In doing the experiment described in a related post, I've tried to understand the various datums, datum tags, and other mysterious designations on the data sheets. I thought I'd summarize what I think I know about datum realizations. Please correct me where I'm wrong.
You may also want to read articles on the NGS web site, such as
http://www.ngs.noaa.gov/PUBS_LIB/develop_NSRS.html
http://www.ngs.noaa.gov/INFO/OnePagers/NewDatumsOnePager.pdf
http://www.ngs.noaa.gov/faq.shtml
Following regional triangulations begun by Hassler, the US Coast & Geodetic Survey connected across the continent in the 1800's and eventually did a least-squares adjustment that resulted in a national datum. After some iterations it was called the North American Datum of 1927 (NAD27). It used the Clarke ellipsoid model of the earth and was defined relative to Meades Ranch.
After several decades of use and the gathering of more data (including early satellite data), a new datum was defined. It used an ellipsoid earth model that fits the whole world better than Clarke and (with minor differences) is essentially the earth model used for GPS. It was defined as the North American Datum of 1983 and became available in 1986, hence NAD83(86).
The topographic maps of the country were prepared in NAD27 coordinates. You will find that even a recreational GPS can see the difference between that and NAD(83); in my area it is about 50 feet so you need to account for it when using topo map with GPS.
GPS operates with a datum that is used internationally. The US military calls their version WGS84 and update it to match the international datum every several years. NGS chose to fix NAD83 to the North American tectonic plate to minimize coordinate changes. The two datums drift apart by a couple centimeters per year. Additionally, it was learned that the initial realizations of those two datums were not as close as hoped, so the difference is more like a couple meters. Most recreational GPS units ignore the difference
Since then additional GPS data has been used to improve the NAD83 realization. A bunch of state-by-state updates were done in the 1990's under the name High Precision GPS Network (HPGN), renamed High Accuracy Reference Network (HARN) which you see on signs as you recover some stations. Some states had two versions, so the latest in any state is called the Federal Base Network (FBN), a label you see on data sheets. You also see Cooperative Base Network (CBN), which is a densification of the high accuracy stations.
The 199x FBN adjustments updated all horizontal control stations in the data base. They used the old triangulation data along with GPS data on some, but a minority, of the old tri-stations. Additional HARN stations resulted from GPS measurements on elevation bench marks that had no accurate lat-lon.
HARN/FBN was still NAD83 because it used the same definitions, but the realization was different because it used additional measurements that shifted things a bit, hence names like NAD83(1995). I think of realizations like a fish net. After you tie down the lat-lon of some points on the net, there is still some sag or wandering by the rest of the net. Each realization gets its points better aligned, but not all points have equally good data.
The next realization (update) was NAD83(2007) with the goal to remove inconsistencies between the state adjustments. This time only GPS data was used, so triangulation stations that didn't have GPS data were omitted, and you still see their NAD83(199x) coordinates at the top of their data sheets. Those old coordinates typically have a fraction of a foot to even a few feet of local warpage that is not modeled. Intersection stations are going to be less accurate than triangulation stations.
The latest is NAD83(2011), which you see on data sheets having good GPS data. If a project design is based on an older realization, then measurements tied to the adjusted monuments can be converted accurately enough to 2007 and FBN, but stations that were not occupied with GPS will usually not match well when converted other realizations.
(corrections welcome)
There was a great cartoon about this:
"how does a geographer find a wife?"
"Datum"
Carry on.....!
Lecture notes for a class on this topic are here: http://geodesyattamucc.pbworks.com/f/25Feb2010_NAD.pdf
A few minor clarifications: HPGN was High Precision Geodetic Network, the FBN, CBN and UDN designations were intended as a hierarchy of stations with the FBN intended to be a nationwide 100 km spacing network where all points were to have high precision positions, well-determined orthometric heights, gravity measurements and to be revisited periodically to update and/or verify values. The CBN (Cooperative Base Network) was to permit densification at a tighter spacing. UDN (User Densification Network) was intended to support localities who wanted establish lower accuracy networks tied to the national network. While the FBN was to be a federal responsibility funding did not permit the plan to be fulfilled.
The original implementation of NAD83 did NOT include any GPS observations. It did include Doppler. As users adopted GPS it was found that the new measurements were better than the network would support hence the HPGN/HARN campaign. As this was a cooperative enterprise there were great differences in numbers and spacing of network points, observations were made on a state-by-state basis during a span of about five years. The HARN was intended to provide high accuracy horizontal positions; accurate heights were not a goal.
With advances in GPS hardware and software and models it became possible to obtain good ellipsoid heights. Therefore the FBN resurveyed campaign was undertaken. It was likewise a cooperative effort with states participating at different levels and at different times. Unlike the earlier HARN surveys CORS sites were available for these surveys.
As you indicate the 2007 and 2011 adjustments were intended to provide a nationwide solution and consistency with the CORS. Conventional terrestrial observations were not used in adjustments since NAD 83(1986). The points using these techniques were updated to be consistent with the nationwide system. Their inherent reduced accuracy cannot be fixed as the observations are not accurate enough.
As the choice of sites to use in these campaigns emphasized accessibility, GPS suitability, and some spacing criteria, most triangulation stations have not been included in these surveys. If suitable for GPS they could always be observed and submitted to OPUS-DB or included in a project submitted to the NGS.
Ground monuments should always be suspect as they are subject to all manner of disturbance.
Cheers,
DMM
It would be nice if OPUS derived positions of classic triangulation stations were appended to the data sheets.
Scott Zelenak, post: 371094, member: 327 wrote: It would be nice if OPUS derived positions of classic triangulation stations were appended to the data sheets.
It would be very nice if a lot more had OPUS positions, but that would require a big budget or a lot of voluntary submissions. Since a lot of OPUS shared solutions are not on marks with existing data sheets, I'm not sure how practical it would be to sort them out so you didn't have to look for them separately.
From what I gather, Bluebooking would get them on data sheets, but that is a lot more effort.
GeeOddMike, post: 371075, member: 677 wrote: If suitable for GPS they could always be observed and submitted to OPUS-DB or included in a project submitted to the NGS.
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In my original post change "early satellite data" to "pre-GPS satellite doppler data"
to prevent people misinterpreting it.
If the points were submitted by their PIDs any station could be identified and repeat solutions averaged perhaps.
I wouldn't use numbers on a datasheet for a Tri station that wasn't occupied for anything but NAD27 control, which is what those monuments are. It's rare that a Tri station was included into the HARN network, most were bench marks. That's why the HARN network has been so valuable, it is a network with accurate verticals while OPUS/CORS has tended to be so inaccurate vertically.
Shifts for a nearby HARN that is a tri station and first order bench:
NAD27-NAD83(86): 0.11"S, 2.25"W or about 10' south and 160' west
(86)-(93): 1.5'north, 1.7' west,,,,,,,,,this was the big GPS shift, most people began using the NAD83(93) values and stuck with them.
(93)-(07): .09'N, .04'W
(07)-(11): .02'N, .07E
As you can see there have only been the three new calculations, there were other published shifts but those were changes in the ellipsoid heights (94), (01).
I know of one municipality that begin broadcasting a radio signal for GPS near the beginning, they call it NAD83(88) which I assume means it is on the 1986Epoch.
They have so much data on that system that they don't wish to shift completely over with their new base which is a new CORS point west of town and will offer "shift" numbers to allow users to keep on the old system and still use the new one.
Of course it's the vertical component that is the sticker, the old verticals work (HARN) and are on NAVD88 which the new CORS will not match with Geoid12b
It is not my experience that most HARN points are/were benchmarks. Perhaps in some areas that is the case not where I've worked. The impetus to establish a height system based on high accuracy ellipsoid heights coupled with a high-precision geoid model, was to deal with issues like the decimation of monuments due to construction, geophysical changes and the like as well as known, unresolvable problems with NAVD88 (especially the continental tilt in this network).
While many happily use local benchmarks and assume the disagreement between the differential leveling height determinations and the GPS-derived orthometric heights is due to bad GPS or geoid models, I wonder. Many benchmarks have never been leveled to after they were established. Many are in locations where they are likely subject to local movement or disturbance. As indicated above, the national height system (NAVD88) has known defects.
It has been my experience that many users of benchmarks rely on one benchmark whose height is never validated by checking to others. The quality of equipment used, observation procedures and "adjustments" leave much to be desired.
As for the various datum tags, the original NAD 83 (which did not include GPS) is labeled NAD 83(1986). It does not have an epoch associated with it. Subsequent state-by-state HARN were established during various years. In order to differentiate them from NAD83(1986) date tag reflecting the date of observations were added. It was not until advancements in equipment, modeling and software occurred that the possibility of determining high accuracy ellipsoid heights became a priority. This prompted an additional project, also undertaken on a state-by-state basis, that had the primary goal an improvement in ellipsoid heights.
The HARN and FBN observations have been used in combination with CORS to provide a nationwide consistent network. The fact that there were two: 2007 and 2011 reflects the fact that we are constantly learning how best to deal with data. See Dr. Milbert's report on the 2007 survey.
While not mentioned in my earlier post, the importance of the ITRF should be acknowledged. The gravimetric geoid model coupled with ITRF coordinates will provide a more consistent height system. The current hybrid geoid model is distorted by the GPS on BM data set as well as transformations between NAD83 and ITRF.
GeeOddMike, post: 371167, member: 677 wrote: The current hybrid geoid model is distorted by ... transformations between NAD83 and ITRF.
Could you please explain how the transformation affects heights? I think of NAD83 as only describing the horizontal.
Computations of the gravimetric geoid are performed in ITRF (a three-dimension system). Determining an orthometric height from an ellipsoid height requires a model of geoid-ellipsoid separation. The hybrid geoid is provided for use with NAD83(2011). I refer to the hybrid model as being distorted in the sense that the GPS on BM points are used to create a surface best fitting NAVD88.
While ellipsoid heights can be used for many useful things they cannot capture fluid flow. In the words of the old engineering prof, we want solids to flow downhill.
