Adjusted

[+Neweyess]

The following quotes are found at the Geocashing site?

 

"On the benchmark's page, look at the line under the marker's coordinates. If it says "location is SCALED", then the coordinates were probably originally obtained from a map and won't be very accurate. "

 

"Control points that are used for horizontal control have much more accurate coordinates. (In some cases, those coordinates can seem almost hyper-accurate.) If, on the benchmark page, it says "location is ADJUSTED", then you are looking for a horizontal control point. Most of these have been included in a nationwide network adjustment to obtain state-of-the-art precision, and the coordinates can be much more accurate than any handheld GPS can measure (within a few millimeters)."

 

My question is how was the adjustment made? More specifically, does it mean that a professional has been to the site to get the GPS coordinates personally --or not necessarily? My reason for asking is, if a datsheet shows no entry during the time GPS technology was available, yet it's adjusted, does that mean a professional visited the site anyway and there is no log for it --or is there some other way to adjust without going to the site. Also, would NGS adjust based on a report received from one of us?

 

Neweyess

[Bill93]

Before GPS they achieved super precision with super human efforts using high grade theodolites (on towers as needed) and massive amounts of office computation to adjust the data until all the measurements fit as best possible. There is some good history somewhere on the NGS site.

[mloser]

The adjustment mentioned is to NAD83 (North American Datum 1983) datum, which was a change to use a different ellipsoid, GRS80, (essentially, the earth's shape) based on the central reference point being the center of the earth, and not the Clarke base station at Meade's Ranch in Kansas. The shift of datum resulted in a grid shift that varied depending on the location in the US and the shape of the ellipsoid used at that location.

 

For a bit more information on the grid systems can be found at http://www.towermaps.com/nad.htm. There is no magic about this site--it is just the first one I came up with on a Google search.

[+BuckBrooke]

I've noticed adjustments in the New Mexico surveyor's office for Nad83(1986) and NAD83(1992), so they've had adjustments since 1983, presumably as the data gets better.

[Z15]

ADJUSTED = Least squares adjustment.

(Rounded to 5 decimal places.)

 

This 272 page report describes it.

[+rogbarn]

I've noticed adjustments in the New Mexico surveyor's office for Nad83(1986) and NAD83(1992), so they've had adjustments since 1983, presumably as the data gets better.

Does the date in parentheses have some special significance? Is it just the date of the adjusetment? Is an adjustment made in 1986 somehow different than one made in 1992 if they both use NAD83? I've never paid much attention to those dates and I'm wondering if I'm missing something.

[Z15]

NAD83 (1986) indicates positions on the NAD83 datum for the

North American Adjustment, completed in 1986.

NAD83 (nnnn) indicates positions on the NAD83 datum for the

North American Adjustment, but readjusted to a State High

Accuracy Reference Network (HARN) on the date shown in (nnnn).

NAD83 (CORS) indicates positions which are part of the CORS

network.

[caseyb]

I am curious... might this be the type of thread that would be appropriate for the NGS forum? I am honestly not sure how people feel about it. Although it was handled very well by the folks here, I feel bad for not seeing it.

 

-Casey-

[2oldfarts (the rockhounders)]

I am curious... might this be the type of thread that would be appropriate for the NGS forum? I am honestly not sure how people feel about it. Although it was handled very well by the folks here, I feel bad for not seeing it.

 

-Casey-

 

Casey,

 

This information is used by all of the Benchmarkers no matter how they log their recoveries. It is very interesting & is being handled by the Professional Surveyors. No problem here at all. But, thank you for asking.

 

Shirley~

[+Black Dog Trackers]

Here is my little explanation of the term 'least squares adjustment' with approximately zero math.

 

The method of least squares is a statistical solution to a problem of how to get the 'best' answer from an inherently imperfect measuring process such as surveying.

 

A computer program is really needed to perform least squares adjustments for any network bigger than 3 or 4 points because it involves matrix algebra, which means lots of equations to solve.

 

Enter the survey data (lengths and angles between the points) along with the coordinates of at least one of the points, and a least squares adjustment program will give the statistically best estimate of the true coordinates of each of the points.