The ellipse is an oval, with a major axis the longer axis , and a minor axis the shorter axis. If you rotate the ellipse around one of its axes, the shape of the rotated figure is a spheroid. For the earth, the semi-major axis is the radius from the center of the earth to the equator, while the semi-minor axis is the radius from the center of the earth to the pole.
A particular spheroid is distinguished from another by the lengths of the semi-major and semi-minor axes. For example, compare the Clarke spheroid with the GRS spheroid and the WGS spheroid, based on the measurements in meters below. A particular spheroid can be selected for use in a specific geographic area, because that particular spheroid does an exceptionally good job of mimicking the geoid for that part of the world.
A datum is built on top of the selected spheroid, and can incorporate local variations in elevation. With the spheroid, the rotation of the ellipse creates a totally smooth surface across the world.
Since this doesn't reflect reality very well, a local datum permits local variations in elevation to be incorporated.
The underlying datum and spheroid to which coordinates for a dataset are connected can change the coordinate values. An example using coordinates within the city of Bellingham, Washington follows. The X-Coordinate is the measurement of the angle from the Prime Meridian at Greenwich, England, to the center of the earth, then west to the longitude of Bellingham, Washington. The Y-Coordinate is the measurement of the angle formed from the equator to the center of the earth, then north to the latitude of Bellingham, Washington.
If the surface of the earth, at Bellingham is bulged out, the angular measurements in decimal degrees from Greenwich and the equator will become slightly larger. An illustrative example using the city of Bellingham, Washington, follows. It is apparent that while NAD83 and WGS84 express coordinates that are nearly identical, NAD27 is quite different, because the underlying shape of the earth is expressed differently by the datums and spheroids used.
The longitude is the measurement of the angle from the prime meridian at Greenwich, England, to the center of the earth, then west to the longitude of Bellingham, Washington. The latitude is the measurement of the angle formed from the equator to the center of the earth, then north to the latitude of Bellingham, Washington. If the surface of the earth at Bellingham is bulged out, the angular measurements in decimal degrees from Greenwich and the equator will become slightly larger.
If the surface at Bellingham is lowered, the angles will become slightly smaller. Regardless of how an aircraft is recording measurements, it is not possible for a person to hike to an altitude, only an elevation.
Orthometric height is defined as measured distance between the geoid and the topographic surface. Elevation measurements are created from and stored within vertical datums for accurate Z-values within a geographic coordinate system for accurate analysis.
Ellipsoid height is defined as the measured distance between the reference ellipsoid and the topographic surface. GPS receivers use ellipsoidal height since the calculation is easier to obtain on the fly. If the GPS knows where it is in relation to the XY positions on the reference ellipsoid, it can easily calculate it's height above that established zero. For a GPS reciever, to calculate elevations based on the geoid is much more labor intensive.
Geoid separation geoid height is defined as the measured difference between the reference ellipsoid and the geoid. Geoid separation is used to increase the accuracy of GPS measurements in the post-processing phase of data collection since the GPS receiver uses elevation established between the reference ellipsoid and the topographic surface, but accurate measurements are between the geoid and the topographic surface or the orthometric height.
In order to convert from ellipsoidal height to orthometric height, the geoid separation needs to be a known value. Notice how these height determination definitions can be related back to the kinds of vertical datams. Over time, data collection technology improves, while at the same time land and ocean masses are in constant flux - moving, shifting, and boogieing down. As a response to these changes, datum measurements regarding the precision of control points must also be changed to keep up with the shifting world.
While it might seem rather obvious, it is a gazillion times easier to move the mathematical points on the datum then to go out, dig up, and physically replace the benchmarks.
And since we know that benchmarks are used for several different datums, moving them to repair the accuracy of one datum would be catastrophic to all of the others. A datum shift is when the coordinate associated with a benchmark and the resulting control points is adjusted or changed based upon either better surveying techniques, better mathematical calculations, or adjustments for continental shift.
Datum shifts can be major , noted with a two-digit year following the datum name — i. WGS84 Major datum shifts are extremely involved, and usually include a major survey project and mathematical calculations, while minor datum shifts are usually completed when just a few control points are deemed incorrect. A control marker set by the National Geodetic Survey to determine latitude and longitude.
Active Oldest Votes. Improve this answer. Padmanabha Chowdhury 1 1 gold badge 2 2 silver badges 14 14 bronze badges. OK, I think I half understand. So how is the 'fixing point s ' defined?
Is that usually specified somehow? I've added some for older datums. I don't feel competent to describe how the ECEF datums are done. You might want to take a look at this pdf. Mirimu Gerald Mirimu Gerald 1. Sign up or log in Sign up using Google. Sign up using Facebook.
0コメント