It is called pseudo because it is measured in angular units (degrees) rather than linear units (meters). This is just latitude and longitude represented as a simple grid of squares. So when you tell ArcGIS to make a flat map with a GCS, it is forced to choose a projection! So it draws using a pseudo Plate Carrée projection. Remember that it is impossible to draw the round earth on a flat surface without a projection. In Map Properties, expand the Geographic Coordinate System list and choose any one. Semimajor Axis, Semiminor Axis, and Inverse Flattening define the size of the spheroid.Ĭan’t we choose a GCS for our map instead of a PCS? I’ve made maps before that were in WGS 1984 and they drew just fine, didn’t they?
The Datum defines which model is used to represent the earth’s surface and where that model is positioned relative to the surface.It typically runs through Greenwich, England, but it doesn’t have to. The Prime Meridian is an arbitrary line of longitude that is defined as 0°.Coordinates in a GCS are recorded in an Angular Unit, usually degrees.Let’s look at the properties of the WGS 1984 geographic coordinate system: Remember that a PCS is just a GCS that has been projected. A PCS also contains a Geographic Coordinate System! In this case, it’s WGS 1984.
QGIS VS ARCGIS HOW TO
They tell you how to distort the earth-how to tear and stretch that orange peel-so the parts that are most important to your map get the least distorted and are displayed best on the flat surface of the map. You can get close, but only if you start tearing the peel apart. Imagine peeling an orange and trying to lay the peel flat on a table. You can’t draw the round earth on a flat surface without deforming it. The earth’s surface-and your GCS-are round, but your map-and your computer screen-are flat. Once your data knows where to draw, it needs to know how. You still need to know which GCS it is in before you know where it is on Earth. The coordinates 134.577☎, 24.006°S only tell you where a location is within a geographic coordinate system. The GCS is what ties your coordinate values to real locations on the earth. Australian Geodetic Datum 1984 is designed to fit the earth snugly around Australia, giving you good precision for this continent but poor accuracy anywhere else. There are many different models of the earth’s surface, and therefore many different GCS! World Geodetic System 1984 (WGS 1984) is designed as a one-size-fits-all GCS, good for mapping global data. But in order to draw a graticule, you need a model of the earth that is at least a regular spheroid, if not a perfect sphere. Because the planet spins, the poles are a bit closer to the center of the earth than the equator is. There are high mountains and deep ocean trenches. It’s a lumpy, bumpy, and uneven rounded surface. Well it turns out the earth isn’t a perfect sphere. So why isn’t knowing the latitude and longitude of a location good enough to know where it is? How can location A and location B in the Australia example both be correct?
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