Adjusted how?

[+dukeofurl01]

I was looking at marks in the area, and came across this one: LT0676 and it says that it's adjusted, but how can that be since it hasn't been visited since 1949? Who adjusted it? Doesn't it take a survey grade GPS to adjust it?

[+billwallace]

On the datasheet, just below the CURRENT SURVEY CONTROL section are these two lines

 LT0676.The horizontal coordinates were established by classical geodetic methods
LT0676.and adjusted by the National Geodetic Survey in March 1994.

classical geodetic methods would be occupying the station and taking measurements

It looks like the NGS adjusted it ... my guess would be that in 1949 the coordinates were determined for/in the NAD27 datum then later they were converted/adjusted to NAD83 - I think adjusted can also refer to the process of distributing the error in a control network across all the stations in the network.

 

A little further down the datasheet in the SUPERSEDED SURVEY CONTROL section it looks like it was adjusted once before. - probably the original NAD83 conversion? I wonder what they did in 1994?

 LT0676                          SUPERSEDED SURVEY CONTROL
LT0676
LT0676  NAD 83(1986)-  40 03 06.70799(N)    121 22 45.62310(W) AD(1984.00) 2
LT0676  NAD 27      -  40 03 07.11800(N)    121 22 41.72600(W) AD(       ) 2
LT0676  NGVD 29 (07/19/86) 2159.2    (m)         7084.     (f) VERT ANG     

Ah yes from dsdata.txt - least squares adjustment ... that is a process/method of distributing the error in a control network (my take anyway)

ADJUSTED = Least squares adjustment.
                      (Rounded to 5 decimal places.)

Just how many flavors of NAD83 are there?

[+dukeofurl01]

Well, as a cartographer and GIS person, I can say with assurance, there is actually only one NAD83, but many ways of using it.

 

But taking coordinates that were imprecise in NAD27, and converting them to NAD83, they're still going to be imprecise. I've been in the habit of not taking HH2 coordinates for marks that say Adjusted, but now I'm wondering...

 

Where do you see 1994?

 

I don't understand the AD(1984.00) 2 part, and have never heard of NGVD 29, and don't quite get the information that is after that.

[+LSUFan]

 

I don't understand the AD(1984.00) 2 part, and have never heard of NGVD 29, and don't quite get the information that is after that.

 

Duke, That stands for the National Geodetic Vertical Datum......and in simple terms concerns the height (or elevation) of the benchmark in relation to an established point (which was sea level).

 

You may find the answers to a lot of this here:

 

http://www.ngs.noaa.gov/faq.shtml

 

and here

 

http://en.wikipedia.org/wiki/Benchmark_%28surveying%29

Edited February 4, 2011 by LSUFan

[Bill93]

There is one definition of NAD83, but many "realizatons" of it, as shown by the designation in parenthesis. When they do a new realization, they use the best and latest data available for some stations (nowadays CORS station history and VLBI data) and combine it with the original old theodolite observations from the 1930's for other network stations using least squares to get a best fit to make the adjustment. This gets us NAD83(1986), NAD83(1992), NAD83(1996), NAD83(NSRS2007) etc.

 

My tentative understanding of a realization is like a rubber sheet map that wants to be the perfect NAD83, but, since it has to be pinned down to the real world, there is a small amount of stretching where there was observational error. Each later realization has less stretching because it uses better data and is the best estimate available, but still isn't perfect.

 

Vertically we have National Geodetic Vertical Datum of 1929, NGVD29 and North American Vertical Datum of 1988, NAVD88. They are based on leveling runs across the country taken over the last century. They are truly different vertical datums because the definition changed. Basically NGVD29 was a best fit with the data they had for the US in the early 1930's, and NAVD88 was a best fit using additional data in the US and Canada (and maybe Mexico) and fitted to all of North America.

 

NGS is working toward a new vertical system based on GPS measurements instead of the optical leveling runs, but will have to gather a lot more gravity data (search GRAV-D) in order to be able to convert from height above the ellipsoid to orthometric height, which is what we ordinarily use as a measure of elevation. Speaking of which, it would be nice if someone more knowledgeable than me checked and clarified my wikipedia edits of orthometric height and dynamic height.

Edited February 4, 2011 by Bill93

[+Rotareneg]

But taking coordinates that were imprecise in NAD27, and converting them to NAD83, they're still going to be imprecise. I've been in the habit of not taking HH2 coordinates for marks that say Adjusted, but now I'm wondering...

 

You have to remember that they didn't directly measure the coordinates of the marks, they just measured the angles to other marks and then used primative computers to do lots of tedious calculations to come up with the final coordinates. With modern computers they are able to recalculate the coordinates, still using the original measured angles, by performing many more calculations than was possible before.

Edited February 4, 2011 by Rotareneg

[+dukeofurl01]

There is one definition of NAD83, but many "realizatons" of it, as shown by the designation in parenthesis. When they do a new realization, they use the best and latest data available for some stations (nowadays CORS station history and VLBI data) and combine it with the original old theodolite observations from the 1930's for other network stations using least squares to get a best fit to make the adjustment. This gets us NAD83(1986), NAD83(1992), NAD83(1996), NAD83(NSRS2007) etc.

 

My tentative understanding of a realization is like a rubber sheet map that wants to be the perfect NAD83, but, since it has to be pinned down to the real world, there is a small amount of stretching where there was observational error. Each later realization has less stretching because it uses better data and is the best estimate available, but still isn't perfect.

 

Vertically we have National Geodetic Vertical Datum of 1929, NGVD29 and North American Vertical Datum of 1988, NAVD88. They are based on leveling runs across the country taken over the last century. They are truly different vertical datums because the definition changed. Basically NGVD29 was a best fit with the data they had for the US in the early 1930's, and NAVD88 was a best fit using additional data in the US and Canada (and maybe Mexico) and fitted to all of North America.

 

NGS is working toward a new vertical system based on GPS measurements instead of the optical leveling runs, but will have to gather a lot more gravity data (search GRAV-D) in order to be able to convert from height above the ellipsoid to orthometric height, which is what we ordinarily use as a measure of elevation. Speaking of which, it would be nice if someone more knowledgeable than me checked and clarified my wikipedia edits of orthometric height and dynamic height.

 

This one is interesting: http://www.ngs.noaa.gov/GRAV-D/pubs/GRAV-D_v2007_12_19.pdf

[AZcachemeister]

One does not question HOW the NGS adjusted the co-ordinates for a mark.

 

One only accepts that the co-ordinates are WAY more accurate than ones (consumer grade) GPSr could possibly display.

[CallawayMT]

As AZ states, the classical monument coordinates will be better than any resource grade GPS values that you collect.

 

Past survey-grade GPS occupation of these type of classical stations usually have fallen around the 3 foot error ellipse from HARN or CORS station survey ties.

 

Kurt

[foxtrot_xray]

One does not question HOW the NGS adjusted the co-ordinates for a mark.

 

One only accepts that the co-ordinates are WAY more accurate than ones (consumer grade) GPSr could possibly display.

 

.. Wait. I thought that's what we do here? Question everything?

[DaveD]

The National Geodetic Survey and our predecessor agency the U.S. Coast & Geodetic Survey has observed several million individual observations including: triangulation and traverse direction (angle) measurements, high accuracy distances from rods, tapes and electronic (microwave, laser and infrared) measurement system, height differences from leveling, gravity observations and global positioning system vectors. When an data element such as the horizontal position or the orthometric height are designated as "ADJUSTED" that means that the value was determined through a very rigorous analysis of the observations, application of appropriate standard errors and adjustment of these data as part of either a national, regional or local least squares adjustment of the observations. All such observations are maintained by NGS in our database as part of the National Spatial Reference System and are available to the public on demand (we seldom get such requests). The term "ADJUSTED" along with the corresponding order of accuracy provides the user with a level of confidence in the quality of the data element to meet the needs of their surveying/cartographic/GIS applications.

 

We constantly tell users to challenge these data. Many control points are in places effected by a high degree of plate tectonic motion while many, if not most, of the geodetic positions and heights are the result of observtions taken more than 40 years ago and have not been validated by contemporary measurements. Unless a mark has been phyically moved a considerable distance it's not likely that you will detect a significant difference with the resource/recreation grade GPS receivers. The modernization of GPS and development of other constellations such as the Russian GLONASS, European Union GALILEO and Chinese COMPASS will greatly enchance the capability of hand-held devices. Some believe that future receivers may easily achive 10 cm with no augementation (e.g. WAAS, NDGPS).