[Proj] NGS Control Points

[Richard Greenwood]

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