Mapping data is available from St. Mary’s County DW&T offices for a fee of $200. A package of 1 DVD and 3 CD’s is provided, with AutoCAD files of the 1993 and 1998 5’ topographic contours in AutoCAD format, as well as the 2004 2’ topography described below. This data is provided in the Maryland State Plane NAD 83 Feet horizontal coordinate system. The vertical datum is NGVD 29 for 1993 maps, and NAVD 88 for later maps.
Other County mapping.
In November 1992, the Department set the first 130 precise Global Positioning Stations (GPS) using numbered brass caps embedded in concrete monuments in accordance with the National Geodetic Survey (NGS) Blue-Booking Standards. Through subsequent densification the County currently has 283 monuments. It is this Department’s objective to continue with GPS network densification until all new subdivision and site plan surveys can feasibly tie into same and form a County-wide network. A listing of GPS points and their respective locations is available to the public at no cost.
GIS Diagram Accuracy and Control
Horizontal control for the GPS network is the North American Datum of 1983 (NAD 83) and vertical control is NGVD 1929. A tabulation of latitudes, longitudes, northings, eastings, reach descriptions and elevations (expressed in both meters and in feet) is available for distribution. GPS specifications require first order accuracy of 10 parts per million, which is 1:100,000 for horizontal control. Vertical elevations were maintained to 0.05 foot between points and 0.1 foot throughout the County-wide project area.
Digitized topography is available at a scale of 1”= 200’ with five (5) foot contour intervals on 24” x 36” sheets for $325 per sheet. Individual grid sheets are
available at $20 per grid. Planimetric (without contours) data may be obtained at no cost. The information on these sheets is not to be used in place of field run topography for subdivision, site plan or roadway design. Horizontal control of the planimetric maps is accurate to a 40th of an inch, which meets the fourth edition of the National Map Accuracy Standards. Vertical control is within a half of a contour interval or 2 ½ feet. The low altitude topographic information was used by the Emergency Management Agency for the completion of the Emergency 911 Uniform Addressing Program. Updated topographic information will be available as funds permit with emphasis placed on the development districts.
On March 27, 1998, analog aerial photography was obtained for low altitude (7,200 feet at a nominal scale of 1:14,400) and high altitude (20,000 feet at a nominal scale of 1:40,000). A digital elevation model was collected using an optical stereo-plotter and analytical triangulation performed to ensure image accuracy. Over 472 low altitude black and white photographs at a scale of 1”=1200’ are available for perusal at the Department of Public Works and Transportation. Digital Aerial Ortho-Photography is available in ArcMap format from the Department of Information Technology for the years 1998, 2003, and 2007.
This phase of the mapping is performed after all aerial photography and ground control is received and checked for completeness. All control points are located and symbolized on their respective photo contact prints. Certain points (pass points and tie points) are then accurately marked on the diapositives (using a WILD PUG4 Point Transfer Instrument) and recorded using a 60 micron drill bit which removes a portion of the emulsion from the diapositive. The points are measures using first order analytical stereo plotters and checked to ensure the required tolerance limits have been met. A final “bundle block” adjustment is applied which incorporates the surveyed ground control, the camera calibration data and photogrammetric measurements made on the stereo plotters. This final step ensures proper accuracy of the data.
In April 2004, under contract to the Maryland Department of natural Resources, Sanborn Map Company conducted a LIDAR overflight of St. Mary’s County at an elevation of 4500’, with a 498 meter flight line spacing and 50% overlap. An average of one sample was collected every 1.3 sq. meters. The horizontal position is accurate to 1 meter, and the vertical accuracy to 9.8cm for 95% of all points collected. This data was post-processed by Spatial Systems, and St. Mary’s County subsequently funded conversion to 2’ contour elevations maps in both ArcMap and AutoCAD formats.
Light Detection and Ranging (LIDAR) is a method of locating objects on the ground using aerial-borne equipment. It is similar to RADAR or SONAR in that the two-way travel time of an energy beam reflected off an object is precisely measured, but this technology uses laser light instead of radio or sound waves. This technology has proven very useful in remote sensing of the earth. It can be used for determining elevations of both the earth's surface and items (natural and man-made) on the surface. Analysis of LIDAR data is used in detailed modeling of the earth's surface for drainage and floodplain studies, determining how a new structure will affect views from various locations, shoreline erosion studies, and other reasons. This data set contains only the bare earth mass points. Bare earth points represent ground features. Features that are above ground, such as buildings, bridges, tree tops, etc, have been eliminated. Mass points may appear to have "holes" with no elevation values in areas where the surface could not be determined, such as very dense forests or urban areas.
The LIDAR-derived data were collected by the Maryland Department of Natural Resources in support of shore erosion studies along the shorelines of the Chesapeake Bay. It also supports the Federal Emergency Management Agency's specifications for mapping floodplains. These efforts required detailed elevation data and models, such as those available from LIDAR sensing. The data have also been made available to aid in other projects that require detailed surface, vegetation and/or structure elevations.
These data are part of a suite of spatial data products generated using the LIDAR technology. First and Last returns are generated. From these data, Bare-Earth Mass Points that represent the earths surface are produced. A gridded Digital Elevation Model (DEM) is produced from the Bare Earth Mass Points. Imagery is also created that represents the intensity of the LIDAR return; it is a 1-meter resolution product that resembles a panchromatic Digital Orthophoto Quad (DOQ). The Anne Arundel/Howard Counties and Charles / St. Mary’s Counties, Maryland QA Reports may be viewed at:
Road Inventory Efforts
In order to comply with Government Accounting Standards Board (GASB) and National Pollution Discharge Elimination System (NPDES) requirements, and to assist Public Works staff in inventorying and maintaining its facilities, GIS mapping has been developed for the following features: Traffic Signs, Guardrail, Curb and Gutter, Sidewalk, Pavement Condition, and Storm Drain and Stormwater Management structures. Through a private vendor, an imaging software system called Road View was developed, and each public road driven and photographed. The software tracks the location of the camera on the roadway, and four images are displayed (front, back, left and right road edges) at regular intervals along the roadway. A GIS map displays the location of the camera on the road network. Each asset is located with a symbol on the GIS map, with a clickable link to the database which describes the asset. Staff will be updating these databases as an ongoing effort. Although not available for public viewing at this time, this roadway inventory will directly benefit the general public through more efficient tracking and maintenance of its road network.
Screen Shot of GIS Road View Software Showing Typical County Road
Contact the Department of Public Works & Transportation at (301) 863-8400 for information on ordering photographic reproductions, enlargements, or topographic/planimetric products. Please refer to our County Mapping Products for additional information a copy of our
County Mapping Products Order Form.
Additional GIS data products.
The first GPS satellite was launched in 1978.
The current system is composed of second generation GPS satellites, called Block II.
The first Bloch II satellite was launched in 1989.
The Defense Department declared GPS fully operational in 1995.
When the system was first introduced, miscalculations were programmed into GPS transmissions to limit the accuracy of non-military GPS receivers. This operation was cancelled in May 2000.
There are 24 GPS satellites in orbit at this moment.
The 24 satellites cost an estimated $12 billion to build and launch.
Each satellite weighs about 1,735 lb. (787 kg).
The satellites are in orbit about 11,000 mi. (17,703 km) above the Earth.
A satellite takes 12 hours to orbit the Earth once.
The Russians have a system identical to the U.S. system called GLONASS. It is not yet fully operational.
How GPS Works
Global Positioning System (GPS) consists of 24 earth-orbiting satellites. These satellites allow any person who owns a GPS receiver to determine his or her precise longitude, latitude and altitude anywhere on the planet. For as little as $100, you can know exactly where you are and where you have been. For anyone who has ever been lost -- while hiking in the woods, boating in the ocean, driving in an unfamiliar city or flying a small airplane at night -- a GPS receiver is a miracle. When you use GPS receiver, you're never lost!