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Verticon vs. Adjusted Altitude

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Was looking for a few benchmarks for a friend to hunt - and I saw something I don't remember coming across before (though I may have and just not noticed).

 

It said the altitude was Adjusted. I have more often seen the term Verticon.

 

Is Verticon the same as "Scaled" ?? and should "Adjusted" altitude also be pretty spot on (gps error notwithstanding of course)?

 

Thanks in advance - I hope everyone is well.

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Was looking for a few benchmarks for a friend to hunt - and I saw something I don't remember coming across before (though I may have and just not noticed).

 

It said the altitude was Adjusted. I have more often seen the term Verticon.

 

Is Verticon the same as "Scaled" ?? and should "Adjusted" altitude also be pretty spot on (gps error notwithstanding of course)?

 

Thanks in advance - I hope everyone is well.

 

Adjusted means the elevation was calculated based on the network of other stations connected to it by leveling and taking into account the measurement errors of each station and the closing error of the leveling loop.

 

Vertcon means the elevation was calculated using the value the station had under the older 1929 datum (converting to the current 1988 datum) , and it is rounded to 1 decimal place (.1 meters = about 3.6 inches). This value is less accurate than an adjusted value but still much better than a scaled value.

 

So in order of accuracy:

 

Adjusted - the best

Vertcon - so so (.1 m)

your GPS (maybe 3 m)

Scaled - lousy (nearest contour on a USGS map)

Edited by Papa-Bear-NYC

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Thanks Papa - and yup - I should have used the word elevation - not altitude. Ooooops!

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VERTCON is a NGS software utility For more info, look here

 

Computes the modeled difference in orthometric height between the North American Vertical Datum of 1988 (NAVD 88) and the National Geodetic Vertical Datum of 1929 (NGVD 29) for a given location specified by latitude and longitude. This conversion is sufficient for many mapping purposes.

Edited by Z15

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Adjusted means the elevation was calculated based on the network of other stations connected to it by leveling and taking into account the measurement errors of each station and the closing error of the leveling loop.

 

Vertcon means the elevation was calculated using the value the station had under the older 1929 datum (converting to the current 1988 datum) , and it is rounded to 1 decimal place (.1 meters = about 3.6 inches). This value is less accurate than an adjusted value but still much better than a scaled value.

 

So in order of accuracy:

 

Adjusted - the best

Vertcon - so so (.1 m)

your GPS (maybe 3 m)

Scaled - lousy (nearest contour on a USGS map)

 

Re-activating this one to try to get my head straight and come up with correlation numbers for a return visit to Lees Ferry (with John of 2OF's); especially since POSTED is not covered above.

 

C/P from my reply/question in his "Now what will I do?" thread earlier this week.

 

Well, first I need to figure out (preferably have one of the experts here tell me) if there is any correlation/accuracy between VERTCON & POSTED elevations.

 

BM 12 GP0234* NAVD 88 - 959.02 (+/-2cm) 3146.4 (feet) VERTCON

M 7 GP0233* NAVD 88 - 960.279 (meters) 3150.52 (feet) POSTED ***new info: GP0233_STAMPING: 3147.684 M 7 1921

GAGE BM 1 GP0236* NAVD 88 - 952.024 (meters) 3123.43 (feet) POSTED

 

end C/P

 

Could also tie to:

GP0603* NAVD 88 - 961.3 (meters) 3154. (feet) VERTCON

 

2OF's have also logged E 7 1921 and their photo shows a stamped elevation of 5074.800, maintaining the three feet +/- correction applied to M 7 1921.

 

5068bc82-9bd2-4a78-982b-c1aae5b06ef9.jpg

 

GP0199* NAVD 88 - 1547.683 (meters) 5077.69 (feet) POSTED

GP0199_STAMPING: E 7 1921

 

C/P from e-mail to John:

 

The reason I have reservations about the different elevations is the mess I found last December up in Wyo.

 

OX0679* NAVD 88 - 1774.6 (meters) 5822. (feet) VERTCON (original ETNA)

AA2119* NAVD 88 - 1772.2 (meters) 5814. (feet) GPS OBS (ETNA RESET)

OX0336* NAVD 88 - 1772.191 (meters) 5814.26 (feet) ADJUSTED ( D 67 1934 - the only one that I did a NGS recovery on)

 

OX0679''REFERENCE MARK NO. 1 IS A STANDARD C. AND G.S. BENCH MARK DISC

OX0679''SET IN THE TOP OF A 6 INCH SQUARE CONCRETE POST WHICH PROJECTS

OX0679''4 INCHES. IT IS 2 FEET WEST OF FENCE CORNER AND APPROX. THE

OX0679''SAME ELEVATION AS THE STATION MARK. IS STAMPED D 67 1934.

 

Did not note in my field book; but recollection and photos would put D 67

20978eb2-ad76-48c3-88aa-0190d39b17bc.jpg

more than a foot above ETNA RESET. The original ETNA was certainly not eight feet higher.

 

The three datasheets differ in the station to RM distance also."

 

end C/P

 

I don't pay much attention to DS elevations, but noticed that all above are only *accurate* to the the nearest foot.

 

Thanks for any assistance, kayakbird

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Mike,

I found this reference to POSTED elevations on the NOAA website here

Begin C/P

Benchmarks in other categories were not retrieved:

 

P - POSTED - Force Fix to NAVD88.

N - Determined by Single Spur.

O - From Horizontal Branch but Other Agency.

 

Briefly, "adjusted" benchmarks form the bulk of NGS data. Using more recent software, these level surveys were checked and adjusted into the network. "Hand keyed" benchmarks refer to historical data (typically associated with the NGVD 29 datum) that have been adjusted and keyed manually, but have not been processed through the full set of more recent data checking and adjustment software. "Computed from nearby bench marks" refers to the same historical data as "Hand keyed", but are incomplete in some respect, most likely due to superseded and/or missing adjusted heights. "Reset" benchmarks denote geodetic leveling over short distances to establish a replacement mark for a benchmark, and usually have only one network point connection. "Readjusted due to earth movement" benchmarks have elevations computed from the most recent leveling measurements in areas of known vertical motion. "From horizontal branch" benchmarks represent short level tie data measured by NGS in the course of performing horizontal control surveys.

 

 

For the categories that were not retrieved: "Posted" benchmarks were withheld from the NAVD 88 general readjustment due to excessive misclosures. After the readjustment, the troublesome survey lines were fit to the network, and the points were flagged. "Determined by single spur" benchmarks are established from only one network point, and are not considered sufficiently reliable for this data set. "From horizontal branch but other agency" benchmarks are short level ties performed by other agencies when conducting horizontal control surveys. Due to issues of data reduction, this category was not retrieved. In addition, control points obtained from standard trigonometric leveling were not considered to be of sufficient accuracy. And, benchmarks established from GPS surveys were not used. While such orthometric heights can be accurate, a data set independent of any underlying geoid model was desired.

End C/P

So, what I gather from the above is that POSTED elevations would fall below GPS obs. on PapaBear's accuracy list. I would surmise that D 67 has the most accurate elevation of the three, and that if it was used to level to Etna Reset, that information did not make it onto the Datasheet. Instead, the published Etna Reset elevation was derived from GPS observations.

As for the original Etna elevation: it is based on vertical angle observations, probably from distant locations, which cannot be relied on in many cases. Applying the Vertcon correction to this value was probably not a good idea, as it gives a false sense of the elevation's accuracy.

So I guess the lesson here is don't put too much faith into elevations that are POSTED.

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