History always seems to cause problems. In this case, the concepts of latitude and longitude have been used since antiquity. When it was discovered in the 17 th century that the earth was not a intro, but was flattened somewhat, the old definitions of latitude remained.
All the old assumptions could no longer be used, but the values measured were defined to be correct. Therefore many definitions were effectively changed. For example, this means that the line defining latitude no longer went to the center of the earth. Latitude was defined by the up/down direction as measured on the surface.
The use of the locally sensed Up, in astronomy and surveying also led to subtile effects on how height were measured. Because the local horizon was the reference (as defined from the local Up), small variations in gravity caused the reference surface for height to bend and twist.
Until the satellite era, all heights on the earth
were measured with respect to a surface call the Geoid, which was very
poorly know. Luckily measurement were repeatable, and the same
no matter what direction you went. Going around in a circle always
gives the same height - but at the 10 to 30 meter level the reference
surface was unknown. It did vary slowly though, so this was imperceptible
to the causal observer. This height is commonly called Mean Sea Level
(MSL) height. It has the more scientific name of orthometric height.
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