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You might be talking about the difference between geodetic (or ellipsoidal)
latitude and geocentric latitude. Have a look at the figure on this page
 to see if it helps.
-- Richard Langley
   Professor of Geodesy and Precision Navigation

On Sun, 23 Mar 2008, Lu Abel wrote:

>
>This seems quite silly, but I realize that I don't know the "official"
>definition of a degree of latitude.
>
>I'm sure most on this list know that the earth is an oblate spheroid --
>it's fatter than it is tall.   This means if I cut the earth in half
>through its poles, the resulting cross-section looks like an ellipse,
>wider than it is tall, rather than a perfect circle.   And this
>elliptical cross-section can lead to two possible definitions of a
>degree of latitude.
>
>If take a cross-section of the earth and draw an angle one degree up
>from the equator, is the place where this line intersects the surface of
>the earth the first (degree) parallel?   Or is the first parallel one
>ninetieth of the way from the equator to the pole?
>
>Years ago I took an offshore navigation course that taught the various
>"sailings," including use of the Meridional Parts table from Bowditch to
>determine a rhumb line course when traversing long distances (especially
>those with dramatic north-south differences).   (Meridional Parts give
>the "stretch" in the latitude scale required at various latitudes to
>create a Mercator chart).   As I recollect, the meridional parts down
>near the equator are actually slightly less than 1.00000, which would
>indicate that the first of my two definitions is the correct one.
>
>I know there are some experts in cartography on the list, I'm sure this
>is trivial for them.
>
>Lu Abel
>
>>
>


===============================================================================
 Richard B. Langley                            E-mail: lang@unb.ca
 Geodetic Research Laboratory                  Web: http://www.unb.ca/GGE/
 Dept. of Geodesy and Geomatics Engineering    Phone:    +1 506 453-5142
 University of New Brunswick                   Fax:      +1 506 453-4943
 Fredericton, N.B., Canada  E3B 5A3
     Fredericton?  Where's that?  See: http://www.city.fredericton.nb.ca/
===============================================================================

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