![]() Some reference ellipsoids developed more recently, such as the World Geodetic Survey (WGS84), are suitable to model the Earth as a whole. Mapping tasks involving a high degree of positional accuracy rely on a horizontal datum that `fits’ local areas on the Earth’s surface. Figure 1 shows the difference between how the Everest 1830 reference ellipsoid `fits’ the Earth’s surface of India and the Clarke 1866 reference ellipsoid `fits’ the Earth’s surface of North America. Table 1 shows several common reference ellipsoids and the parameters that define their mathematical relations. Since the 1800s, dozens of reference ellipsoids were computed that fit the Earth’s shape (particularly local areas) better and provided a higher degree of accuracy in coordinate positioning than a spherical assumption. For illustrative purposes (Van Sickle, 2010, p.9) describes the overall impact of this bulge on the Earth’s dimensions as follows: if the "Earth were built with an equatorial diameter of 25 ft, the polar diameter would be about 24 ft 11 in, almost indistinguishable from a sphere." With such a model, the polar radius is smaller than the equatorial radius creating a flattening at the poles and a bulging along the equatorial zone. Also called a reference ellipsoid, it is defined by two parameters: the equatorial and polar radius. Richter’s discovery was integrated by Sir Isaac Newton when, in 1687, he suggested that due to Earth’s gravitational differences and rotation, its shape was better described as an oblate ellipsoid. One insight, discovered by Jean Richter in 1671, was the variation in gravity across the Earth’s surface. In order to develop a reference ellipsoid, several insights into the Earth were necessary. ![]() Other models such as the reference ellipsoid have been developed to better represent Earth’s shape. Of course, the Earth is not a perfect sphere and such an assumption does not provide for a high degree of positional accuracy. The spherical assumption is usually applied to thematic maps especially at small scales. One model, a sphere, is the simplest in that all points on the Earth’s surface are the same radius from its center. One outcome has been the development of different Earth models. Throughout history, the exactness of the Earth’s size and shape has been speculated upon a great deal by many. On the other hand, a vertical datum provides a position with respect to an elevation origin such as mean sea level. A horizontal datum fixes a position on the Earth’s surface related to the origin of latitude and longitude. With respect to the Earth, two datums exist: horizontal and vertical. Generally speaking, a datum is a base value to which all other values relate. To be usable, for example in a GIS environment, a coordinate value referencing a position on the Earth’s surface needs to be tied to a datum. But, what is a datum and how `much’ is incorrect? What follows is a brief overview of horizontal and vertical datums and their importance to mapping activities. While Google Earth is cast on the WGS84 datum what happens if the coordinates are, for example, cast on another datum? The location mapped in Google Earth would be incorrect. If these coordinates are entered into Google Earth, then a point near State College, Pennsylvania appears. Furthermore, N and W are assigned to 40 47′ 36" and 77 51′ 36", respectively. If the coordinates are geodetic, what is the associated datum?įor now, assume 40 47′ 36" is latitude and 77 51′ 36" is longitude. Are these geodetic or spherical coordinates Ĥ. What direction is attached to the coordinates (e.g. Which value is latitude & which value is longitude Ģ. Unfortunately, uncertainty is high because the following details are not provided:ġ. Consider this simple question: where is 40 47′ 36" & 77 51′ 36" exactly located? These coordinates could pinpoint several places. However, defining the exact location on the Earth’s surface is not necessarily straightforward. For more information check out īy its very nature, geospatial data is tied to the Earth’s surface. As a way to test understanding, each week participants are given practical assignments that link the concepts to handling spatial data. ![]() This ten-week course presents individual concept galleries that provide background on each topic. This course is designed for professionals who work with spatial data but are not confident in their understanding of these topics. Penn State’s World Campus offers "Map Projections for GIS Professionals" a course on understanding the role that datums, map projections, and grid systems play in spatial data. Often, a better understanding of horizontal and vertical datums can reduce the difficulties. In the mapping professions, handling spatial data can be challenging (e.g., data alignment). A 917Kb PDF of this article as it appeared in the magazine complete with images is available by clicking HERE
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