[Proj] NGS Control Points

[Robert McFaul]

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.

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 these differences so small that one need not worry? If I calculated this
correctly the difference is around 3.5 feet for the latitude. I'm not a
geodecist and realize that the process is not absolute.

Why the difference?

Thanks.

Robert McFaul
Dynamic Graphics, Inc.
Alameda, CA. USA