How to Order GIS Data Products

from The Maryland-National Capital Park & Planning Commission
Information & Publications
8787 Georgia Avenue
Silver Spring, Maryland 20910

BACKGROUND

The implementation of a geographic information system (GIS) permits The Maryland–National Capital Park and Planning Commission (M–NCPPC) to make computer–generated map products available for public use. GIS data products are ideal for a variety of land planning applications engaged in by engineers, surveyors, developers, civic associations, and private citizens. This policy grants specific limited usage rights (not ownership or intellectual property rights) to customers in consideration for nominal fees. Customers purchase these rights for use on a specific project, subject to the conditions of the GIS Data Products License Agreement, which customers are required to execute at the time products are ordered.

This document details the composition of the base maps and provides conditions and precautions for their use. A GIS Data Products Order Form and a GIS Data Products License Agreement must be completed to obtain GIS data. Before ordering any GIS data, please read this entire document. Questions should be referred to Information and Publications, (301) 495–4610.

Base Maps
The County land base is comprised of over 660 base map tiles, each covering a 4000’ by 6000’ area, the perimeter of which is based on the Washington Suburban Sanitary Commission (WSSC) grid tile layout (the WSSC grid tile layout is depicted on Alexandria Drafting Company (ADC) map books in red, along with grid designations on the page margins). Base maps are referenced to the Maryland State Plane Coordinate System NAD83/91 (HPGN) feet grid.

Digital copies are available in either DXF, Arc Export (v. 7.1), or ArcView Shape File (v. 3.0) formats. File sizes range from approximately 50 kilobytes to over 5 megabytes, depending on the type of data requested, and the location.

Planimetric/Topographic Base Maps
Planimetric/topographic maps were compiled at a scale of 1:2,400 (1"=200’) with a contour interval of 5 feet. They were prepared using 1:14,400 scale aerial photographs captured from a flight altitude of 1,200’. Topographic information shown was compiled by stereo photogrammetric methods. Contouring was generated by stream digitizing.

Basically, any permanent physical features visible from above appear on the maps. This would include buildings, roads, water bodies, wooded areas, railroads, parking lots, retaining walls, and so forth. Topography is indicated at 5’ contour intervals, with spot elevations shown at grade breaks.

The details of the file structure of the planimetric/topographic base map can be found in Planimetric/Topographic Map Composition.

Property Base Maps
These maps depict the approximate locations of property boundary lines including street right of way, subdivision lot, and deed parcel boundaries. Property annotation such as street names, lot and block numbers, subdivision names, and parcel identifiers is included. The maps were compiled by digitizing the tax maps of the Maryland State Department of Assessment and Taxation. The boundary linework was then "occupationally adjusted" to best fit roads, buildings, and fence lines shown on the planimetric/topographic base maps.

Property base maps are updated on a continual basis. As each new deed or subdivision goes on record, the corresponding property base map is updated. The date of each map’s latest update appears in the revision table marked ‘PROPERTY’ located in the title block.

The details of the file structure of property base map can be found in Property Map Composition.

USE OF THE DATA

Ownership
County GIS data products are copyrighted, proprietary information, under the ownership of M–NCPPC. Customers acquire no ownership rights through this policy. Rather, they are granted limited license to use M–NCPPC GIS data for specific purposes authorized in the Permitted Uses section of the License Agreement. M–NCPPC GIS data may not be sold or given to others (except to Authorized Operators as identified in the License Agreement), and may not be incorporated (in whole or part) into other maps, brochures, or documents (except as specified in this policy), without separate express written permission from M–NCPPC.

Customers intending to incorporate any M–NCPPC GIS data into their plans (whether by hand tracing, scanning, or importing of digital data) must place a statement of the following form in a conspicuous location on the face of such documents.

The customer should tailor the note slightly so as to reflect any of the GIS base map data used, and may insert the phrase ‘in part’ between the words ‘based’ and ‘on’, if applicable.

Accuracy
M–NCPPC makes no guarantees as to the accuracy and completeness of the GIS data. The variety of sources and vast size of the database makes it very difficult to quantify the accuracy of the maps. A planimetric/topographic verification project using GPS (global positioning system) technology was conducted (1997) to test the positions of over 1,400 random points selected from some 43 map tiles. Although the results concluded that the maps meet National Map Accuracy Standards, this project involved a relatively small sampling of the millions of features shown throughout the County, and thus, provides only a general confirmation of the accuracy of the planimetric/topographic base.

As such, the following statements are made:

- Planimetric/topographic maps compiled under our specifications generally depict planimetric features within 5 feet of their true location. Contours will generally be depicted to within 2–1/2 feet of their true elevation, and spot elevations to within 1/2 foot of their true elevation.

- Property maps reflect data from several sources and the accuracy varies accordingly. While each new subdivision is mapped using survey coordinates shown on record plats, the majority of the data was digitized from tax maps, a source of which the accuracy is impossible to quantify. Therefore, all property line data should be interpreted as approximate.

- Since printed paper map plots are generated from base map data, they will only be as accurate as the planimetric/topographic and property data used to compile them. Orthophoto images have approximately the same level of accuracy as the planimetric/topographic base maps.

Suitable/Unsuitable Applications
The scale and contour interval at which the base maps were compiled make them ideal for general planning purposes. Recommended applications include preliminary and pre–preliminary plan preparation, watershed analysis, forest stand delineation plans, and other applications where the maps are displayed at their compilation scale (1"=200’), or reduced to smaller scales. Due to the accuracy limitations previously discussed, customers are strongly encouraged to field check any GIS data they intend to use in their plans.

M-NCPPC base maps are not suitable for use as boundary or topographic land surveys. Property lines are depicted in their approximate locations and generally do not reflect actual field surveys. Likewise, the planimetric and topographic data is not sufficiently accurate for engineering site design of roads, buildings, or utilities. Customers interested in accurate field surveys should seek the services of a reputable professional engineering or land surveying firm.

The availability of digital data greatly facilitates manipulation of map data, but can also invite the possibility of misuse. Enlargement of base maps to larger scales should be approached with caution, as any discrepancies or errors will be proportionately increased. As such, enlargements to scales greater than 1"=100’ are not recommend. Also, further interpolation of contour or spot elevation data to produce smaller contour intervals or additional spot elevations is strongly discouraged. This practice yields results which are unreliable, and introduces a false appearance of improved accuracy. M-NCPPC will not accept plan documents on which this practice has been employed.

Special Exceptions / Situations
Organizations with a 501(c)(3) status can purchase GIS data at a 50% reduced fee. These organizations will need to supply proper identification of their organization’s status when submitting their data request. A signed Licensing Agreement must also be submitted with the data request. All other provisions regarding the distribution of GIS map products will apply.

Business entities who are engaged in a professional relationship with a member of the MC:MAPS Consortium should get their GIS data request filled by that agency. A $10.00 per tile copying charge will be collected from the business entity for submitting such a request to another Consortium agency. A letter verifying the business relationship on the agency’s letterhead must accompany the data request. The business entity must also sign a License Agreement when submitting the data request.

The MC:MAPS Consortium will discuss any potential data exchange, whether with a private or public entity, on a case–by–case basis.

HOW TO ORDER
To order GIS data products, a copy of the GIS Data Products Order Form and GIS Data Products License Agreement must be completely filled out and signed for each project. GIS data orders require a minimum of 48–hours processing time.

Finally, the GIS Data Products License Agreement must be completed. It provides the terms and conditions under which the customer is allowed to use M–NCPPC GIS data. The License Agreement is a legally binding contract, and must be signed, dated, and attached to the completed order form at the time the GIS data product is ordered. Payment for GIS products is due at the time of ordering, and may be made by cash, check, or major credit card.

>Contact GIS

>Back to: > Our website - home > GIS - home