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NAD83 to NAD27

Discussion in 'General Land Surveying' started by SIR VEYSALOT, Oct 16, 2013.

  1. SIR VEYSALOT

    SIR VEYSALOT 5-Year Member

    Joined:
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    Can anybody provide a source to convert NAD 83 LAT/LON to NAD 27 LAT/LON. We are currently using Carlson 2012 in MAP 2012. I know its possible in MAP but not sure of the process and CORPSCON is down. (Not sure if CORPSCON will do it) We primarily collect in NAD83. Thanks
     
  2. hubermar

    hubermar 3-Year Member

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    May 23, 2012
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    Location:
    North Carolina
    I apologize for the Corpscon server being down. But i have created a temporary alternative at http://corpscon.silkwerks.com/

    Keep in mind that Corpscon converts between NAD83(86) or the NAD83(HARN) and NAD27. it does not handle NAD83(2007) or NAD83(2011). Not yet anyway!
     
  3. Mark Mayer

    Mark Mayer 5-Year Member

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    > Keep in mind that Corpscon converts between NAD83(86) or the NAD83(HARN) and NAD27. it does not handle NAD83(2007) or NAD83(2011). Not yet anyway!
    Also remember that the operation is a sort of rubber sheet translation, not a mathematical conversion.
     
  4. MightyMoe

    MightyMoe 4-Year Member

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    Also remember that the operation is a sort of rubber sheet translation, not a mathematical conversion.


    Boy is that ever correct. We did a bunch of testing in the early days and found the NAD83-NAD27 conversions to be between .5' o 6' "off" in this area.

    Usually they were under 2' "wrong". In talking to other surveyors they claimed they've seen 15' of "error" in the conversions; I never did, but then again after a while I quit looking or caring, once I knew how bad it could be I knew enough to never put much stock on the programs and use them for what they are-a guesstimate.

    A lot of programs and devices do the conversions, Corpscon, Trimble, handheld GPS, any data collector that I've had, Autocadd....
     
  5. Kris Morgan

    Kris Morgan 5-Year Member

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    Rusk, Texas
    I downloaded CORPSCON years ago and have it on the desktop. Post the values in 83 and I'll post them in 27.
     
  6. Mark Mayer

    Mark Mayer 5-Year Member

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    OK, OR, WA
    > Boy is that ever correct. We did a bunch of testing in the early days and found the NAD83-NAD27 conversions to be between .5' o 6' "off" in this area.
    In many cases it is partly a product of the quality of the NAD27 coordinates that were derived using "classical geodetic methods" vs. GPS derived NAD83 coordinates. Classical Geodetic methods attempted to meet 1: 100,000 closure, which means around a foot of error between stations 20 miles apart. The there is the 1:10000 closure of infill surveys between, and the NAD27 coordinate reported on some local point might be several feet off the theoretically correct figure. That's hard to appreciate in an age when we can do better than that with our iphones.

    > A lot of programs and devices do the conversions, Corpscon, Trimble, handheld GPS, any data collector that I've had, Autocadd....
    I think they all, including CORPSCON, rely on NADCON ...
     
  7. GeeOddMike

    GeeOddMike 4-Year Member

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    Location:
    Florida
    Aloha,

    As the NGS site remains unavailable, I take the liberty of providing a link to an article on NADCON published in Professional Surveyor. It is a PDF of the file sent to them. http://geodesyattamucc.pbworks.com/w/file/53304733/NADCON_ProfSurv_article.pdf

    Remember that NADCON, especially for NAD 27 to NAD 83(1986) was intended for use by mappers and others working at low orders of accuracy.

    As indicated elsewhere, NADCON algorithms are used in CORPSCON. Actually all there algorithms are from NGS. They designed the useful interface.

    Aloha,

    DMM
     
  8. MightyMoe

    MightyMoe 4-Year Member

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    Remember that NADCON, especially for NAD 27 to NAD 83(1986) was intended for use by mappers and others working at low orders of accuracy.

    Oh, if only you could get that message out to everyone. Many engineers and designers think that the conversion programs are accurate, no amount of explanations, facts, examples, can break through that mind set.

    If you truly want NAD27 then survey in NAD27, It's not that hard.;-)
     
  9. base9geodesy

    base9geodesy 2-Year Member

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    Location:
    Silver Spring, MD
    Great comments, especially from Mark, Mike, Norman and MightyMoe. I had the honor of being part of the NGS NADCON evaluation and implementation effort. In the lower 48 states the standard deviation of the transformation between NAD 27 and NAD 83 (1986) is .15m (.5 ft). Anyone who finds differences in the range of 1ft or more typically either started from poor NAD 27 positions or did something wrong. The differences can be as high as 1 m (3.2 ft) in remote parts of Alaska where control is especially sparse. The standard deviation from NAD 83 (1986) to NAD 83(HARN) is .06 m (0.2 ft). As Mike and MightyMoe rightly point out numerous articles and over 100 NGS workshops and seminars to surveyors around the country on this topic have stressed the point that the tool was designed to facilitate transformations in mapping and charting and THE only way to accurately reflect coordinates/heights from one datum or realization to another is to readjust using original observations and control values in whichever reference frame you're trying to go to.

    I am not aware of any transformation tool anywhere in the world that covers continental scales attempting to relate a classical horizontal datum with a more contemporary Earth centric system that comes anywhere close to what NADCON does. The solution developed by the Defense Mapping Agency (DMA) (which is embedded in virtually every GPS receiver on the planet) to cover the same area has a standard deviation of something like 9.3 m (31 ft)!

    Given the negative connotation of the term rubber sheeting as basically a WAG, I wouldn't use it to describe NADCON. This tool was developed by looking at the 2-D positional changes (latitude and longitude) for more than 150,000 stations with NAD 27 positions that participated in the original NAD 83 adjustment. Because the control points are scattered randomly NADCON uses gridded values of these changes at 15' intervals. The value of the positional shift at any given grid node is influenced most strongly by the those stations closest to that point with increasingly less influence the further away a station is. The same process and in general the same stations were used in creating the grids to go from NAD 83(1986) to NAD 83(HARN). As Mark indicated NGS has created a beta tool that will go from NAD 83(HARN) to NAD 83 (2007) and from NAD 83(2007)to NAD 83 (2011). You can access from their web page as soon as they're allowed to go back to work. NGS would really like your comments on these tools. They are limited in the input to data formated as *80* records that NGS uses in the Blue Book so good luck.
     
  10. John Hamilton

    John Hamilton 4-Year Member

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    As Dave says, normally it should work quite well. I agree that it is usually better than 0.15 m. But, one must consider the source of the NAD27 coordinate being used or compared. USGS typically did 1:10,000 work, so a USGS station 20 miles from a first order NGS station could be off 10 feet and still meet the spec (usually was much better, but it could happen). And, the way some private surveyors (and government agencies) did control in the old days means the coordinate could be way off (azimuth bias accumulates with distance), etc.

    Also, one should be aware that NAD is way different than NAD27. Around my area it is about 100 feet if I recall correctly.
     
  11. base9geodesy

    base9geodesy 2-Year Member

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    John, I assume you mean NAD 83 to NAD 27 was about 100' in your area - which is correct. NAD to NAD 27 in the Pittsburg region is more like 3-10 ft.
     
  12. Cliff Mugnier

    Cliff Mugnier 4-Year Member

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    Ever notice that the people that know about this stuff ...

    ... are older than dirt?

    I plead guilty to the same crime. Hope this stuff gets archived for the future whippersnappers that didn't live through the dark days of geodetic surveying ...
     
  13. MightyMoe

    MightyMoe 4-Year Member

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    Thanks for the details, although I've seen some monuments that meet the .5' "error" (I really hate that word for this conversion) they are usually NAD27 monuments that were occupied as HARN points or first order NAD27 monuments fairly close to a HARN point.

    I occupied a first order NAD27 control point that checked 3.5' x 2.9' (if I remember correctly) and that's what made me really start looking at the numbers.

    Other points in the area checked from .5' to 2' which I felt was really good for what I was looking for, but I wouldn't want to use them for anything more than a rough coordinate when a simple quick static session would give me very accurate numbers.
     
  14. John Hamilton

    John Hamilton 4-Year Member

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    Dave: SHANNON (KY3090):

    NAD (USSD): 40-21-33.53/80-01-26.95
    NAD27: 40-21-33.15538/80-01-25.85590
    NAD83: 40-21-33.39838/80-01-25.03102

    NAD-NAD27: 28.285 m (92.8 ft)
    NAD-NAD83: 45.463 m (149.16 ft)

    There were about 10-20 stations around here with NAD coordinates (you sent me an excel file with the data), and the shifts were fairly consistent, around 90-100 feet. Most of the NAD stations were third order.

    I know that you did a bunch of research on this stuff (NAD/USSD) years ago, where is that info now? You may recall one reason that I asked you about this years ago was that the City of Pittsburgh origin is at 40°26'N/80°00'W on NAD, not NAD27 as many around here assume. I first realized there was something strange when I located (with GPS) the radio antenna on the roof of the University of Pittsburgh Cathedral of Learning (PID KX2072) and the NAD27 coordinates missed the city maps by nearly 100 feet.
     
  15. John Hamilton

    John Hamilton 4-Year Member

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    One of the reasons (among others) for doing the original NAD83 adjustment was because there were problem areas in NAD27 because of the way it was "spliced" together, adding new work onto old work. One of the first GPS jobs was in eastern PA in the mid 80's. We directly GPS'd between two first order triangulation stations that were in different triangulation chains. The misclosure was about 3 feet if I recall correctly. Problems like that distortion are bound to show up in the NADCON data sets, remember that 0.15 m is an rms value.
     
  16. half bubble

    half bubble 4-Year Member

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    I have only really had to deal with this on one job/campaign. I tried Corpscon and found, like others, that it was loose to 0.5-1.0'

    Luckily, I had some old NAD27 coordinates for some of the points, so I used Primacode Transform to get the NAD83 stuff onto NAD27.

    I have had even better success doing that least squares conformal best fit to take NAD27 coords and put them on NAD83 by shooting a few points in GPS and transforming the NAD27 to those points. Good for recon, and darn close to the modern points once you find them.
     
  17. MightyMoe

    MightyMoe 4-Year Member

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    I wondered immediately when I first saw Corpscon how it could possible work. I think it was in the mid 1980's. I was working on an Airport job and the engineer wanted everything in NAD83 and NAD27.

    There was a first order triangulation station on site and I surveyed from it to another one a few miles away for my main traverse. I was using a T2 with a Topcon distance meter.

    I used both sets of numbers on the data sheet, the NAD83 and the superseded 27 values (which matched my old data sheet) to calculate the traverse as a 83 and a 27 line and was quite pleased with the closures and results. Then an almost war began between me and the engineer. He wanted everything to match Corpscon! And nothing did.

    Try as I might to explain to him that Corpscon wouldn't work, he just wouldn't believe me; after all I'm just a surveyor what would I know. I should make it all "fit" the government approved calculation. This lasted weeks, meetings, meetings, calls to Corpscon, me showing him the data sheet and putting the 83 number through the program and getting a different number than the 27 number on the data sheet, nothing would convince him. I finally just said; if you want the 27 numbers to match Corpscon do them yourself but remove my name from the map.
     

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