Editing Geodetic datum (section)

==Plate movement==
The Earth's [[plate tectonics|tectonic plates]] move relative to one another in different directions at speeds on the order of {{cvt|50 to 100|mm}} per year.<ref>{{cite book |author=Read HH, Watson Janet |title=Introduction to Geology |place=New York |publisher=Halsted |year=1975 |pages=13–15}}</ref> <!-- Adapted from http://en.wikipedia.org/w/index.php?title=Plate_tectonics&oldid=226650343 --> Therefore, locations on different plates are in motion relative to one another. 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]], increases by about 0.0014 [[Minute of arc#Symbols and abbreviations|arcseconds]] per year.{{cn|date=May 2021}} These tectonic movements likewise affect latitude.

If a global reference frame (such as [[WGS 84]]) is used, the coordinates of a place on the surface generally will change from year to year. Most mapping, such as within a single country, does not span plates. To minimize coordinate changes for that case, a different reference frame can be used, one whose coordinates are fixed to that particular plate. Examples of these reference frames are "[[NAD 83]]" for North America and "[[ETRS89]]" for Europe.