What is a Benchmark?
A benchmark is geographic point whose coordinates and/or elevation has been measured and recorded to a high level of accuracy. Often benchmarks are often denoted by a metal disk permanently affixed to a solid and stable surface or less frequently by an existing structure such as a church spire or communication tower. Benchmarks are used by surveyors, engineers, map makers, and other professionals who need a highly accurate positional reference for their work. Recently, locating benchmarks has become a popular pastime for the geocaching community.
XMap Openspace Datasets
The benchmarks data includes both horizontal and vertical benchmarks in XMap OpenSpace format, which can be imported into any version of XMap 5 or newer. These OpenSpace files include a complete set of geodetic control stations for each state and are maintained by the National Geodetic Survey (NGS). Each geodetic control station has either a precise latitude/longitude used for horizontal control or a precise orthometric height used for vertical control, or both. The data was derived from the NGS benchmark archive files and thus may not contain newer benchmarks. Recent benchmark additions can be downloaded directly from the NGS Website and appended to the appropriate XMap Openspace layer using the Import… Append function of XMap GIS Editor or XMap GIS Enterprise.
The Benchmarks layer has been classified and symbolized to distinguish horizontal and vertical benchmarks. Additionally, a unique symbol has been assigned to benchmarks which are considered both horizontal and vertical control points. To see an explanation of the symbology in XMap, enable the legend from the Tools menu in the Workspace subtab.
Each layer includes four pre-defined queries that can be run from within the Attribute subtab of XMap: Search by County, Search by ID number, Search by Benchmark name, and Search by USGS Quad. The results of these queries will be highlighted on the map and in the attribute table.
The benchmark data is configured to be visible between data zoom levels 9 and 20. If you zoom the map out beyond level 9, you will not see the benchmarks on the map. Users of XMap GIS Editor and XMap GIS Enterprise can adjust this setting using the layer properties settings. Each benchmark is labeled with its unique ID number. This label also acts as a hyperlink offering direct access to the full datasheet for each benchmark on the NGS Website.
How to Download
Use the following steps to download the benchmark data:
Right-click the "Download Benchmarks" link and select Save Target As... or Save Link As… from the shortcut menu.
Browse to the location where you want to save the compressed file to your hard drive and then click Save. The downloaded file will have a .zip suffix.
After unzipping the file, you will see a folder that contains a complete set of benchmark layers for every state. Also included is a text file that provides instructions on importing the data into XMap, as well as a list of all of the included attributes.
The Openspace benchmark layers include the following attributes:
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. This URL can be accessed by clicking the appropriate label on the map.
DEC_LONG - Decimal equivalent of the LONGITUDE value
DEC_LAT - Decimal equivalent of the LATITUDE value
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 place).
» SCALED = Scaled from a topographic map. (Lat, Lon rounded to 0 decimal places).
ELEVATION - Present if available. The orthometric height in METERS indicating 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 benchmark (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 benchmark (rounded to 1 decimal place).
» GPS OBS = Computed from GPS observations (rounded to 1 decimal place).
» 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 - The ellipsoid height in METERS referenced to GRS80 ellipsoid. Present if available.
ELLIP_SRCE - Ellipsoid Height 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 position.
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.
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.
DIST_RATE - Distribution rate for POSTED and READJUSTED benchmarks which do not have an order and class.
ELLP_ORDER - Order of accuracy for the ELLIP_HT. Should be 1,2,3,4, or 5 if present.
ELLP_CLASS - Class of accuracy for ELLIP_HT Should be 1 or 2 if present.
FIRST_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, FIRST OBSERVED, GOOD, POOR, MARK NOT FOUND, SEE DESCRIPTION, DESTROYED.
LAST_RECBY - Agency which reported the last condition of the mark.
STABILITY - The stability of the mark. May have one of the following four codes: A = Most reliable and expected to hold position/elevation well, B = Will probably hold position/elevation well, C = May hold position, but of type commonly subject to surface motion, e.g. frost heave, etc., D = Mark of questionable or unknown stability.
Benchmark Type – Designation of benchmark as Horizontal, Vertical, Horizontal and Vertical, or Unknown
URL – Link used on access to online datasheet.
Importing into XMap
Follow these steps to import Benchmarks into XMap:
Select the GIS tab and click the Workspace subtab button to the left of the tab area
Click the Layers button above the Workspace table, click Import and select New Layer to open the Import Layer wizard.
Browse to the location where the Benchmark file (.openspace) was saved, select the file, and click Open*.
Select a database from the Database drop-down list in the Import Layer window and click the Next button. The layer import will begin begins.
*To bypass the first three steps of the import process, drag the OpenSpace Transfer file into the map window in XMap.
Searching for Benchmarks in XMap
To Search for a particular benchmark, follow these steps:
Select the Attributes subtab button to the left of the tab area.
Click the Query drop-down list, and choose the appropriate query:
Search by County
Search by ID number
Search by Benchmark name
Search by USGS Quad.
Click the Run button to open the Run Query dialog box.
Enter the necessary information and click Run. The results of the query are highlighted on the map and in the attributes table.
To cancel the query and revert the selected points to their default symbols, choose Cancel Query from the same drop-down list.
Editing the Data
Users of XMap Professional cannot change the symbology of the benchmark points, the style of the label, or any of the other characteristics of the layer; however, users of either the GIS Editor or Enterprise versions of XMap can modify the layer in numerous ways. For instance, the symbol style can be changed, the labels can be removed or edited, additional queries can be built, and selected areas within the layer can be highlighted and used to create a new layer. For details on how to perform any of these functions, refer to the Help system in XMap.