Plate Movement and Longitude
The Earth's various tectonic plates move relative to one another in different directions and at different speeds, on the order of 50 to 100mm per year. As a result, points on the Earth's surface that are located on different plates are always in motion relative to one another over time, for example, the longitudinal difference between a point on the Equator in Uganda, on the African Plate, and a point on the Equator in Ecuador, on the South American Plate, (same latitude) is increasing by about 0.0014 arcseconds per year. It should be noted that these relative tectonic movements effect the measurement of latitude similarly.
If a global reference frame such as WGS84 is used, the longitude of a place on the surface will change from year to year. To minimize this change, when dealing exclusively with points on a single plate, a different reference frame can be used, whose coordinates are fixed to a particular plate, such as NAD83 for North America or ETRS89 for Europe.
Read more about this topic: Longitude
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