11/20/2023 0 Comments Ngs benchmark meaning of condition s![]() NGS data sheets also provide State Plane and Universal Transverse Mercator (UTM) coordinates, the latter only for horizontal control stations. But suffice it to say for the objective here, the NGS data sheet is helpful to learn what control is available in a particular area. Of course, the first retrieval date at the top of the sheet is valuable to know when the point was last recovered. When the data sheet is retrieved online, one can use the link provided to bring them up. Photographs of the station may also be available in some cases. The ellipsoid height per the NAD83 ellipsoid is shown followed by the geoid height where the position is covered by NGS’s GEOID program. It is important to note that NGS uses a clockwise rotation regarding the Laplace correction. These values are followed by the quantity which, when added to an astronomic azimuth, yields a geodetic azimuth, is known as the Laplace correction. They are the same type of X, Y, and Z coordinates presented earlier. ![]() These are right-handed system, 3-D Cartesian coordinates and are computed from the position and the ellipsoidal height. When they are available, earth-centered earth-fixed (ECEF) coordinates are shown in X,Y and Z. And they have no decimal places if they were scaled from topographic map, Scaled, or found by conversion from NGVD29 values using the program known as VERTCON. Stations’ vertical values are given to 1 decimal place if they are from GPS/GNSS observation (Obs) or vertical angle measurements (Vert Ang). When a station’s elevation has been found by precise leveling but non-rigorous adjustment, it is called Computed. Another category is Posted, which indicates that the station was adjusted after the general NAVD adjustment in 1991. There is Adjusted, which are given to 3 decimal places and are derived from least squares adjustment of precise leveling. There are 13 sources of vertical control values shown on NGS data sheets. When a date is shown in parentheses after NAD83 in the data sheet, it means that the position has been readjusted. Horizontal values may be either Scaled, if the station is a benchmark or Adjusted, if the station is indeed a horizontal control point. However, these old values may be shown under Superseded Survey Control. Adjustments to NAD27 and NGVD29 datums are a thing of the past. When the height is derived from GPS/GNSS observation, a geoid model must be used to determine the orthometric height. The orthometric height in meters is listed as “ORTHO HEIGHT” and followed by the same in feet. Under the heading, “Current Survey Control,” you will find the latitude and longitude of the station in NAD83 which is fixed to the North American plate, currently in NAD83 (2011), and its height in NAVD88. Even though the station is located in the area covered by the quad sheet, it may not actually appear in the map. Geological Survey (USGS)-7.5 minute quad name follows. The PID is also found all along the left side of each data sheet record and is always two upper case letters followed by four numbers. Either of these may be used to search for the station in the NGS database. This is followed by the station’s designation, which is its name, and its Point Identification, PID. There are several, and among them are Continuously Operating Reference Station, Federal Base Network Control Station, and Cooperative Base Network Control Station. Then the station’s category is indicated. The first line of each data sheet includes the retrieval date. ![]() In addition to the latitude and longitude, the published data include the state plane coordinates in the appropriate zones. There is a good deal of information about the passive survey monuments on each individual sheet. That utilization should be informed by an understanding of the datasheet that accompanies each station and is easily available online. They can provide reliable control when properly utilized. Monuments that are the physical manifestation of the National Spatial Reference System (NSRS) and can be occupied with survey equipment are known as passive marks. These sheets have a great deal of excellent information concerning the coordinates, the quality, and many of the details about existing control points that can be used as checkpoints in any GPS/GNSS survey. You can retrieve sheets that are similar to what you see in the illustration. Brass tablets and brass caps have been set around the United States by the National Geodetic Survey also known as passive marks. ![]() One of the things that is very useful is understanding the existing control that is currently available. ![]()
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