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.

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:

1. to support a spatially oriented graphic system that automates the production of Master Title and Use Plats.
2. to use the geographic coordinates to relate legal land descriptions, ownership and status information to map-referenced points on the earth's surface.
3. to provide the limits, or boundaries, for resource data collection.
4. display rectangular survey data to the parcel level.
5. display metes and bounds survey data, specifically mineral surveys, homestead entry surveys, riparian boundaries, and tract surveys.
6. GCDB is set up to incorporate future surveys designed for greater accuracy.

With the above cautions in mind, the uses and advantages of the GCDB are described below.

Uses of the GCDB

1. record keeping - e.g. ownership, rights, exchanges, sales, designations, easements, etc.
2. mapping
3. graphics
4. preliminary corner search
5. planning and other land management purposes

1. depicts the cadastral land net to the parcel level.
2. shows special surveys.
3. creates townships as units of data.
4. provides a template for each township.
5. is dynamic, and can be updated as new survey and control information are added to the existing template.
6. will become increasingly accurate over time.
7. supports the establishment of partnerships between federal agencies, and state and local governments to collect, maintain and use the data, and reduce duplication of effort.
8. makes full use of technology to assist with distributing data.
9. utilizes an open data base structure

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:

1. Latitude and Longitude
2. UTM
3. State Plane Coordinate System

Horizontal control is derived from:

1. Digitized 7.5 minute topographic map
2. GPS positions
3. Geodetic control

GCDB Coverages
The following coverages are created by the GCDB Data Preparation processes:
 

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:

1. Township - contains a single feature whose spatial extent is that of the township.  Descriptive information includes the State, Principal Meridian and Township.
2. Section - contains features that represent the sections within the township. Each feature is described by the section number.
3. Quarter - contains features that represent quarter subdivisions of the sections.  Features are attributed with a two-letter, directional code describing the quarter

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:
 

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:
 

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 http://lm0500.blm.gov/.
 

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.