{"id":10428,"date":"2015-01-11T21:39:25","date_gmt":"2015-01-12T03:39:25","guid":{"rendered":"http:\/\/gisgeography.com\/?p=10428"},"modified":"2024-03-10T07:14:32","modified_gmt":"2024-03-10T12:14:32","slug":"ellipsoid-oblate-spheroid-earth","status":"publish","type":"post","link":"https:\/\/gisgeography.com\/ellipsoid-oblate-spheroid-earth\/","title":{"rendered":"Ellipsoid\/Spheroid – Our Oblate Spheroid Planet Earth"},"content":{"rendered":"\n
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\"Oblate<\/figure>\n<\/div>\n\n\n

We say Earth is a sphere. But it’s not exactly a perfect one.<\/p>\n\n\n\n

It’s an oblate spheroid that bulges at the equator<\/strong> and is somewhat squashed at the poles<\/strong>.<\/p>\n\n\n\n

In fact, it bulges about 14 miles out more at the equator compared to pole-to-pole.<\/p>\n\n\n\n

Because of the field of geodesy<\/a>, we’ve gained a much better understanding of the shape of our planet. Let’s delve into this a little deeper.<\/p>\n<\/div><\/div>\n\n\n\n

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What is an Ellipsoid in GIS?<\/h3>\n\n\n\n

Geodesists have adopted an ellipsoid model to determine latitude and longitude coordinates<\/a>. The major axis of an ellipse is the equatorial radius. The minor axis is from the poles to the center.<\/p>\n\n\n

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\"Axis<\/a><\/figure>\n<\/div>\n\n\n

Geodesists use reference ellipsoids to specify point coordinates such as latitudes (north\/south), longitudes (east\/west), and elevations (height).<\/p>\n\n\n\n

The most common reference ellipsoid in cartography and surveying is the World Geodetic System (WGS84)<\/a>. The Clarke Ellipsoid of 1866 was recomputed for the North American Datum of 1927 (NAD27)<\/a>.<\/p>\n\n\n\n

When comparing NAD27 and NAD84, latitude and longitude coordinates can be displaced to the degree of tens of meters (with the same latitude and longitude coordinates). The image below shows an exaggerated semi-major axis and semi-minor axis. <\/p>\n\n\n

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\"Semi-Minor<\/figure>\n<\/div><\/div><\/div>\n\n\n\n
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How Do Horizontal Datums Relate to Ellipsoids?<\/h3>\n\n\n\n

Horizontal datums give us the capability to measure distances and directions across the surface of the earth. Most horizontal datums<\/a> define a zero line at the equator from which we measure north and south (latitudes).<\/p>\n\n\n\n

There is also a zero line at the Greenwich Meridian<\/a> from which we measure east and west (longitudes). This standardized reference point simplifies navigation and geographic coordination, serving as a fundamental reference for global positioning<\/a> and mapping systems.<\/p>\n\n\n

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\"Prime<\/figure>\n<\/div>\n\n\n

Together these lines provide a reference for latitude and longitude<\/a> expressed in decimal degrees. These latitudes and longitude positions (Geographic Coordinate Systems) are based on spheroid or ellipsoid surfaces that approximate the surface of the earth \u2013 a datum.<\/p>\n\n\n\n

We reference all coordinates to a datum. This datum serves as a crucial baseline<\/strong> for establishing the accurate positions of geographic features and ensuring consistency in spatial data analysis and interpretation.<\/p>\n\n\n

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\"Latitude<\/figure>\n<\/div>\n\n\n

A datum describes the shape of the Earth in mathematical terms. A datum defines the radius, inverse flattening, semi-major axis, and semi-minor axis for an ellipsoid.<\/p>\n\n\n\n

Here is the WGS84 datum:<\/p>\n\n\n\n