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Did someone call for a geodesist? ;-)
This article might help:
http://www2.unb.ca/gge/Resources/gpsworld.january97.pdf
It really depends on the datum AND map grid used. See Figure 1 in particular. 
And as for differences between astronomically defined coordinates and 
geodetic coordinates referred to, say, WGS84 (nominally geocentric), there 
may be differences in the Greenwich meridian used (one through an actual 
telescope at Greenwich used in the past for definition of GMT or one about 
100 m to the east in the case of the WGS84 0-degree meridian). There will 
also be small differences due to deflection of the vertical and polar motion.
-- Richard (Langley)


On Tuesday, December 17, 2013,351, at 8:45 PM, Gary LaPook wrote:

> Well, this pretty explains why.
> 
> http://en.wikipedia.org/wiki/Geodesy
> 
> gl
> 
> From: Chris Presmeg <0886045615{at}neomail.co.za>
> To: garylapook---.net 
> Sent: Tuesday, December 17, 2013 12:56 PM
> Subject: [NavList] Re: A question for the geodisists
> 
> 
> What has Geodesy to do with it?
> ----- Original Message -----
> From: Gary LaPook
> To: CHRISTOPHER PRESMEG
> Sent: Tuesday, December 17, 2013 1:07 PM
> Subject: [NavList] A question for the geodisists
> 
> On July 2, 1937 Amelia Earhart was attempting to find Howland island. Her    
navigator, Fred Noonan, was using celestial navigation to fly the standard 
single line of position landfall approach to the island. (see: 
https://sites.google.com/site/fredienoonan/discussions/navigation-to-howland-island 
)
> 
> The Pacific Sailing Directions (1885) gives the location of Howland as 0° 
49' north, 176° 40' west. Bowditch (1920) gives it as 0° 49' 00" north, 176° 
43' 09" west. Bowditch (1938) gives it as 0° 48' north, 176° 38' west. This 
is still listed as the correct coordinates in H.O. 126.
> 
> Some have argued that Noonan was working with the old coordinates and was 
aiming for 0° 49' 00" north, 176° 43' 09" west and so missed the island by 
about five nautical miles. However, the correct coordinates were given on the 
June 1937 edition of the local chart, 1198, so we know that the correct 
coordinates had been found sometime prior to Earhart's departure from the 
states so I would think that the updated information would have been provided 
to her, considering that the orders to assist her came directly from the 
White House and the importance of the correct information would have been 
apparent to anyone. 
> 
> I have looked at Howland on Google Earth and it now appears that it has 
moved to the east another mile! And the coordinates found on the most recent 
chart now    show it also at 0° 48' 00" north, 176° 37' west, one mile east 
of he published location. How does the island keep moving? If this is the 
true location then Noonan may have been aiming another NM too far to the 
west. There is a note on the newest chart that says:
> 
> 
> So it appears that the most recent movement resulted from changing the datum 
to    WGS 84 but it seems to me that an adjustment of an entire mile to the 
east is very large compared to other such shifts. So, if Noonan was using the 
old coordinates would he have been aiming six NM west or just five? And I 
think that I remember this correctly, that the geodetic gratical used for map 
making places the landmarks where they would be placed by celestial 
navigation. So if the island really is a at 176° 37' west then this might 
help explain why they couldn't find the island. I thought that the greatest 
difference between the geodetic and the geographic coordinates, as far as 
celestial navigation is concerned, is in latitude and that changing to WGS 84 
wouldn't have such a large effect on longitude. Can someone explain how 
shifting from one datum to another would cause this type of error in the 
celestial navigation.
> 
> 
> 
> gl
> 
> Attached File: 
> 
> (img/125785.howland pages from 1920 american_practical_navigator_an_epitome.jpg: Open and save)
> 
> Attached File: 
> 
> (img/125785.howland position_page_1.jpg: Open and save)
> 
> Attached File: 
> 
> (img/125785.howland position_page_2.jpg: Open and save)
> 
> Attached File: 
> 
> (img/125785.howland ge.jpg: Open and save)
> 
> Attached File: 
> 
> (img/125785.howland chart 83116.pdf: Open and save or View online)
> 
> : https://navlist.net?i=125785
> : https://navlist.net?i=125799
> 
> 
> : https://navlist.net?i=125803
> 

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| Richard B. Langley                            E-mail: lang@unb.ca         |
| Geodetic Research Laboratory                  Web: http://gge.unb.ca/     |
| 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.fredericton.ca/       |
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