic) preserve scale – equidistant preserve direction – azimuthal preserve great circles - gnomic preserve circular shapes - stereographic What are map projections? There are two types of coordinate systems Geographic – use latitude and longitude coordinates on the surface of a sphere Projected – use mathematical conversion to transform latitude and longitude coordinates to a flat surface. ArcMap has over a hundred projections Albers – equal area conic projection Shape and linear scale Minimally distorted in the region between the standard parallels Areas proportional to the same areas on the earth Direction Locally true Distance Most accurate in the middle latitudes Robinson – world projection Compromise projection used for world maps Shape Distortion is very low within 45° of the origin and along the equator Areas Distortion is very low within 45° of the origin and along the equator. Direction Generally distorted Distance Scale is constant along any given latitude and for latitude of the opposite sign Limitations Useful only for world maps State Plane Not a projection it is a coordinate system that divides the fifty U.S. states, Puerto Rico, and the U.S. Virgin Islands into more that 120 numbered sections, referred to as zones. Designed for large-scale mapping in the U.S. UTM – Universal Transverse Mercator Shape Accurate representation of small shapes Areas Minimal distortion within each UTM zone Direction Local angles are true Distance Scale is constant along the central meridian Limitations Error and distortion increase for regions that span more than one UTM zone Map Projections vs. Datum Transformations A map projections is a systematic rendering from 3-D to 2-D Datum transformations are from one datum to another, 3-D to 3-D Changing from one projection to another may require both Datum To more accurately represent locations on the earth’s surface A datum links a spheroid to a particular portion of the earth’s surface. Datum Most commonly used datums in North America North American Datum (NAD) 1927 using the Clarke 1866 spheroid NAD 1983 using the Geodetic Reference system (GRS) 1980 spheroid World Geodetic System (WGS) 1984 using the WGS 1984 spheroid Input geographic coordinate system NAD 1927 Output geographic coordinate system WGS 1984 Datum The coordinates for a location will change depending on the datum and spheroid on which those coordinates are based. Georeferencing – what? Aligning geographic data to a known coordinate system so it can be viewed, queried, and analyzed with other geographic data Georeferencing may involve shifting, rotating, scaling, skewing, and in some cases warping, rubbersheeting, or orthorectifying the data Georeferencing – why? Raster data is commonly obtained by scanning maps or collecting aerial photographs and satellite images Scanned map datasets don't normally contain spatial reference information aerial photography and satellite imagery, sometimes the location information delivered with them is inadequate and the data does not align properly with other data you have. Thus, to use some raster datasets in conjunction with your other spatial data, you may need to align, or georeference, to a map coordinate system. Georeferencing - Transformation Coordinates in the source system Coordinates in the target system