LTE: Letters to the Editor
Professional Surveyor Magazine - April 2014
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NGS appreciates the feedback from our user community, such as that provided by Michael Daly as well as Chuck Ghilani’s response (Letter to the Editor, Feb 2014
). We completely agree that reference frame changes should be not only theoretically correct but partnered with practical information and education. We will strive to improve this situation.
Thankfully things are not as bleak as Professor Ghilani states—specifically, we do have transformation software between the different realizations of NAD 83. When the GEOCON software was released in 2012 this filled the initial gap between the “HARNs” and the NSRS2007 realizations. GEOCON11 filled the gap between NSRS2007 and 2011. To sum up, the transformation software available through NGS is:
- NADCON : NAD 27 to NAD 83(1986) or US Standard Datum to NAD 83(1986)
- NADCON : NAD 83(1986) to NAD 83(“HARN”)
- GEOCON : NAD 83(“HARN”) to NAD 83(NSRS2007)
- GEOCON11: NAD 83(NSRS2007) to NAD 83(2011)
Where “HARN” indicates the most recent post-1986, pre-2007 coordinates on a point without consideration of whether they are from a HARN (High Accuracy Reference Network), an FBN (Federal Base Network), or a mix of both. This shorthand use of “HARN” is satisfactory for many states where the FBN and HARN are indistinguishable at the few cm level, but in other states a future version of GEOCON will more adequately disentangle these two realizations in those states where the HARN and FBN are not only more distinct but treated as such by the user communities of those states.
All of these software tools are available on the NGS website at geodesy.noaa.gov
Furthermore, and in response to Professor Ghilani’s other comments:
- Our geodetic control datasheets provide current and superseded control information for each published station, which includes the epoch and/or adjustment dates for the datums and realizations applicable to each station.
- HTDP (http://geodesy.noaa.gov/TOOLS/Htdp/Htdp.shtml) provides transformations between NAD83 and all WGS84 realizations.
- It is not quite correct to say NAD 83(NSRS2007) and NAD 83(2011) are “based on ITRF datums” though it is correct to say that their coordinates are derived from GPS vectors tied to CORS whose NAD 83 coordinates have a direct mathematical relationship to the various ITRFs.
National Geodetic Survey
I want to thank you for your History Corner article in the March 2014 issue, “Community Service and the Pioneer Surveyor: E.B. Camp” It was well written and historically important to Manatee County as well as the surveying profession!
Todd E. Boyle, RSM
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