The National Geodetic Survey (NGS) serves as the Nation's depository for geodetic data. The NGS distributes geodetic data worldwide to a variety of users. These geodetic data include the final results of geodetic surveys, software programs to format, compute, verify, and adjust original survey observations or to convert values from one geodetic datum to another, and publications that describe how to obtain and use Geodetic Data products and services.
Lesser quality Latitude, Longitudes may also be preset in the dataset. These are identified by a POS_SRCE attributes HD_HELD1, HD_HELD2, or SCALED. These lesser quality positions are described at: <http://www.ngs.noaa.gov/cgi-bin/ds_lookup.prl?Item=SCALED>
Vertical control stations (those with precise Orthometric Heights) were established in accordance with FGDC publications "Standards and Specifications for Geodetic Accuracy Standards" The final Orthometric Height of these stations were in most cases determined by a least squares adjustments of the vertical observations but in some cases may have been keyed from old survey documents. Vertical control stations have Orthometric Heights displayed to 2 or 3 places and are identified by attribute ELEV_SRCE of ADJUSTED, ADJ UNCH, POSTED,READJUST,N HEIGHT,RESET,COMPUTED
Lesser quality Orthometric Heights may also be preset in the dataset. These are identified by a POS_SRCE attributes GPS_OBS, VERT_ANG, H_LEVEL, VERTCON, SCALED. These lesser quality orthometric heights are described at: <http://www.ngs.noaa.gov/cgi-bin/ds_lookup.prl?Item=SCALED>
IMPORTANT - Control stations do not always have both precise Latitude, Longitude AND precise Orthometric Height. A horizontal control station may have a orthometric height associated with it which is of non geodetic quality. These types of heights are displayed to 0, 1, or 2 decimal places. Worst case being off by +/- 1 meter. LIKEWISE - A Vertical control station may have a Latitude, Longitude associated with it which is of non geodetic quality. These types of Latitude, Longitudes are displayed to 0, 1 or 2 decimal places. Worst case being off by +/- 180 meter. Refer to <http://www.ngs.noaa.gov/cgi-bin/ds_lookup.prl?Item=SCALED> for a description of the various type of methods used in determining the Latitude, Longitude, and Orthometric Height.
Attribute POS_CHECK and ELEV_CHECK indicate whether or not an observational check was made to the position and/or orthometric height. Care should be taken when using "No Check" coordinates.
If attribure ELEV_SRCE = 'VERTCON' then the Orthometric Height was determined by applying NGS program VERTCON to an Old NGVD 29 height. In most areas VERTCON gives results to +/- 2 cm. See <http://www.ngs.noaa.gov/TOOLS/Vertcon/vertcon.html> for a more detailed explaination of VERTCON accuracy.
Ellipsoid Heights are also present in the dataset. The ellipsoid heights consist of those determined using a precise geoid model, which are displayed to 2 decimal places and are considered good to +/- .005 meters, and those displayed to 1 decimal place and are considered only good to +/- .5 meters
Horizontal positional accuracy statements pertain to horizontal control stations only. i.e. Those with geodetic quality Latitude, Longitudes. Positional Accuracy explanation contained in "Geometric Geodetic Accuracy Standards and Specifications For Using GPS Relative Positioning Techniques," FGCS (formally FGCC) publication version 5.0, 1989, (See table 1, p6). with the exception of Order A and Order B which have an accuracy of as described at: <http://www.ngs.noaa.gov/cgi-bin/ds_lookup.prl?Item=HORZ%20ORDER>
Vertical positional accuracy statements pertain to vertical control stations only. i.e. Those with geodetic quality Orthometric Heights. Orthometric Height accuracy explanation is contained in FGDC publication "Standards and Specifications for Geodetic Control Networks" with the exception of vertical control of class 0 POSTED and READJUSTED Heights which are described at: <http://www.ngs.noaa.gov/cgi-bin/ds_lookup.prl?Item=VERT%20ORDER>
Ellipsoid height accuracy explanation contained in "Geometric Geodetic Accuracy Standards and Specifications for Using GPS Relative Positioning Techniques," FGCS (formally FGCC) publication version 5.0, 1989. (see table 1, p6). The accuracies for ellipsoid heights are described at: <http://www.ngs.noaa.gov/cgi-bin/ds_lookup.prl?Item=ELLP%20ORDER>
The NGS' final product is the geodetic data sheet. Geodetic data sheets are comprehensive summaries of all published information for a given geodetic reference point, including: the geographic position and/or height based on the current reference datum, condition of the survey mark when it was last visited, a description of where the point is located and how to reach it, and an explanation of the terms used in the data sheet. In support of these geodetic data, the NGS provides software, publications, and various user services, including geodetic advisor program, instrument calibration, surveying standards, and technical workshops.
This dataset contains certain information extracted from the above mentioned data sheet.
Geodetic Data - vertical positional datum conversion, use program VERTCON (version 2.0)
These programs provide indirect spatial reference data and are available from NOAA, National Geodetic Survey at <http://www.ngs.noaa.gov/PC_PROD/pc_prod.shtml>
The current attributes and their meaning are shown below.
#FeatureId Temporary unique ID assigned to this station.
DATA_DATE- The date when this information was retrieved from the NGS database.
DATA_SRCE- Data Source where the information for the mark came from. You should use this link to obtain a full datasheet for the mark or obtain the datasheets from <http://www.ngs.noaa.gov/cgi-bin/datasheet.prl> if you intend to use the data for survey control.
DEC_LONG- Decimal equivalent of the LONGITUDE Always displayed to 10 decimal places, but you should see POS_SRCE and POS_ORDER to determine the true accuracy.
DEC_LAT- Decimal equivalent of the LATITUDE
PID- Permanent Identifier assigned by NGS to each mark
NAME- Station Name (a.k.a. Designation)
STATE- State Code
COUNTY- County Name
QUAD- USGS Topographic Quad Map Name
LATITUDE- Latitude in Deg-Min-Sec format
LONGITUDE- Longitude in Deg-Min-Sec format
POS_DATUM- Datum of the LATITUDE,LONGITUDE Should always be NAD83
DATUM_TAG- Datum Tag of the LATITUDE,LONGITUDE 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.
POS_SRCE- Position Source for the LATITUDE,LONGITUDE
ADJUSTED = Least squares adjustment. (Lat,Lon Rounded to 5 decimal places.)
HD_HELD1 = Differentially corrected hand held GPS observations. (Lat,Lon Rounded to 2 decimal places.)
HD_HELD2 = Autonomous hand held GPS observations. (Lat,Lon Rounded to 1 decimal places.)
SCALED = Scaled from a topographic map. (Lat,Lon Rounded to 0 decimal places.) ELEVATION- Present if available. The Orthometric Height in METERS inicating the height above the Geoid.
ELEV_DATUM- Datum of the ELEVATION
ELEV_SRCE- Elevation Source for the ELEVATION
ADJUSTED = Direct Digital Output from Least Squares Adjustment of Precise Leveling. (Rounded to 3 decimal places.)
ADJ UNCH = Manually Entered (and NOT verified) Output of Least Squares Adjustment of Precise Leveling. (Rounded to 3 decimal places.)
POSTED = Pre-1991 Precise Leveling Adjusted to the NAVD 88 Network After Completion of the NAVD 88 General Adjustment of 1991. (Rounded to 3 decimal places.)
READJUST = Precise Leveling Readjusted as Required by Crustal Motion or Other Cause. (Rounded to 2 decimal places.)
N HEIGHT = Computed from Precise Leveling Connected at Only One Published Bench Mark. (Rounded to 2 decimal places.)
RESET = Reset Computation of Precise Leveling. (Rounded to 2 decimal places.)
COMPUTED = Computed from Precise Leveling Using Non-rigorous Adjustment Technique. (Rounded to 2 decimal places.)
LEVELING = Precise Leveling Performed by Horizontal Field Party. (Rounded to 2 decimal places.)
H LEVEL = Level between control points not connected to bench mark. (Rounded to 1 decimal places.)
GPS OBS = Computed from GPS Observations. (Rounded to 1 decimal places.)
VERT ANG = Computed from Vertical Angle Observations. (Rounded to 1 decimal place; If No Check, to 0 decimal places.)
SCALED = Scaled from a Topographic Map. (Rounded to 0 decimal places.)
U HEIGHT = Unvalidated height from precise leveling connected at only one NSRS point. (Rounded to 2 decimal places.)
VERTCON = The NAVD 88 height was computed by applying the VERTCON shift value to the NGVD 29 height. (Rounded to 0 decimal places.)
ELLIP_HT- Present if available. The ellipsoid height in METERS referenced to GRS80 ellipsoid.
ELLIP_SRCE- Ellipsoid Ht Source for the ELLIP_HT Should always be GPS_OBS when present.
POS_ORDER- Order of accuracy for the LATITUDE,LONGITUDE Should be one of the following- A,B,1,2,3
Order and class for Orders 1, 2, and 3 are defined in the Federal Geodetic Control Committee publication "Standards and Specifications for Geodetic Control Networks".
In addition- Order A stations have a relative accuracy of 5 mm +/- 1-10,000,000 relative to other A-order stations.
Order B stations have a relative accuracy of 8 mm +/- 1- 1,000,000 relative to other A- and B-order stations.
POS_CHECK- Y=Observational Check was made for the position, N=NO Observational Check was made for the positions ELEV_ORDER- Order of accuracy for the ELEVATION Should be 1,2, or 3 for Vertical Control Stations. Will be blank for stations used for Horizontal Control only.
Also see attribute DIST_RATE which is used for some vertical control stations.
Elevation order and class for 1, 2, and 3 are defined in the Federal Geodetic Control Committee publication "Standards and Specifications for Geodetic Control Networks". In addition-
Vertical control which were determined only for the purpose of supplying a height for Horizontal Distance Reductions are assigned an order of 3.
Class 0 is used for special cases of orthometric vertical control as follows-
Vertical Order/Class Tolerance Factor -------------------- ---------------- 1 class 0 2.0 mm or less 2 class 0 8.4 mm or less 3 class 0 12.0 mm or less
ELEV_CLASS- Should be 0, 1, or 2 See details under ELEV_ORDER
ELEV_CHECK- Y=Observational Check was made for the orthometric height, N=NO Observational Check was made for the orthometric heights VERTCONED- Y=Orthometric Height was determined by applying VERTCON to an old NGVD 29 height. N=Orthometric Height determined by observations. DIST_RATE- Distribution rate for POSTED and READJUSTED benchmarks which do not have an Order and Class are as follows "Posted bench marks" are vertical control points in the NGS data base which were excluded from the NAVD 88 general adjustment. Some of the bench marks were excluded due to large adjustment residuals, possibly caused by vertical movement of the bench marks during the time interval between different leveling epochs. Adjusted NAVD 88 are computed for posted bench marks by supplemental adjustments.
A range of mean distribution rate corrections is listed for each posted bench mark in the data portion of the publication. A summary table of the mean distribution rates and their codes is listed below. The mean distribution rate corrections which were applied to the original leveling observations is a good indication of the usefulness of the posted bench marks' adjusted NAVD 88 heights.
Distribution Distribution Rate Code Rate Correction ------------ --------------- "a" 0.0 thru 1.0 mm/km "b" 1.1 thru 2.0 " "c" 2.1 thru 3.0 " "d" 3.1 thru 4.0 " "e" 4.1 thru 8.0 " "f" greater than 8.0 mm/km
POSTED BENCH MARKS SHOULD BE USED WITH CAUTION. As is the case for all leveling projects, the manditory FGCS check leveling two-mark or three-mark tie procedure will usually detect any isolated movement (or other problem) at an individual bench mark. Of course, regional movement affecting all the marks equally is not detected by the two- or three-mark tie procedure.
ELLP_ORDER- Order of accuracy for the ELLIP_HT Should be 1,2,3,4, or 5 if present.
The following ellipsoid height order and class relative accuracy standards have not yet been adopted by the Federal Geodetic Control Subcommittee, but are currently in use by NGS-
Ellipsoid Height Maximum Height Classification Difference Accuracy ------------------ ------------------- 1 class 1 0.5 (mm)/sqrt(km) 1 class 2 0.7 2 class 1 1.0 2 class 2 1.3 3 class 1 2.0 3 class 2 3.0 4 class 1 6.0 4 class 2 15.0 5 class 1 30.0 5 class 2 60.0
The ellipsoid height difference accuracy (b) is computed from a a minimally constrained correctly weighted least squares adjustment by- b = s / sqrt(d)
where b = height difference accuracy s = propagated standard deviation of ellipsoid height difference in millimeters between control points obtained from the least squares adjustment. d = horizontal distance between control points in kilometers
ELLP_class- Class of accuracy for ELLIP_HT Should be 1 or 2 See details under ELLP_ORDER
1_RECV- Date when the station was first monumented or in the case of landmarks, first observed.
LAST_RECV- Date when the station was last recovered.
LAST_COND- Last recovered condition of the mark. Should be one of the following- MONUMENTED 1 OBSERVED GOOD POOR MARK NOT FOUND SEE DESCRIPTION DESTROYED
LAST_RECBY- Agency who reported the last condition of the mark.
STABILITY- The stability of the mark may have 1 of 4 codes as indicated below- A = MOST RELIABLE AND EXPECTED TO HOLD POSITION/ELEVATION WELL B = PROBABLY HOLD POSITION/ELEVATION WELL C = MAY HOLD, BUT OF TYPE COMMONLY SUBJECT TO SURFACE MOTION - E.G. FROST HEAVE, ETC D = MARK OF QUESTIONABLE OR UNKNOWN STABILITY