
Latitude doesn’t exactly mean what I thought  - wglb
http://www.johndcook.com/blog/2011/09/17/latitude-doesnt-exactly-mean-what-i-thought/
======
CoreDumpling
Programmers beware! You need to be especially careful with converting geodetic
lat/lon/alt coordinates to and from other systems, particularly with the
Earth-centered, Earth-fixed system (basically a Cartesian x,y,z coordinate
grid). Do it incorrectly, and you might find yourself buried under several
meters of dirt ;)

All too often I've come across code that started out by assuming a spherical
Earth (wrong!) or trying to compensate by multiplying by a factor to adjust
for the oblation (still wrong!). Doing this calculation accurately is very
much non-trivial and still a topic of active research in geodesy.

Fortunately, there's libraries to do the hard work for you:

<http://earth-info.nga.mil/GandG/geotrans/>

<http://geographiclib.sourceforge.net/>

~~~
colanderman
It gets better still! Did you know that GPS data and KML (Google Earth's
format), despite both using the WGS84 spheroid, are not compatible? GPS
computes altitude above the spheroid (shape of the earth), while KML requires
altitude above the geoid (shape of the sea). These differ by several hundred
feet in many areas. (The geoid is 30 m below the ellipsoid where I am.)

But wait there's more! Just using lat/long and ignoring elevation? Guess what,
the verticals for GPS and KML differ too! GPS looks down a line normal to the
spheroid; KML looks down a line normal to the geoid (in the direction of
gravity). Those are almost always different. So unless your GPS readings were
taken at sea level, your data's _still_ wrong!

The picture here explains what's going on:
<http://en.wikipedia.org/wiki/Geoid#Description> The "plumb lines" are what
are used by KML; the other lines are what are used by GPS. Notice how they
point to different locations...

(P.S. I'm not a GIS-icist, but this comes from experience of collecting and
correcting GPS data and reading spec sheets. I'd love if an actual GIS or
Google person told me I was wrong but I'm not holding my breath.)

------
yannis
Sure it doesn't but interestingly enough the article does not go into its
roots. In France after the Revolution the Metrical or Decimal system of
measures was introduced. The fundamental standard adopted was that of a
quandrant meridian; This quandrant was divided into ten millions of equal
parts, and one parts or divisions was called the Metre, which was adopted as
the unit of length and painfully measured by triangulation by M.M. Delambre
and Mechain by measuring an arc of the meridian between the parallels of
Dunkirk and Barcelona!

~~~
mturmon
Actually, your fact above seems unrelated to the point of the OP, which is
purely geometrical and in no way involves meters or miles.

It sounds more like you want to tell the story of the original definition of
the meter.

~~~
vorg
> It sounds more like you want to tell the story of the original definition of
> the meter

Give the guy some latitude.

------
Hoff
If you're curious about the history of this topic, Dava Sobel's _Longitude:
The True Story of a Lone Genius Who Solved the Greatest Scientific Problem of
His Time_ is well worth reading.

<http://en.wikipedia.org/wiki/Longitude_(book)>

------
michaelochurch
This somewhat counter-intuitive definition is a result of the need for
backward compatibility with existing latitude data, taken long before people
had any idea of the Earth's oblation.

Originally, sextants were used to measure latitude based on solar elevation.
This definition of latitude is easily measured by sextant: 90 degrees minus
the maximum (i.e. noon) solar altitude on the equinox.

------
nraynaud
basically don't do projection/unprojection/coordinate transformations on your
own, if you really want to write the code use the specs directly (good luck!)
or use a lib if you can.

Best thing: stick to WGS84+webmercator as much as you can (some people can't
help it: continents drifts, legacy compatibility etc.) Geography geekness is
fun for geographs and pisses everybody else in the world (and I'm doing GIS
stuff for a living).

~~~
colanderman
Except we can't, because standards dictate stupid this. My example: GPS data
gives altitude above the spheroid; KML requires altitude above the geoid.
_This means that even 2D GPS coordinates that were not taken at sea level
cannot be used directly in KML and hence Google Maps._

Converting this requires projecting spheroid data into 3-space and
reprojecting it onto the geoid (which is a complex geometrical shape). Neither
of these steps is trivial. Of course it's easy if you've got proj4; but this
kind of stuff just does not ship with Android.

~~~
deadmansshoes
There's always <http://proj4js.org/>

------
niels_olson
The first part of this is key: geocentric vs geographic. There are actually
several corrections needed if you try to combine the two in celestial
navigation.

------
aneth
Wouldn't this be to ensure that one degree of latitude is always approximately
the same distance? (i.e. one mile)

~~~
michaelochurch
No. It has to do with how coordinates were calculated before we had good data
on the Earth's oblation. Latitude was calculated based on the solar elevation
(measured by sextant) at noon. For example, at 40 degrees north (by
definition) the sun will be at 50 degrees at noon on an equinox.

Longitude is a different story; it's relatively recent that we've been able to
calculate that. Sailors in the 17th century had sextants and solar tables and
could figure out their latitude easily, but had no idea where they were in
terms of longitude.

~~~
dredmorbius
Replying off the top of my head, but IIRC once astronomical phenomena became
known and observable, these could provide constant "clocks" which could be
verified at different points on the Earth's surface. In particular,
transits/occlusions of planets, moons, or stars could be viewed from different
points. While observations regarding the Earth's moon are subject to some
parallax, by the time you're looking at other planets (or their lunar
systems), the error is minimal.

We're talking Keplerian / Gallilean times onward (telescopic observation and
orbital mechanics) for the most part, though some observations may have been
possible / performed in earlier times. Turns out that Gallileo and Halley
proposed such methods in 1612 and ~1683.

Accurate navigational chronometers came along in the late 18th century.

