Differences between revisions 2 and 3
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A - a relative accuracy of {{{5 mm +/- 1:10,000,000 relative to other A-order stations.}}} ||<-2 ( tablestyle="font-size:9pt;" rowstyle="font-size:11pt">'''GPS Standards'''||
||'''Horz. Order'''||'''Definition'''||
||AA||a relative accuracy of {{{3 mm +/- 1:100,000,000 relative to other AA-order stations.}}}||
||A||a relative accuracy of {{{5 mm +/- 1:10,000,000 relative to other A-order stations.}}}||
||B||a relative accuracy of {{{8 mm +/- 1: 1,000,000 relative to other A- and B-order stations.}}}||
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B - a relative accuracy of {{{8 mm +/- 1: 1,000,000 relative to other A- and B-order stations.}}} ||<-2 ( tablestyle="font-size:9pt;" rowstyle="font-size:11pt">'''Classical standards'''||
||'''Horz. Order'''||'''Definition'''||
||First||1: 100,000 relative to other stations of the same order.||
||Second-order, class I||1: 50,000 relative to other stations of the same order.||
||Second-order, class II||1: 20,000 relative to other stations of the same order.||
||Third-order, class I||1:10,000 relative to other stations of the same order.||
||Third-order, class II||1:5,000 relative to other stations of the same order.||
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First - The classical relative accuracy specifies a distance tolerance. For instance, if two first-order points are separated by 10km, then the distance between them will be accurate to 10 cm, in principle. To estimate the absolute accuracy of a position, a resonable estimate for a "classical" station would be based on the distance from the station to the nearest CORS, HARN, or FBN station that existed at the time of adjustment within the same state as the station. For instance, here is a portion of a datasheet:
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Second - {{{
 LY2625 DESIGNATION - BEVANS
 LY2625 PID - LY2625
 LY2625 STATE/COUNTY- NJ/SUSSEX
 LY2625 USGS QUAD - CULVERS GAP (1997)
 LY2625
 LY2625 *CURRENT SURVEY CONTROL
 LY2625 ___________________________________________________________________
 LY2625* NAD 83(1996)- 41 12 07.74590(N) 074 51 22.66670(W) ADJUSTED
 LY2625* NAVD 88 - 235. (meters) 771. (feet) SCALED
 LY2625 ___________________________________________________________________
 LY2625 LAPLACE CORR- 0.90 (seconds) DEFLEC99
 LY2625 GEOID HEIGHT- -32.54 (meters) GEOID03
 LY2625
 LY2625 HORZ ORDER - FIRST
 LY2625
 LY2625.The horizontal coordinates were established by classical geodetic methods
 LY2625.and adjusted by the National Geodetic Survey in September 1999.
}}}
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Third - The nearest high-accuracy station in New Jersey in 1999 was LY2920, located 25 km away. That would make the absolute position of LY2625 accurate to about 0.25 meter.

A forthcoming adjustment of all GPS stations, scheduled for completion in 2007, will list absolute accuracies on the datasheet. Unfortunately, classical stations will be excluded from that adjustment, so estimates such as the above will still be necessary.

When used with respect to survey ["control"], order is a specification of the accuracy and precision of the measurements that were made. The ["NGS"] has standards that must be met for the different orders of accuracy.

On an NGS datasheet, one of the lines will say "HORZ ORDER" (geocaching datasheets do not have this specification line). The levels of order and their accuracy are:

GPS Standards

Horz. Order

Definition

AA

a relative accuracy of 3 mm +/- 1:100,000,000 relative to other AA-order stations.

A

a relative accuracy of 5 mm +/- 1:10,000,000 relative to other A-order stations.

B

a relative accuracy of 8 mm +/- 1: 1,000,000 relative to other A- and B-order stations.

Classical standards

Horz. Order

Definition

First

1: 100,000 relative to other stations of the same order.

Second-order, class I

1: 50,000 relative to other stations of the same order.

Second-order, class II

1: 20,000 relative to other stations of the same order.

Third-order, class I

1:10,000 relative to other stations of the same order.

Third-order, class II

1:5,000 relative to other stations of the same order.

The classical relative accuracy specifies a distance tolerance. For instance, if two first-order points are separated by 10km, then the distance between them will be accurate to 10 cm, in principle. To estimate the absolute accuracy of a position, a resonable estimate for a "classical" station would be based on the distance from the station to the nearest CORS, HARN, or FBN station that existed at the time of adjustment within the same state as the station. For instance, here is a portion of a datasheet:

 LY2625  DESIGNATION -  BEVANS
 LY2625  PID         -  LY2625
 LY2625  STATE/COUNTY-  NJ/SUSSEX
 LY2625  USGS QUAD   -  CULVERS GAP (1997)
 LY2625
 LY2625                         *CURRENT SURVEY CONTROL
 LY2625  ___________________________________________________________________
 LY2625* NAD 83(1996)-  41 12 07.74590(N)    074 51 22.66670(W)     ADJUSTED  
 LY2625* NAVD 88     -       235.     (meters)     771.     (feet)  SCALED    
 LY2625  ___________________________________________________________________
 LY2625  LAPLACE CORR-           0.90  (seconds)                    DEFLEC99
 LY2625  GEOID HEIGHT-         -32.54  (meters)                     GEOID03
 LY2625
 LY2625  HORZ ORDER  -  FIRST
 LY2625
 LY2625.The horizontal coordinates were established by classical geodetic methods
 LY2625.and adjusted by the National Geodetic Survey in September 1999.

The nearest high-accuracy station in New Jersey in 1999 was LY2920, located 25 km away. That would make the absolute position of LY2625 accurate to about 0.25 meter.

A forthcoming adjustment of all GPS stations, scheduled for completion in 2007, will list absolute accuracies on the datasheet. Unfortunately, classical stations will be excluded from that adjustment, so estimates such as the above will still be necessary.

order (last edited 2009-03-21 21:03:19 by localhost)