The Geographic Coordinate Data Base (GCDB) is a digital coordinate-based representation of the Public Land Survey System (PLSS). GCDB coordinates portray the legal land parcels of the PLSS and tie land description, records, parcel information, and resource data to positions on the ground.
(A GCDB section with corner point identifiers, subsections, and associated attributes.
Illustration from BLM Utah State Office presentation "What Is The GCDB".)
GCDB is an integrated network of survey records, horizontal control, land descriptions, and estimates of positional accuracy. Control comes from digitized USGS quads, GPS points, and geodetic reference. The GCDB data is collected by the BLM Survey Offices to provide a high-quality framework for Geographic Information Systems.
GCDB is a cadastral reference theme and a common foundation upon which all data themes can be registered and integrated.
General requirements for the GCDB within the BLM are:
With the above cautions in mind, the uses and advantages of the GCDB are described below.
Uses of the GCDB
Geographic Measurement Management software (GMM) is used to compute the geographic coordinates of PLSS corners using official cadastral survey record data, and to provide an estimate of the dependability of positions. GMM utilizes a Least-square Adjustment to determine the best fit between available survey and control records. Error estimates are assigned to survey records and control, based on the quality/reliability of the data.
GCDB computed coordinate values for PLSS corner positions and monuments are referenced to NAD 27:
Horizontal control is derived from:
The following coverages are created by the GCDB Data Preparation processes:
||ARC/INFO FEATURE CLASS|
|LABELS_GCDB||Area points, Survey lines||Points, Arcs, and Nodes|
|GCDB||Land areas, Survey lines||Polygons, Regions, Arcs, Nodes, and Labels|
The CTRL coverage is a point coverage that contains control locations for the specified township. Control point features are described by elements such as a point identifier, elevation and reliability measurements. The Control coverage contains only those points that are used as control points.
The LABELS_GCDB coverage contains label (point) and survey line features that provide land descriptions for a given area. The LABELS_GCDB coverage provides the basis for the GCDB coverage polygon and region topology. This coverage contains a complete graphical representation of the GCDB files including all label points and survey lines.
The GCDB coverage contains the region features (township, section, and quarter) with a single label point connected to land description attributes that relate specifically to that point. The GCDB Coverage is a region coverage with the following three region subclasses:
Shown below is a view of several quarter-quarter sections in a GCDB coverage from southwest Colorado. Note the attributes listed on the left which provide detailed information about the yellow-selected area. The numbers shown, such as 100700, are GCDB corner point identifiers. This coverage was downloaded as an e00 file from the BLM GCDB web site, imported into ArcView, and displayed as shown below.
The table below describes some of the attributes shown
above, as noted in the GCDB specification:
|Nominal Location||Nominal Location A - Q|
|Survey_type||Survey Type A - Z|
|Survey Number||Survey number|
|Conflict_cd||Conflict code - (reserved for future use)|
|Prinmer_cd||Principle Meridian code|
|Tier_frac||Tier fractional code. 0 = Not a fractional tier, 1 = ¼ tier, 2 = ½ tier, 3 = ¾ tier.|
|Range_fraq||Range fraction. 0 = Not a fractional range. 1 = ¼ range, 2 = ½ range, 3 = ¾ range.|
|Range_dir_cd||Range direction code, E, W|
GCDB Corner IDs
The GCDB point identifier system is a six digit number composed of a three digit prefix (Xxx) followed by a three digit suffix (Yyy). To give an example, the general guidelines for the PLSS survey point IDs are:
|XxxYyy - XxxYyy|
|000000 - 000009||Reserved for software codes|
|095000 - 704999||Rectangular Surveys|
|705000 - 799999||Boundaries with Mileposts|
|800000 - 836999||Riparian Boundaries|
|837000 - 899999||Tracts|
|900000 - 999989||Mineral Surveys, Homestead Entries, etc|
|999990 - 999999||Reserved for software codes|
The southwest corner of each township is assigned a point ID of 100100. Remaining section corners in a township are assigned an X value in the prefix (Xxx) and a Y value in the suffix (Yyy) indicating how many section east and north the corner is from the southwest corner of the township. So a point ID of 400400 indicates a section corner three miles east and three miles north of the southwest corner of the township, which is the corner of sections 15, 16, 21, and 22.
Subsection corners are incremented numerically in greater detail. The following illustrates the basic GCDB ID system for quarter-quarter sections, specifically in this example, Section 8.
More on the GCDB corner IDs can be found at the BLM
Wyoming GCDB web site.
Where To Get GCDB Data
GCDB data can be obtained via download from the GCDB web site. The main GCDB web page is at http://www.blm.gov/gcdb/.
To go directly to the site for downloading GCDB data,
you can go to
The BLM Land Survey Information Web Site
Another source of GCDB data, made available in the Spring of 2002, is the BLM's Land Survey Information web site.
"The Land Survey Information (LSI) Web site contains a
collection of geographic information from the Bureau of Land Management's
(BLM) Geographic Coordinate Data Base (GCDB) representing the Public Land
Survey System (PLSS) of the United States. The Land Survey information
is available to download free of charge."
From the LIS web site
The LSI web site provides an ESRI ArcIMS interface as a means of locating and downloading GCDB data. This allows you to find and select townships to download through graphic selection.
When you zoom in far enough on the initial map of the Western U.S., the display reveals townships, thematically colored to indicate availability of GCDB data. You can then easily select and download GCDB data from the area you are interested, as shown in the example below of some townships in northern Arizona.
Obtaining GCDB Data from the BLM's LSI site. The yellow highlighted
areas above are the currently selected townships, which can be downloaded
by clicking on the SHP or Flat File buttons at the bottom left.
|Sources Of Information Used For This GCDB Section
BLM GCDB Web Site: http://www.blm.gov/gcdb/
BLM Oregon GCDB Web Site: http://www.or.blm.gov/OR957/GCDB/
BLM Wyoming GCDB Web Site: http://wgiac.state.wy.us/wsdc/blm/gcdb/
BLM LSI Web Site: http://www.geocommunicator.gov/lsi/.
GCDB Data Preparation June 2000 Geographic Coordinate Data Base, BLM working document.
Geographic Coordinate Data Base Coverage Format V.1.03.00 - 06122001 Description Document, BLM working document.
Users Guide For The Geographic Coordinate Data Base, BLM Wyoming State Office, Milbert Krohn, August 1995.
What Is The GCDB? BLM Utah State Office PowerPoint presentation.
Continue to Part Three: Standards
Return to Part Three main page
Table Of Contents - Cadastral Information For GIS Specialists
Links to the other Cadastral Courses:
Learning The Cadastral Data Content Standard
County Recorders And The Cadastral Data Content Standard
Surveyors And The Cadastral Data Content Standard
Presented by the United States Department of the Interior Bureau of Land Management, and
the Federal Geographic Data Committee Cadastral Subcommittee