Howard County Geodetic Network

 

Description

                    Currently the network consist of 745 stations divided in two sub-networks: A

Horizontal Geodetic Network , contains  453 stations and provides both horizontal and vertical information, and a Vertical Network with 292 stations and provides only vertical information .

                    The Horizontal Network maintains the horizontal coordinates (Northing, Easing,

Latitude, Longitude, Scale Factors , value of the Convergence of Meridians and Ellipsoidal Height) in Maryland State coordinates in both NAD27 and NAD83 Datums, the elevations also are provided in reference to NAVD88 and NGVD29.

                   The Vertical Network (Benchmarks) maintains only the vertical information (elevations) of its stations in both vertical datums.

                   Physically each station is usually a brass or aluminum disc , stamped with the Howard County Survey Division, the name of the station and a stamped cross or  other mark to indicate the exact geometric point represented by the station. These disks are embedded atop of a three feet deep column of concrete, buried in the ground so the disk ends up about one inch below the ground surface. Some stations set by other institutions, but incorporated to our system may be slightly different, but in order to be included they must have minimum characteristics of stability and durability.

                  The Coordinate information is obtained after a combination of G.P.S. and traditional survey observations (Digital leveling) and rigorous office computations and adjustments based in the standards and guidelines for Geodetic Control Networks provided buy the National Geodetic Survey (NGS).

                 The accuracy of the Howard County Network is:

                 Uncertainty in the Horizontal positions : about 1cm (First Class)

                 Uncertainty in the Elevations : about 2.5cm (2^nd Class)

                There is no difference in the vertical accuracies of the elevations provided by either sub-network.

                 All the information is kept in a CPU in an ACCESS database maintained by The Howard County Survey Division.

    GENESIS:

                    Howard County started its first geodetic network in 1971 by subcontracting the build up to a local Engineering company: Greenhorne and Ohmara . The job was performed by areas (phases) and took about five years and finished with a little more than one thousand station. The information about these stations is kept in the office of the Survey Division, but the network is not maintained anymore. Currently more than half of these stations have been destroyed or paved over.

                 By 1989, as the state of Maryland would change its official system of coordinates in 1992 from NAD27 to NAD83 datums, the County was facing  the dilemma of performing a great number of new observation and a gigantic transformation of coordinates in order to adapt the old network to the new system, or using the new GPS techniques and profiting of the availability of the at the time new national High Accuracy Reference Network (HARN),

start from scratch and build a new network already in the new NAD83 coordinates.

                Between 1990 and  mid 1992 the County selected  42 stations well distributed  over the area of the County, and with them created an initial network using pure GPS observation.

This network was reported to NGS for inclusion in the National Survey Reference System (bluebooking). The network was accepted by NGS and all the stations were included in the national database with a First Class classification.

    Using that initial network as frame, the County has continued building up by continuously replacing lost stations and setting new ones. In the mid nineties the system was improved by combining it with a network of digital levels covering the whole area. In the year 2000 the frame of bluebooked stations was renovated and expanded by re-observing the original stations with a batch of new ones to a total of sixty one bluebooked stations at the present times.

   The whole network is continually readjusted in order to incorporate ne stations and observation.

How to Obtain information

 

  The information about each station is published in a sheet (recovery card) which displays all

the previously mentioned information plus a rough sketch of the location of the station and indications how to find it.

   These cards  can be obtained at the office of the Howard County Survey Division at::

        9250 Bendix Road

       Columbia , Md 21045

 

   By phone to : Division Chief : Keff Hobbs (410)-313-6120

                         Geomatic Engineer: Ralph Mercado (410)-313-6103

                        Division Secretary : Arline Bennett (410)-313-6119

   By fax: (410)-313-6106

   e-mail: rmercado@co.ho.md.us

 

   The Survey Division also publishes an index map of the County displaying the location of all the stations.

    The GIS