Geographic Coordinate Data Base (GCDB)
What is GCDB? The Bureau of Land Management (BLM) Geographic Coordinate Data Base (GCDB) is a collection of geographic information representing the United States Public Land Survey System (PLSS) and other official surveys. The GCDB data is computed from BLM’s official survey records including Cadastral survey plats and field notes supplemented with local survey records and geodetic control information.
Who uses GCDB data? Since the 1990’s the GCDB flat files have been used by Federal agencies for making maps using computer-aided drafting applications. Soon many land surveyors began using the GCDB flat files to help them locate survey corners on the ground. Over time, the GCDB data became available via the Internet in geographic information system (GIS) formats and many local governments and private companies began using the GCDB in their in-house GIS applications. During the last few years geospatial products like the Cadastral National Spatial Data Infrastructure (CADNSDI) file geodatabases have been developed and distributed on-line. Today the typical GCDB user might be anybody with a global positioning system (GPS) unit, a smart phone or an Internet connection and an interest in maps.
Tell me more. BLM began collection of the geographic coordinate information in 1989 and the data collection effort continues today. The best positions for the survey points are determined by:
- Collecting the locations of all township, section, aliquot part, government lot and special survey corners in a township,
- Incorporating horizontal control positions from published sources and GPS observations to tie the survey framework to the earth,
- Adjusting the GCDB using the least squares method of adjustment to determine the best geographic positions for all of the survey points in the dataset,
- Assigning the official land descriptions to every land unit depicted in the GCDB,
- Translating the resulting framework into GIS formats for use with spatial applications.
To find out more about GCDB file formats and data elements check out the GCDB User’s Manual and the GCDB Standards web page.
Which GCDB format is best? Where available, the best GCDB datasets for GIS/geospatial work are the CADNSDI file geodatabases. The CADNSDI databases conform to the newest Federal Geographic Data Committee (FGDC) guidelines for geospatial data and contain the most up-to-date GCDB data. In areas where the GCDB data has NOT been collected, the CADNSDI datasets are supplemented with PLSS data from alternate sources. The resulting seamless (or almost seamless) statewide datasets are reviewed by the PLSS data steward with jurisdiction and approved as the best available PLSS data before the datasets are published.
Where can I get the data? CADNSDI file geodatabases for Arizona, California, Colorado, Idaho, Montana/North Dakota, Nevada, New Mexico, Utah and Wyoming are available for download from the PLSS Data Download site on the GeoCommunicator Download Public Land Survey System (PLSS) Data web site. CADNSDI databases for Oregon and Washington are expected to be available soon.
Many of the BLM state offices have posted their GCDB flat files on their state office websites. Links to these websites are provided on the GCDB Offices page.