LTE: Letters to the Editor
Professional Surveyor Magazine - August 2013
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Jolly Olde Zero Longitude Line
As an “old” surveyor, on land and sea, on a trip to Jolly Olde England
(2011) I too had a burning desire to go to the Greenwich Observatory and stand on the zero longitude line, one foot on each side. It’s a serious uphill hike to get to the Observatory for an old, out of shape flatlander like me. I am so, so glad I made it to the top of that hill, and no one in my party (no surveyors or sailors) had a clue why anyone would want to go up that hill and stand on that line. I will never forget the thrill.
Joe T Smith, PE, PLS
Baton Rouge, Louisiana
Unstable Greenwich Meridian
I found your article [“Defining Surfaces”] informative but generating several questions.
First regards ECEF Coordinates. Do these change for every revision of the ITRF and DOD’s WGS 84 redefinition? I suspect they do.
Second, your article pointed out that the Greenwich Meridian has moved 200 feet. Is this due to England drifting [continental drift]? If not, can you explain the source of the drift?
Finally, are we (the profession “we”) maintaining the Conventional Terrestrial Pole for the foreseeable future as a standard?
Personal observation: If the Greenwich Meridian is not stable, then nothing is and it seems absurd to tie physical objects to these coordinate systems.
I enjoyed your book on computations. I share it with another surveyor for his use on occasion.
Michael Daly, PE & LS
Gallup, New Mexico
In answer to your questions:
1. Yes, the coordinates change with every revision, which means you need to keep track of this information on past and present surveys since control (NGS or DoD/GPS) will have different coordinates.
2. Since this was established in 1600s, I believe, yes it is due to continental drift. Ecuador has monuments for both the original definition of the equator and its present location (personally saw this on Samantha Brown’s travel show) :-)
3. I am not on the inside on the CTP but I have not heard anyone talking about it changing.
4. You comment is correct. I have stated to the people in charge at the NGS that they should stop changing the coordinates since we all work in a relative system anyway and then announce the next big change in datums in 2022 (or so), like what happened in 1986 with NAD83. I realize that this is confusing users of the systems but also recognize that the DoD has had five different coordinates for GPS and they won’t listen to anyone. :-) In fact, it is hard to find info on when the DoD changed their coordinate systems. The people from NGS say that the surveyors are requesting these changes but I have never heard this personally. Maybe a few are calling and asking why things don’t agree. I don’t know since I am not an insider. We all just have to live with it and keep track of our metadata for any project so that we can recreate it in the future if necessary.
I'm writing to thank you for your new series of articles in Professional Surveyor, "Where Theory Meets Practice," and to ask a couple questions that they have brought to my mind.
I am somewhat of a dinosaur (a practicing surveyor in my mid-60s) who has willingly embraced GNSS and GIS and am, as a result, struggling to play catch up with the basics of geodesy and beyond. Prior to this, most of us plane surveyors were more than happy to live and work in our flat world of local coordinates, but, I suspect that those days are gone forever. While I understand that your articles can't take the place of a college-level course or even an in-depth seminar, they are a great introduction/review of the basics that we need to understand. Otherwise, we will just be 'button pushers' like the vast majority of those who are beguiled by the capabilities of GPS, not professionals.
Chuck: I agree with your last statement. Unfortunately, I am afraid some surveyors are simply button pushers. This is why I am doing this series.
My first question relates to astronomic north. Since I started my own practice in 1989, I have referenced 99% of my surveys to astronomic north by taking sunshots. Even with GNSS, I still occasionally use them 'as a check' (surveyors are always suspicious of new technologies). Is the data for the sun in published ephemeris form such that the resulting 'north' will be the Conventional Terrestrial Pole or is it the projection of the earth's polar axis on the day of the solar observation? If not, would applying the Laplace correction convert astronomic north to the CTP? I have always suspected that the difference might be minimal and have never certified my meridian reference 'to the second', but, as they say, I gotta know.
Your astronomic azimuths are slightly different from the CTP for two reasons. First they are related to the instantaneous position of the Earth’s pole at the time of the observation. The IERS organization keeps track of this info and it is always very small (<1”). You can view the xy position of the pole here.
The second depends on where you live, and that is deflection of the vertical at the location you take the observation. These typically are under 5” but can be over 10” in some locations. The NGS has a deflec12A model and others here. Both of these corrections involve equations to correct an observed astronomical azimuth. Just to be complete, the equations are (I’m not recommending you do this!):
For CTP , which is always under observational errors. (xp and yp) are from IERS
For deflection of the vertical (Laplace correction)
- z = Zobs − Ψ = Zobs + ξ cos α + η sin α (use A from above or observed azimuth for alpha in this equation to correct zenith angle)
- α = A − η tan φ − (ξ sin α − η cos α)cot z (z is corrected zenith angle from 1. Again observed will be very close)
However, a solar observation is typically of less accuracy than both of these corrections combined, so, I don’t see a problem or need to correct them. Especially on a boundary survey. Simply reference your meridian as an “Astronomic North observed on mm/dd/yyyy” and you will leave all the information that someone may need in the future to re-establish your azimuth.
By the way, I congratulate you on observing Astro Azimuths. They are and always will be reproducible to the accuracy of the observations, which means your surveys are set for perpetuity. GPS is not as good for establishing azimuths due to the error in positioning over what is typically a short line.
My second question is regarding citing the epoch of GNSS derived positions since I understand that they change over time. I have attached an OPUS report for a static observation I did to use as a control point (base station) for GNSS RTK surveying and terrestrial (total station) surveys. Should one cite the "Ref Frame: NAD_83..." as the epoch or the "IGS08...." (or both) as the epoch for the work? and, would this epoch reference apply to the State Plane Coordinates as well, since these are what I show on my survey plat?
Cite the Reference Frame of the coordinates that you used and the appropriate epoch. The NAD 83 (2011) coordinates are in the left column and the IGS08 coordinates are in the right. These are the XYZ and lat, lon, height coordinates. So it depends on which coordinates you used. If you are using the SPCS coordinates then you are in NAD 83 (2011) (epoch 2010). If you are using some legacy control monument coordinates from an NGS datasheet then you would use NAD 83. Since they are still in the original system NAD83 system. Only the HARN and CORS stations (via OPUS) are being updated. So cite the reference frame of the coordinates you use.
I understand that you are likely on summer vacation, and appreciate your taking the time to consider this inquiry. Again, thanks for your years of service to the surveying profession.
I am happy to respond to a “dinosaur” that still considers learning important. :) Anytime.
Glen Yasharian, PLS
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