![]() Conversely, the Mollweide projection gives accurate areas, while directions are distorted. For example, the Mercator projection is best used for marine navigation, because directions are correct, while other qualities such as area are distorted. There are numerous projections, each optimized for a particular purpose. The reason why this matters is because if you don't know what datum your data is in, you or someone else will have to guess which datum the data are in, and this greatly increases the probability of positional errors when you import your data into the Land Mapper or GIS.Ī map projection is simply a method for converting, or projecting, the three-dimensional surface of the Earth onto a two-dimensional plane (for example, a piece of paper or a computer monitor) for the purpose of making a map and/or performing distance, direction, and area computations. This is due to the different ellipsoids associated with the datums. Consider the following example:Īll three coordinates above identify the same physical point on the Earth's surface, but the seconds value varies. And since the ellipsoids have different numerical values, a latitude and longitude value for a particular datum will not have the same value in a different datum. Why does this matter?Īll geographic coordinates (latitude and longitude) are derived from an ellipsoid. Have their ellipsoids sized and positioned to better match a particular region of the Earth. Is sized and positioned to best represent the surface of the Earth on a global scale. Illustration of the differences between local and global datums. Conversely, the ellipsoids of global datums such as WGS84 are positioned to be closer to the center of the Earth, which gives the best approximation of the Earth's surface on a global scale. ![]() These are local datums, because they are optimized for use only in a certain part of the world. The Clarke 1866 and GRS 1980 ellipsoids are positioned such that the surface of the ellipsoid better matches the North American continent. The position of the ellipsoid with respect to the center of the Earth (indicated by the eccentricity ) also varies among datums. You can see that there are slight differences in the values (indicated by bold type). For example, here are the parameters for several datums that are commonly used in the United States: So, what is the "set of numbers that define the size, shape, and position of the ellipsoid"?Īn ellipsoid can be mathematically described by four parameters: semi-major axis (the equatorial radius), semi-minor axis (the polar radius), the degree of flattening, and the ellipsoid's position with respect to the center of the Earth. The Earth represented as an oblate spheroid The example of the deformation of the spinning bowling ball is exactly what happens to the Earth - its rotation about the polar axis causes an equatorial bulge, with a resultant flattening at the poles. This flattened shape is called an oblate spheroid (basically, a three-dimensional ellipse). The result would be a sphere that appears to be squashed, similar to what would happen to a round balloon if you squeezed it from both sides simultaneously. If you could spin a bowling ball so fast that centrifugal force were able to deform the ball, its equatorial radius would be slightly larger than 4.25" and its polar radius would be slightly less than 4.25". That is, its equatorial radius of 4.25" is the same as its polar radius. ![]() Simply put, a datum is a set of numbers that define the shape, size, and position of an ellipsoid which best approximates the true surface of the Earth, either locally or globally.Ĭonsider a bowling ball. To learn more, refer to the ArcGIS Help Library. The discussion below will give you a basic understanding of essential geodesy concepts. When working with the Land Mapper, or any geographic data, it is essential to have a basic understanding of these concepts. The ability to accurately determine locations on the surface of the Earth is fundamental to GIS and GPS, and this is accomplished by using various datums, map projections, and coordinate systems. Accurate maps would be impossible to make.We could not figure out precisely where we are.
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