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| − | | + | #REDIRECT [[World Geodetic System 1984]] |
| − | ==The Earth is Not Round!==
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| − | [https://gis.utah.gov/nad83-and-webmercator-projections/ '''The Earth is Not Round! Utah, NAD83 and WebMercator Projections''']<br> | |
| − | By the Utah Automated Geographic Reference Center
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| − | ===Anchor Point Datum===
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| − | From the 'The Earth is Not Round!' article we read:
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| − | {{cite|The UTM NAD83 projection uses the GRS80 ellipsoid and a center-of-the-earth anchor point as its datum}}
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| − | The '''anchor point''' ellipsoid associated with NASD84 is necessary to interconnect with other spherical models, in order to interchange data.
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| − | First it talks about the spherical model:
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| − | {{cite|Geographic coordinates use latitude and longitude values to define positions on the 3D surface of the earth, which is of course, best modeled as an ellipsoid, not a sphere. The ellipsoid and its accompanying anchor point that ties it in to the real world, are known collectively as the WGS84 datum.}}
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| − | Note "real world."
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| − | Then it talks about the flat model:
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| − | {{cite|UTM NAD83 is a projected coordinate system that represents physical locations abstracted to a '''flat, cartesian coordinate system.''' The UTM NAD83 projection uses the GRS80 ellipsoid and a center-of-the-earth '''anchor point''' as its datum}}
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| − | The passage is speaking of an anchor point to connect the two types of systems together.
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| − | The accompanying image is the flat map with anchor point:
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| − | [[File:NAD83 Datum.png]] | |
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| − | ==Spherical Coordinates Projected To a Plane==
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| − | [https://www-group.slac.stanford.edu/met/Align/GPS/CCS83.pdf '''The California Coordinate System''']
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| − | {{cite|The State Plane Coordinate System was established to provide a means for transferring the geodetic positions of monumented points to plane coordinates that would permit the use of these monuments in plane surveying over relatively large areas without introducing significant error.
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| − | A plane-rectangular coordinate system is by definition a flat surface. '''Geodetic positions on the curved surface of the earth must be “projected” to their corresponding plane coordinate positions.''' Projecting the curved surface onto a plane requires some form of deformation. Imagine the stretching and tearing necessary to flatten a piece of orange peel. In California the Lambert Conformal map projection is used to transform the geodetic positions of latitude and longitude into the y (Northing) and x (Easting) coordinates of the CCS83.}}
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| − | We read that the geographic (spherical) coordinates are projected '''''to''''' the plane coordinates. Not the other way around.
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| − | [http://wvgis.wvu.edu/data/otherdocs/standardsandpubs/wv_coordinate_systems_jan02.html '''West Virginia GIS Technical Center''']
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| − | {{cite|Geographic Coordinate System (GCS): An unprojected coordinate system that uses latitude and longitude to define the locations of points on a sphere or spheroid. The use of longitude and latitude is encouraged for general reference and distribution of national framework data because it provides a seamless coordinate system for most of the United States. '''Geographic coordinates can be readily projected onto a planar coordinate system to display data properly or measure distances accurately.''' The Geographic Coordinate System is the recommended coordinate system for unprojected GIS data sets that cover the entire geographic extent of West Virginia.}}
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| − | We read that the geographic (round earth) coordinates can be projected onto a planar (flat earth) coordinate system to accurately display distances.
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| − | The reader should ponder how spherical coordinates projected onto a planar coordinate system could display data and distances accurately if the earth is a globe. Why should these systems require that? Are naps with spherical coordinates are not possible?
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| − | If the earth is a globe, the opposite should be true. Projecting spherical coordinates onto a plane should make data and distances more inaccurate. Not accurate.
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| − | ==Flat Maps as Standard==
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| − | From https://www.e-education.psu.edu/geog862/book/export/html/1644 we read:
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| − | {{cite|Welcome to Lesson Six of this GPS course. And this time, we'll be talking about two coordinate systems. And I have a little bit of discussion concerning heights. We've touched on that a little bit. Now these coordinate systems that we're going to discuss are plane coordinate systems based upon the fiction that the earth is flat, which, of course, immediately introduces distortion. '''However, much of GIS work—and GPS work as well—is done based upon this presumption.'''}}
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| − | Another comment: https://www.pobonline.com/articles/86585-geodetic-surveying-made-plain
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| − | {{cite|A decade or two ago, I was the instructor for some introductory surveying courses at a community college. In the first class we would define “plane surveying” as surveying that did not take into consideration the curvature of earth, and “geodetic surveying” as that which did. That was about the only time the two categories received anywhere near equal attention. Oh, we would point out (still in that first lecture) some of the real-world evidence of a non-flat earth, such as the fact that the length of an 11.5-mile arc on the earth's surface is only five hundredths of a foot longer than its subtended chord, or that the sum of the angles in a spherical triangle on the earth’s surface having an area of 75 square miles is only one second greater than the angle sum of the same size plane triangle. (Even today I get a kick out of those dramatic tidbits. I’m sure a real geodesist would roll his or her eyes at such trivia!) But those examples were used as reasons for not focusing—no, for not mentioning—geodetic concepts for the rest of the semester. We would end the ten-minute token nod to geodetic surveying with the tongue-in-cheek remark, “Therefore, for this class, we will respect the time-honored principle handed down through the ages—that the earth is flat.” And, truth be known, that’s probably not a bad narrowing of a dauntingly broad subject.
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| − | But more than narrowing, it was simply that few rank-and-file surveyors in private practice ever had occasion to use geodetic concepts in their daily work. Even when we dutifully enrolled in the occasional State Plane Coordinate seminar, the principles quickly faded from memory from lack of use. If a surveyor from the general population had ever even heard of the geoid and its relationship to the ellipsoid, and where mean sea level fit into the picture, hearing about it was as far it went.}}
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| − | We see that flat maps are the standard.
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