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VERTCON


trmcconn
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The data sheets for most triangulation stations list latitude and longitude as "adjusted" and elevation as "scaled", meaning that the latter is estimated (usually from aerial photos.) I noticed in one datasheet that scaled is replaced by VERTCON. What does this designation mean? More generally, I wonder how common it is to have stations for which both coordinates and elevation are adjusted? The location of such a station in space relative to the datum would be precisely known, and maybe such a station would be useful even in the age of GPS. (The endpoints of the original line on Long Island would qualify as examples, although they no longer exist:(, since they were at sea level. )

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That means the mark had an NGVD29 elevation, but it wasn't considered good enough data to be put into the NAVD88 adjustment. Look at the SUPERSEDED data down lower on the data sheet for the NGVD29 value.  VERTCON applies an interpolated difference between the two datums to arrive at a usable, but not super precise elevation in the newer NAVD88 datum.

 

There are some stations that have classical observation data for both horizontal and vertical, but it isn't common because of the way sites were selected.  Triangulation stations were usually on high points whereas elevation marks were along railroads or roads where possible to take advantage of easy terrain.  When GPS is used, those tri stations may end up with 3-dimensional coordinates.

Edited by Bill93
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Thank you, Bill93! As always your answer is helpful and informative.

 

A good example of the type of mark I had in mind is station RED STONE (PG1953). It is located at a high point, but it is the high point of the University of Vermont campus and levelling surveys were also run nearby. 

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VERTCON – North American Vertical Datum Conversion

The NAVD 88 height was computed by applying the VERTCON shift value to the NGVD 29 height (displayed under SUPERSEDED SURVEY CONTROL.)

 

VERTCON_sm.png

Edited by Z15
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