Modern surveys, including the most recent of Mt Whitney determine the position of the top of Whitney in meters from the center of the earth in three axes (x, y, z). Elevations are the differences between the position of the peak and the position of a zero-elevation reference surface between the peak and the center of the earth.
The current official form of the reference surface is known as a geoid. The recent changes to the elevation of Mt Whitney are due to increasingly accurate measurement of the position of the geoid and not to changes in the position of the peak of Mt Whitney relative to the center of the earth. The geoid models have been updated recently because there are regions with changes in the position of the geoid referenced to "fixed" markers and because the achievable accuracy in calculating the geoid has increased.
The current system of vertical controls: NAVD88 (North Amaricam Vertical Datum 88) was based on surveyed extensions of sea level based on sites where there were seas available to measure. Since then, more accurate methods based on gravitic measurements have been used to correct and update the surveyed values.
Last June in the Webcam thread on WPSMB I quoted the NGS Whitney site accessed June 12, 2009 quoted below. It used a model called Geoid03 as the current vertical reference. Since then the geoid has been updated to Geoid09 as can be seen in the section of NGS data for Whitney accessed on Jan 21, 2010. The difference of 0.08 meters does not alter the 14,505' elevation value for Whitney.
The following urls provide more general and specific information of the topic and Mt
Whitney.
General Info.:
http://answers.google.com/answers/threadview/id/260547.htmlhttp://www.dot.ca.gov/hq/row/landsurveys/SurveysManual/04_Surveys.pdf see figure 4-1 on page 4-6
http://www.absoluteastronomy.com/topics/Mount%20Whitney see Geography - Elevation Measurements
Recent Updates to NAVD88:
http://www.ngs.noaa.gov/GEOID/GEOID03/http://www.ngs.noaa.gov/GEOID/GEOID09/http://www.ngs.noaa.gov/GEOID/USGG2009/faq_2009.shtmlWhitney data Posted to WPSMB webcam thread June 29, 2009:
1 National Geodetic Survey, Retrieval Date = JUNE 12, 2009
GT1811 *****************************************************************
GT1811 DESIGNATION - WHITNEY
GT1811 PID - GT1811
GT1811 STATE/COUNTY- CA/INYO
GT1811 USGS QUAD - MOUNT WHITNEY (1994)
GT1811
GT1811 *CURRENT SURVEY CONTROL
GT1811 ___________________________________________________________________
GT1811* NAD 83(1992)- 36 34 42.89133(N) 118 17 31.18182(W) ADJUSTED
GT1811* NAVD 88 - 4421.0 (meters) 14505. (feet) VERTCON
GT1811 ___________________________________________________________________
GT1811 EPOCH DATE - 1991.35
GT1811 LAPLACE CORR- -9.63 (seconds) DEFLEC99
GT1811 GEOID HEIGHT- -25.24 (meters) GEOID03
Recent access to NGS Whitney data:
1 National Geodetic Survey, Retrieval Date = JANUARY 21, 2010
GT1811 ***********************************************************************
GT1811 DESIGNATION - WHITNEY
GT1811 PID - GT1811
GT1811 STATE/COUNTY- CA/INYO
GT1811 USGS QUAD - MOUNT WHITNEY (1994)
GT1811
GT1811 *CURRENT SURVEY CONTROL
GT1811 ___________________________________________________________________
GT1811* NAD 83(1992)- 36 34 42.89133(N) 118 17 31.18182(W) ADJUSTED
GT1811* NAVD 88 - 4421.0 (meters) 14505. (feet) VERTCON
GT1811 ___________________________________________________________________
GT1811 EPOCH DATE - 1991.35
GT1811 LAPLACE CORR- -10.35 (seconds) USDV2009
GT1811 GEOID HEIGHT- -25.32 (meters) GEOID09
GT1811 HORZ ORDER - SECOND
GT1811
Dale B. Dalrymple
