Letters to the Editor

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Improved Content

Dear Mr. Sandler:

If I may be so bold to do so from my “worm’s-eye” perspective I would like to let you know that it appears to me that the content of Professional Surveyor magazine has improved appreciably with the last two issues; e.g. the articles by Charles Ghilani and Dave Doyle in particular have been very informative and up to real professional standards, something which was not generally the case in the past.

Regards,
Henry Brecher
Research Associate (Retired)
Byrd Polar Research Center
The Ohio State University


Chilling Control

Angus, I read your tunnel article quickly and well appreciate the precision of the work. As a check surveyor for the project some time ago I ran levels to control points around Queens and discovered to my dismay that a contract survey/engineer had taken 1 NGS benchmark and transferred that elevation to each distant point by GPS without taking geoid separation into account. The worst readings were 0.2 feet in error. Chilling.

Walt Dunlap
Cumberland County, Maine


NGS Outreach

Hi Dave,

Congratulations - Great text (in Vol 33, No. 8); can’t wait for more!

First thought in regards to webinars—is there an email list that provides delivered notices?

Likewise too, is there any way to know when and where an NGS spokesperson will be doing presentations in association with the coming changes (less tentatively scheduled?) for 1.1.2022?

Consider inclusion of the means (email notice is fine with me) for surveyors to be notified in your next article, soon I hope.

Keep `em coming :)

Kelly Bellis
Ellsworth, Maine



Kelly,

I really appreciate your comments. 

You can find the listing of upcoming NGS webinars at
www.ngs.noaa.gov/corbin/calendar.shtml.  As to when and where there may be NGS speakers, well that’s typically going to be up to each state surveying society to invite them or someone else with similar knowledge to their conferences.

I’ll be doing one on the new vertical datum next and will be referencing your previous articles on GRAV-D.

Cheers
Dave D.


Dear Sir,

I think the summary “We Really Do Need to Change” [from Dave’s article on “A New Datum”] is excellent.

Bob A. Smith, LLS
Raymond, New Hampshire


Proper Name for a Datum

PSM:

I would like to request that David Doyle comment on the proper name for a datum. The Texas Board of Professional Land Surveyors allows reference to a datum for Basis of Bearings, and, because we work for a municipal government and review a large number of metes & bounds descriptions, we see all manner of reference. To settle this, we contacted the National Geodetic Survey, and were informed that the datum in our area is property referenced as the “State Plane Coordinate System, Texas North Central Zone 4202, North American Datum of 1983.” Having an authority such as Mr. Doyle weigh in on this would help us close the discussion.

Scott Holt
Dallas, Texas



Dear Mr. Holt,

Thank you very much for your regard for my opinion.  What NGS recommended is correct but it should always also include the adjustment realization.  You could also abbreviate it to NAD 83.  While the rotational or bearing orientations may not be too radically different from one realization to another the positional difference from NAD 83 (1986) to NAD 83 (2011) in Dallas County is in the range of 1.6 ft.



My recommendations are:
“State Plane Coordinate System, Texas North Central Zone 4202, North American Datum of 1983 (XXXX).” or 
“State Plane Coordinate System, Texas North Central Zone 4202, NAD 83 (XXXX).”



Where XXXX equals 1986, 1992, CORS96, 2007 or 2011


1986 - Initial realization of NAD 83

1993 - Texas HARN

CORS96 - Points determined by OPUS solution prior to NAD 83 (2011)

2007 - National adjustment of same year.  Considered to be equivalent to CORS96

2011 - Most current realization of NAD 83



I hope this makes sense.  Let me know if I can be of any further assistance.



Regards,
Dave Doyle




Thank you very much for your assistance.

Actually, there is one more area where your expertise would be quite helpful (and this may, I think, sound a little dumb).  We get quite a few surveyors who contend it is valid to reference State Plane Surface Coordinates.  It is our contention that “State Plane” references grid values only. Anything else is a local projection.  


Mr. Holt,

This is a topic that has driven me crazy for years.  Over the years I’ve given over a hundred workshops/seminars on the development and use of the State Plane Coordinate system (quite a few of them in TX) and in every one I’ve addressed this topic. The use of the term State Plane Coordinates should only be applied to those values that are referenced to the grid as defined either for NAD 27 or NAD 83.  Raising or lowering coordinates to some other reference plane become something else—NOT SPCs. 

I know why surveyors do this and some of their reasons are sound, but just modifying the SPCs to some other surface leads to misidentification and confusion when they are not properly identified, and many if not most times they are not.  I have talked to numerous surveyors who have literally spent days trying to figure out why their values do not match someone else’s by several feet, and this issue is very often one of the culprits.

I tell surveyors if they want to raise their SPCs to ground level then modify the coordinates by adding or subtracting a large value so they won’t be confused later with real SPCs.  I’ve attached several slides from the last SPC presentation that I gave that highlight my recommendations on this topic. I hope this is helpful.

Regards,
Dave Doyle

NGS Chief Geodetic Surveyor (Retired)

Base 9 Geodetic Consulting Services
 
 

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