<HTML dir=ltr><HEAD><TITLE>Re: [Proj] How to transform between NAD83 HARN/HPGN and WGS84</TITLE>
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<DIV dir=ltr><FONT face="Times New Roman" color=#000000 size=2>The NGS website also has the transformation parameters from the various NAD versions to the current WGS84/ITRF.</FONT></DIV>
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<DIV dir=ltr><FONT size=2>C. Mugnier</FONT></DIV>
<DIV dir=ltr><FONT size=2>LSU</FONT></DIV></DIV>
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<FONT face=Tahoma size=2><B>From:</B> proj-bounces@lists.maptools.org on behalf of Aaron Friesen<BR><B>Sent:</B> Fri 20-Feb-09 13:30<BR><B>To:</B> PROJ.4 and general Projections Discussions<BR><B>Subject:</B> Re: [Proj] How to transform between NAD83 HARN/HPGN and WGS84<BR></FONT><BR></DIV>
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<P><FONT size=2>Thank you all for your initial responses...<BR><BR>===== Short follow-up =====<BR><BR>A short follow-up question for those that don't want to read my<BR>long-winded follow-up below:<BR><BR>Do either NADCON or CorpsCon provide datum transforms between a<BR>given NAD83 and WGS84? The websites seem to indicate that they<BR>are only for transforms among various NAD datums and I do not<BR>know of a NAD83 variant that is truly assumed to be equivalent<BR>to WGS84/ITRF00. Please correct me if I am wrong on that.<BR><BR>Thanks again,<BR><BR>Aaron<BR><BR>===== Long-winded follow-up that you may want to ignore =====<BR><BR>Hopefully, you do not mind continuing the thread. I am responding<BR>to multiple people in this message, hoping to keep the thread more<BR>coherent.<BR><BR>Due to non-disclosure limitations, my original description of my needs<BR>only indicated Colorado as a location under NAD83 HARN, thinking I might<BR>be able to extrapolate responses to a more general solution. I apologize<BR>if I wasted your time with my original query.<BR><BR>Let me expand a bit and re-emphasize one point that seems to not have<BR>been addressed (i.e., conversion to/from WGS84).<BR><BR>I will be working with data originating in various parts of the US.<BR>The data represents various assets in the field where sub-foot accuracy<BR>is important (or claimed to be important). The first assumption is that<BR>existing data has the accuracy it claims to have. The second assumption<BR>is that existing data is in the datum it claims to be in. Both of these<BR>assumptions may be incorrect, but need to be assumed to be true for this<BR>discussion.<BR><BR>For existing data, I need to be able to go to the field and locate that<BR>asset within the combined error of the original reading and the current<BR>reading from the GPS device used to locate the asset. For new assets, I<BR>need to be able to take a reading and add that information to the data set. <BR>All GPS field readings for this discussion can be assumed to have sub-foot<BR>accuracy. If they do not, that is an issue between me and the GPS<BR>manufacturer claiming such accuracy. In any case, we are still talking<BR>sub-foot mapping grade GPS, not sub-centimeter survey grade.<BR><BR>The data starts out in various projections (some in State Plane, some<BR>in UTM, some living its entire life as lat/long). I would also like to<BR>assume that any projection does not add significant error. Yes, another<BR>assumption that may or may not be valid, but for now...<BR><BR>The problem is that these various data-sets are in various NAD83 incarnations,<BR>and to take the data into the field or bring back new data from the field, one<BR>or more datum transforms are necessary, getting it to WGS84 and back retaining<BR>its original sub-foot accuracy.<BR><BR>The transform to WGS84 seems to be the point not directly addressed. At least<BR>I did not see it that way. For sub-foot work, the old assumption of NAD83==WGS84<BR>doesn't do it.<BR><BR>> From: C. Mugnier<BR>><BR>> Such questions regarding geodetic-accuracy transformations in Colorado<BR>> are best directed to the U.S. National Geodetic Survey. NADCON indeed<BR>> does have NAD83 HARNHPGN files, and can provide the necessary<BR>> operations either on-line or through a download. The same goes for<BR>> current versions of CorpsCon. Both solutions are not commercial and<BR>> both are free.<BR>><BR>> Go to www.NOAA.NGS.gov and you'll get your problem solved for free<BR>> courtesy of the U.S. Government. A Google search will get you to the<BR>> current version of CorpsCon.<BR><BR>Both NADCON and CorpsCon descriptions talk of conversion between various<BR>incarnations of NAD83, but neither seems to address the transform to/from<BR>WGS84/ITRF00.<BR><BR>> From: Landon Blake<BR>><BR>> When you say NAD83 HARN are you using Colorado State Plane Coordinates?<BR>> Or are you dealing with lat/long values?<BR><BR>All lat/long WGS84 in the field.<BR><BR>> I don't think there isn't a huge difference between NAD83 and WGS84,<BR>> but<BR>> I'd think Proj would have a conversion between the two.<BR><BR>Depending on the NAD83 incarnation, NAD83 to WGS84 can differ by several<BR>meters. Much of the Proj documentation implies that Proj does a null<BR>transformation for NAD83 to WGS84 by default. That may or may not be<BR>true, but the documentation seems to imply it.<BR><BR>With the +towgs84 switch, I might be able to over-ride that, but I'm not<BR>sure this is sufficient to meet my sub-foot requirements and cannot find<BR>any information on what those parameters might be for a given transform.<BR><BR>> If you are dealing with state plane coordinates a local surveyor can<BR>> probably point you to the conversion formulas that you need. Or you<BR>> might check with the state land surveying association in Colorado.<BR>><BR>> If you get stuck, let me know. I can bounce a message off a couple of<BR>> land surveying message boards with Colorado Surveyors listening.<BR><BR>I'm hoping to find a single conversion package (or at least a small<BR>set) to address this problem in the general case.<BR><BR>> I'd be careful using something like Corpscon for data that is sub-foot<BR>> accuracy and needs to stay that way.<BR>><BR>> We wouldn't use that program or any like it to convert data that needs<BR>> to be that accurate. I don't know what the purpose of Aaron's data<BR>> conversion is, but he might need to be careful.<BR>><BR>> Transforming the location of gopher holes using Corpscon is one thing.<BR>> Transforming building corner positions, or the alignment of a new<BR>> pipeline with Corpscon is another thing entirely.<BR><BR>My needs are closer to the pipeline example than the gopher hole one.<BR><BR>> From: Rich Greenwood<BR>><BR>> Corpscon (<A href="http://crunch.tec.army.mil/software/corpscon/corpscon.html">http://crunch.tec.army.mil/software/corpscon/corpscon.html</A>)<BR>> based on NADCON is a good, free, authoritative tool when working in<BR>> the US.<BR><BR>(Sorry for the repeat) But neither Corpscon nor NADCON seem to be able<BR>to get me to WGS84.<BR><BR>> From: Landon Blake<BR>><BR>> I was recommending that one consider the purpose of the data<BR>> transformation before deciding to use ANY transformation package. If I<BR>> was building a nuclear reactor I would want to perform field surveys to<BR>> tie into local survey control.<BR>><BR>> Aaron is probably fine using CopsCon. I just get nervous when I hear<BR>> "sub-foot" accuracy. Usually when you are dealing with coordinates at<BR>> that level of accuracy you are leaving the GIS arena and entering an<BR>> arena that demands field work to reference local control on the ground.<BR><BR>Rest assured, no nuclear reactors are in the works. I understand what<BR>you are saying. Many times, I deal with highly-precise but very inaccurate<BR>data, with GPS readings recorded down to 7 or more decimal places from a<BR>consumer grade device with only 30 meter accuracy.<BR><BR>Thanks again,<BR><BR>Aaron<BR>_______________________________________________<BR>Proj mailing list<BR>Proj@lists.maptools.org<BR><A href="http://lists.maptools.org/mailman/listinfo/proj">http://lists.maptools.org/mailman/listinfo/proj</A><BR></FONT></P></DIV></BODY></HTML>