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GIS Brownbag Series Part 3: Georeferencing and Projections Or… Why does my data not line up?

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G is for Geographic Tied to a location on Earth Georeferencing: tying data to a location on Earth

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Georeferencing Methods 1.Address 322 East Front Street Boise, Idaho USA 2. Linear Referencing 87 meters IDWR is about 2.5 miles north of the Boise Airport

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Georeferencing Methods 3. Geographic Coordinates 43° 36' 49" N 116° 12' 09" W

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T15N T16N T17N T18N R1WR1E T14N T19N R2WR2E Georeferencing Methods 4. Public Land Survey System (PLSS) Township 03N Range 02E Prime meridian

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Shape of the Earth?

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Sphere? “perfect ball” Distance from center to surface is the same everywhere

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Ellipsoid (a.ka. Spheroid) More mass + higher speeds: Earth bulges out at the equator

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Different spheroids have been used to estimate the Earth’s shape. Ellipsoid (a.k.a. Spheroid)

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Spheroid covered with long + lat Can we mix layers that are using different spheroids? Spheroids

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Spheroids Clarke 1866 WGS 84 Boise Layers will be misaligned NO!!

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Shape of the Earth??? Spheroid? “bumps and dimples” : use a geoid

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Why bother with spheroids when Earth is shaped like a geoid? Conclusion: we need a geoid and a spheroid Model the correct shape of the Earth To draw longitude and latitude grid

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Geoids and Spheroids Geoid Spheroid Datum: describes the geoid and how it relates to a spheroid Does this datum work everywhere? Good fit Bad fit

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Datums Many datums to choose from…

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We have not solved a basic problem yet: Earth is round, paper is flat!

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On flat surface need to turn round surface in Cartesian Coordinate System X Y Easiest way: simply plot longitude (X) vs latitude (Y) X Y longitude latitude

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Geographic Coordinate System Does this look good? No: some areas are distorted

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Projections GIS uses complex mathematical models. 3 main types of projections: Cylindrical Conic Planar

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Cylindrical Projection

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IDTM Idaho Transverse Mercator

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Conic Projection

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Planar Projections

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All projections distort reality. Why? Going from 3D to 2D Which projection is this? How is it distorted? Mercator Antarctica is HUGE! Where would this projection be useful? 1. Shape 2. Area 3. Distance 4. Direction

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Comparison of different projections

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Projecting Geographic Coordinates Example: Use GPS to collect spatial data at BSU PointCoordinate 1(X: 564,356; Y: 4,828,149) 2(X: 564,983; Y: 4,827,877) 3(X: 564,267; Y: 4,827,962) GIS does not know in which coordinate system those coordinates fit…

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UTM Zone 11 UTM Zone 10 UTM Zone 12 Idaho StatePlane East PointCoordinate 1(X: 564,356; Y: 4,828,149) 2(X: 564,983; Y: 4,827,877) 3(X: 564,267; Y: 4,827,962) The same coordinates in different projections…

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Defining a Projection SO…We need to tell the GIS which coordinate system we are using!!!

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Sometimes you may want to change the projection of a layer Projecting Data For example: Geographic IDTM

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Coming Soon: GIS Workshop September 2 from 8:30-4:00 Room 150 in the Water Center There is still space! Next brownbag GIS lecture on September 18 Topic TBA

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