the earth is spinning around and orbiting our sun
along with a handful of planets and a few bajillion asteroids
our sun is spinning around and orbiting our galactic center
along with billions and billions of other stars
and clouds of ash dust and gas
our galaxy is spinning around too
along with billions and billions of other galaxies
how do you get your bearings when everything is moving ?
what is your reference?
the planet earth is roughly a sphere
~6,371,000m from the center to its surface
the earth is spinning about an axis
imagined from the north to the south pole
halfway between the poles
around the center of the sphere
is the equator
the angle from the equator to the poles is measured as latitude -
lines of equal latitude circle the earth parallel to the equator
the equator holds the reference position for 0� latitude
making the poles each 90� away from the equator
northward representing positive angles, southward negative angles
meridian lines are drawn from pole to pole
the angle between meridians is measured as longitude -
an arbitrary meridian is chosen as the prime meridian
which holds the reference position for 0� longitude
the prime meridian on earth passes thru a point in greenwich
the surface of the earth is full of hills and valleys
the height of a point is referenced by
the distance above or below the sphere
the height of the sphere is referenced to mean sea level
a common point where all water would come to rest
due to having equal forces of gravity
too sleepy right now to make this any cleaer, brb
the earth is slightly fatter than it is tall
not by much but enough that it can be measured
this 'imperfect' sphere is referred to as a spheroid
shperoids are defined by the radius of the axes at their maximum and minumum lengths:
semi-major axis (a)
semi-minor axis (b)
how 'flat' or 'eccentric' a spheroid is can be compared to other spheroids to clarify their differences
source http://earth-info.nga.mil/GandG/publications/tr8350.2/TR8350.2-b/Sections%201-5.pdf
this image is exaggerated but illustrates that
the earth is slightly fatter than it is tall
wgs defined radii
from the center of the earth to the surface is roughly: (semi-major axis 'a') 6,378,137.0m at the equator (semi-minor axis 'b') 6,356,752.3m at the poles
| Name | Semi-major | Semi-minor | Datum |
|------------|------------|-----------------|-----------|
| Sphere | 6370997 | 6370997 | Geographic|
| Clarke 1866| 6378206.4 | 6356583.8 | NAD27 |
| WGS72 | 6378135 | 6356750.519915 | WGS72 |
| ATS77 | 6378135.0 | 6356750.305 | ATS77 |
| GRS80 | 6378137 | 6356752.3141 | NAD83 |
| WGS84 | 6378137 | 6356752.3142 | WGS84 |
the surface of the earth has many features that are missed by assuming it's perfectly smooth
some ellipsoids can align with hills such that water would be running to a higher elevation
a closer approximation to the actual shape is a geoid there are different geoids made with varying accuracy this is an ongoing measurement
UNDER CONSTRUCTION SRRY
