NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: A question for the geodisists
From: Richard B. Langley
Date: 2013 Dec 17, 22:33 -0400
From: Richard B. Langley
Date: 2013 Dec 17, 22:33 -0400
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) > > : http://fer3.com/arc/m2.aspx?i=125785 > : http://fer3.com/arc/m2.aspx?i=125799 > > > : http://fer3.com/arc/m2.aspx?i=125803 > ----------------------------------------------------------------------------- | 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/ | -----------------------------------------------------------------------------