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Surveyors Information - Geodetic Control Data

Geodetic Survey Control For Maryland

Information on permanently monumented control points including horizontal control, vertical control, High Accuracy Reference Network (HARN) stations, Calibration Baselines and retrieval software can be obtained from the National Geodetic Survey (NGS) web site here: https://www.ngs.noaa.gov/datasheets/index.shtml

To plot NGS published control points in Google Earth there is a free program, DSworld, available from the NGS User Contributed software download page. You can download kml files of control points by state, county, type, etc. link https://www.ngs.noaa.gov/PC_PROD/PARTNERS/index.shtml

Maryland High Accuracy Reference Network (HARN)

Maryland Harn 

The map above shows the location of (High Accuracy Reference Network), or  HARN, points in Maryland. The position of these points was determined by GPS observations and have a stated accuracy of 1.0 to 0.1 parts per million. These monuments were published by the National Geodetic Survey as part of the North American Datum (NAD) of 1983, 1991 adjustment. SHA currently uses these points and this horizontal datum for the basis of our existing projects.


Future Datums? The National geodetic Survey is preparing to publish a new Horizontal and a new Vertical Datum in the year 2022 to replace the North American Datum (NAD) of 1983 and the North American Vertical Datum (NAVD) of 1988. This will have an effect on the State Plane Coordinate System of Maryland as well as the heights, or elevations, that have been based on North American Vertical Datum (NAVD) of 1988. The system will be called the National Spatial Reference System of 2022 (NSRS 2022).

For more information on the new datum(s) click here: https://www.ngs.noaa.gov/datums/newdatums/index.shtml 


MDOT State Highway Administration control points


MDOT SHA uses Static Post-processed Global Navigation Satellite System (GNSS), formerly known as Global Positioning System (GPS), methods to establish survey control points for highway development and system preservation projects statewide. We use 1/2" rebar with caps typically for GPS control points. The horizontal coordinates are in the Maryland State Plane Coordinate System on the North American Datum of 1983, 1991 adjustment (NAD 83/91) and the vertical coordinate, if applicable, is on the North American Vertical datum of 1988 (NAVD 88), unless otherwise noted. There is an extensive network of points. They are available for use by the public surveying community. You can download a KML file to plot MDOT SHA GPS Control points in Google Earth, or other mapping products. The links to the images will not currently work, but you can contact us with a point name and general location and we'll be glad to provide the information on the points.

The file is available for download here: Click Here  (1.95 Mb)

Need additional information, please send email to MDOT SHA Control Points.


EDM Calibration Baselines in Maryland

Calibration Baselines in Maryland 

The map above shows the approximate location of the EDM (Electronic Distance Measuring system) calibration baselines located in Maryland. A calibration baseline consists of a set of very stable survey monuments which are accurately measured using specialized equipment. By comparing their results with the values published for the monuments, surveyors are able to verify the accuracy of their equipment.

Please note that the Appalachian Chapter of the Maryland Society of Surveyors installed a new Calibration Baseline in Hancock Maryland and it has been measured and published.

*Calibration Baselines in Aberdeen and Hagerstown have been destroyed by construction. One or two of the marks in each may remain, but they are unsuitable for Calibration purposes anymore.

Name Location Latitude Longitude
Cambridge CBL Dorchester County 38 35 76 08
Hancock CBL Washington County 39 43 77 44
Howard County CBL Howard County 39 15 76 56
Leonardtown CBL St. Mary's County 38 19 76 33
Pittsville CBL St. Wicomico County 38 23 75 23


The Calibration Baseline data is available here: https://www.ngs.noaa.gov/CBLINES/BASELINES/md