[Proj] NGS Control Points

Wed Jun 22 12:21:54 PDT 2011

I did a search on this "Bench Mark."  It has practically zero records in the National Database other than an elevation listed as an integer (6) and a horizontal position truncated to integer arc seconds.  This is apparently a coordinate that was scaled off of a topographic map and such "positions" are not considered good to better than  ±100 feet.

Now using an official datum transformation package such as NADCON can commonly yield positions to an accuracy of up to ±0.1 foot, but the final value can not be better than the original accuracy of the point.  There's more than one lookup table for NADCPN depending on whether you are using original NAD83 adjustment values, or one of the more specialized adjustments since NAD83 was first published.  As far as I know, there's only two available versions of lookup tables for NADCON: the original adjustment and the HARN adjustment and the latter can only be used in concert with the former - it is a sequential process.

I suspect the point you used as an example is at best an intersection stations and was never occupied.

         Pid    Name                                    Lat        Lon           Elev    O o Hv
  ------ ----------------------------- ---------- ----------- --------- - - --
>HT0774 A C OF SF                     37 46 15. /122 23 53.       6.     1     N 

There's a new adjustment of the NAD83 that is being published on July 15th of this year.  The new "biggie" will be NAD22 in 2022.

Clifford J. Mugnier, C.P., C.M.S.
Chief of Geodesy,
Center for GeoInformatics
Department of Civil Engineering 
Patrick F. Taylor Hall 3531
LOUISIANA STATE UNIVERSITY 
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________________________________

From: proj-bounces at lists.maptools.org on behalf of Richard Greenwood
Sent: Tue 21-Jun-11 14:32
To: PROJ.4 and general Projections Discussions
Subject: Re: [Proj] NGS Control Points

On Tue, Jun 21, 2011 at 10:45 AM, Robert McFaul <robert at dgi.com> wrote:
> Hello,
>
> I have noticed that control points from the National Geodetic Survey (USA)
> NAD83 to NAD27 transform values do not agree with transforms done using BM.

What is "BM"?

> I have applied the 1983 to 1927 shift values provided in the data sheets and
> compared these to an actual transform run. The transform shows a
> difference,
> for instance for point HT0774, of a small amount.
>
>
>    Data sheet (NAD83):   37.77092832 N     -122.39812240
>    Data sheet (NAD27):   37.77098056 N     -122.39707500
>
>    Transform 83 to 27:   37.77099882 N     -122.39704051

Are you using the "conus" grid file? If so, I think you are outside of
its bounds. Another possible source of error is which NAD83 the NGS
datasheet is referring to. Might it be a newer NAD83 such as NAD83/96
or "HARN" or similar?

Rich

--
Richard Greenwood
richard.greenwood at gmail.com
www.greenwoodmap.com
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