Feature: Planned Updates to the NGS Datasheet Format
Professional Surveyor Magazine - January 2012
by Burt Smith
The National Geodetic Survey (NGS
) is always interested in updating its products to better serve the needs of our stakeholders. Recently, NGS developed several modifications to the NGS datasheet—the primary method for accessing the passive control network of the National Spatial Reference System (NSRS
Many developers use NGS datasheets in their products, and NGS recognizes the importance of keeping the public informed of datasheet format changes. Below is a summary of datasheet changes scheduled for implementation in the first quarter of calendar year 2012 and corresponding with the implementation of the National Adjustment of 2011 (NA2011).
The COUNTRY field has been added to the STATE/COUNTY section.
Changes to the Coordinate Box area
In the Coordinate Box, ELLIPSOID HEIGHT has been moved from below the coordinate box to inside the coordinate box and is designated as ELLIP HT.
The geometric EPOCH DATE has been moved from below the coordinate box to inside the coordinate box and is designated as EPOCH.
The DATUM TAG has been added
for “B” and “C” above.
Descriptors are added, and, therefore, hyphens and data have been shifted to the right. Some examples are:
NAD 83(2007) POSITION-
- NAD 83(2007) ELLIP HT-
- NAVD 88 ORTHO HEIGHT-
The Orthometric Height Epoch Date (if applicable) has been removed from the end of the NAVD 88 line to its own line, just below, with the following descriptor:
The orthometric height datum field will now include a hyperlink, in addition to the specific datum referenced by the point. The link will take the viewer to a page on the NGS website explaining the origin of the specific datum used. This datum page will eventually give an explanation for all horizontal and vertical datums.
Changes below the Coordinate Box area:
A note (if applicable) has been added when the geoid height computed from the most current geoid model differs from the geoid height used to determine the published orthometric height, as below:
NOTE: NAVD 88 ortho height was determined from prior model GEOID03.
If applicable, the geoid model used to determine the published GPS-derived orthometric height has been added.
GEOID03 HEIGHT- -22.75 (meters)
The geoid height computed from the most current geoid model now follows the geoid height noted in “H,” above. The descriptor has been changed; therefore, the hyphen has been shifted to the right.
GEOID09 HEIGHT- -22.80 (meters)
X, Y, and Z coordinates follow the published geoid height. The descriptors have been changed; therefore hyphens have been shifted to the right.
Local and network accuracies:
NAD 83(2007) X- -1,994,789.496 (meters) COMP
- NAD 83(2007) Y- -4,697,388.731 (meters) COMP
- NAD 83(2007) Z- 3,815,306.819 (meters) COMP
- LAPLACE CORR: the hyphen has been shifted to the right.
The algorithm used to compute accuracies published as a result of the 2007 Readjustment has been modified. The caption will now read as follows:
FGDC Geospatial Positioning Accuracy Standards (95% confidence, cm)”
Local and network accuracies have been added to the datasheet, with an expanded list of all network connections and their local accuracies, reached through a hyperlink.
In conjunction with FGDC standards, North and East Accuracies have been combined into a single horizontal quantity, denoted as ‘Horiz.’
The median local accuracy is now included, as well as a hyperlink to a complete list of all network connections and their local accuracies.
The median distance between a station and stations directly connected to it has been added and is denoted as “Dist (km).”
Where applicable, the epoch of the orthometric height will be indicated.
Changes to the Superseded Survey Control Section:
The geoid model used to determine the superseded GPS-derived orthometric height will be shown. In order to provide space, the datasheet will no longer provide a meters-to-feet conversion.
Previous datasheet versions looked like this:
NAVD 88 (06/22/05) 1.96 (m) 6.4 (f) GPS OBS
The new version will appear as follows:
NAVD 88 (03/14/97) 1485.51 (m) GEOID96 model used GPS(epoch [if applicable])
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