STRATEGIES
FOR
AUTOMATED PARCEL IDENTIFICATION
FROM DIGITAL RECORD DATA
IN THE
U.S. PUBLIC LAND SURVEY

The Bureau of Land Management is responsible for the collection and maintenance of the Geographic Coordinate Data Base (GCDB). This is a collection of all survey records in the U.S. Public Land System which places error estimates on these records, produces coordinates and error estimates of those coordinates, subdivision to the quarter-quarter section level using the rules of the Manual of Surveying Instructions, 1973, and generation of the parcels created by this process.

The record information has been attributed to varying extents during the data collection and subdivision process. The next step is using these attributes to resolve the legal land descriptions for the created parcels. This process can become quite complex as one has non-standard record information such as meanders, mineral claims, tracts, etc. within the aliquot information.

Geographic Measurement Management (GMM) was initially designed for the data input, coordinate production, and subdivision process for GCDB. Recent additions to GMM have allowed the creation of the parcels and their automatic identification. The procedures for the non-interactive automatic identification process will be discussed.

Introduction

GCDB measurement type data includes bearings and distances derived from field surveys of land boundaries which are specific to the U.S.P.L.S. (Hintz, et al., 1993). Among these can be township and section lines, subdivision of section, meanders, mineral surveys, grants, claims, and various other forms of non-rectangular (metes-and-bounds) surveys. The other type of "measurement" information are control coordinates derived from digitizing of map information (primarily U.S.G.S. quadrangle maps) or coordinates of corners from a field survey process. The control coordinates and bearing-distance information is synthesized via least squares analysis in production of coordinates of all corners in the measurement information.

The coordinates of this process serve as control in all further proportioning and subdivision to the quarter-quarter section level. The special properties of fractional and other unique types of sections are automatically estimated by GMM, and verified by a user (Hintz and Wahl, 1994). GMM enables new survey information to be entered and thus hopefully provide better coordinates via a new least squares analysis of the survey information. The steps required in subdivision have been stored and do not change. Thus re-proportioning and subdivision due to new controlling corners can happen without user interaction.

The next function of GMM is to create parcels (polygons) from the subdivided information and attach a land description to each parcel. The goal of GMM is to automatically assign as many of these descriptions as possible, and leave manual assignment to only very select unusual cases.

Parcel Boundaries by Computation

Before parcels can be created, the lines representing the edges of the parcels must exist. GMM creates these lines through the process of "boundary by computation" during the subdivision procedure.

To begin this process some initial lines must exist. This is the bearing-distance survey information prior to subdivision and consists of the lines as defined in the introduction. On section and township lines this data will usually be to the quarter section level (the original survey) unless further subdivision is indicated by later surveys. Subdivision of section exists only if has been physically performed by a ground survey. Special survey lines (meanders, grants, mining claims, etc.) are also included in this data.

Tie lines to any special surveys (such as a tie to a mineral claim) are identified by the user and removed from this component of processing as they do not represent a parcel boundary. The tie lines are very critical in coordinate generation via least squares analysis as they provide the bearing- distance information relative to the section line data. This level of line information is called the "Raw" data in GMM terminology.

Each proportioning or subdivision process either turns a single line into two lines or creates new lines. The former is typified by the creation of a sixteenth corner by proportion between a section corner and a connected quarter corner. The line connecting the section corner to the quarter corner has now turned into two lines (section corner - sixteenth corner - quarter corner).

In many situations (township lines with closing corners) the described proportion is performed but the section and quarter corner are not directly connected as a closing corner exists between them. GMM automatically recognizes this situation. It decides which side of the closing corner the sixteenth corner will fall on, ensures the sixteenth is properly moved to the section or township line in cases of break in bearing along the line, then correctly makes two lines out of the line the sixteenth corner falls on.

Computation of center of section or center of quarter section is typical of creation of new lines. If the section has not been subdivided as indicated by no data of that type in the raw information, no line will exist between opposite quarter corners.

The intersection process creates four new lines with the example for a center of section being:

1. east quarter corner to center of section
2. center of section to west quarter corner
3. south quarter corner to center of section
4. center of section to north quarter corner

Fractional sections and sections containing meanders or special surveys create unique solutions for section subdivision which boundary by computation must handle. The classic case is lotting against a meander (water). The interior section line intersects the meander line. The interior section line is a new line, and the intersection point on the meander turns the single meander line into two line segments. A lot will exist on both sides of the interior section line bounded on one side by the meander line.

Boundary by computation has enabled GMM to become less dependent on point identification conventions and completely independent of absolute point identification and data ordering. The only place this algorithm fails is on what could be called "implied" lines. These are lines that are not part of the raw data, connect two points in the raw data, but unfortunately the line is not part of any computation. The best example is a very short section abutting a closing line which is simply four lots (either N-S or E-W). Those lots are created by connecting opposite exterior sixteenth or quarter corners but not placing any points inside the section. Therefore no computation defines the line. These lines require some operator input because of the lack of computation defining them.

Parcel Boundaries by Automatic Intersection

After the parcel boundary by computation process is finished it is possible to have intersecting boundary lines where the intersection point has not been coordinated and thus does not exist in the line information. One example of this is a special survey (mineral claim) which crosses a section line but the record data along the section line does not identify it. Another example are interior section lines crossing mineral claims or meanders.

GMM next identifies all of these intersection points, and thus each intersection will turn the individual lines that intersect into two lines (with the intersection point in the middle). A special property of GMM is the recognition of the geodetic aspects of the U.S.P.L.S. All township, section, and special survey lines are lines of constant bearing (therefore curved), while interior section subdivision lines are straight and therefore lines of constantly changing geodetic bearing. The type of line is automatically defined and then the proper type of intersection can occur.

Generation of Parcels

With all intersections defined and thus all parcel boundary segments determined, GMM next creates all parcels (polygons). At this point each parcel simply consists of a list of corner identifications (point ID) and that point's coordinates. A coordinate pair inside the parcel is used to enhance its ability to be located.

These parcels are not really identified though their point ID's, but point ids do provide a variety of inferences as GCDB has certain forms of standard ID conventions which have been discussed in Hintz, et al., (1993). The point ids can allow software to automatically identify the SW 1/4 of the NW 1/4 of section 16. Unfortunately if a closing line exits along the west edge of section 16 that parcel may be a lot.

A fractional section, or one which contains special surveys, is when point ids do not resolve parcel identification. A parcel which contains both standard (township, section, or section subdivision) lines and special survey point ids may be in the special survey (inside), or may be the remainder of the quarter-quarter section and therefore usually a lot (outside). The inside/outside problem cannot be resolved by looking at the parcels which exist after the subdivision process.

Revisiting the Raw Data
for Inside/Outside Identification

The original raw data is a wealth of information which leads to resolving the inside/outside problem. Parcels can be created from the raw line connectivity. If a parcel consists of all standard ids, it is a section or standard subdivision of a section. These parcels are not of consideration in the inside/outside resolution.

All other parcels are either inside of some special survey, or are the remainder (outside) of a section and thus relates to a lot. The easiest identification is of a parcel that is all special point ids - it defines a special survey parcel. A mineral survey which is entirely inside a section will attain this simple identification.

From this point on, GMM follows a series of rules in automatic inside/outside identification. Some examples are:

1. A parcel consisting primarily of meander corners which contains two distinct lines with section line ids is probably "inside". This is typical of a river entering and leaving a section.
2. A parcel consisting of two or more connected components of section lines and one string of special point ids is probably "outside". The special point ids reflect a side of a special survey.
3. Two outside parcels rarely have a common side which is inside a section. That common side should just not exist if both were outside because they would be one parcel with no dividing line. If one parcel has been identified, the identified parcel is of the opposite type.
4. An identified outside connected to an identified parcel by a section line usually indicates the other parcel is outside. The rare exception is when one side of an inside parcel is a section line and is connected across the section line to a lot.
5. Joining inside parcels to identified parcels to create a bigger parcel may result in a parcel with only special point ids. All parcels included in it are inside. This occurs when a series of interconnected mineral surveys crosses a section line.

Needless to say, exceptions exist to all rules and the user is the final judge of the correct assignment of inside/outside. The hardest identification is along the township exterior because the data ends at that place due to the way GCDB data is stored in unique townships.

Merging the RAW parcels
with the subdivision parcels

The subdivision parcels can now be identified as being within a special (inside), lot (outside), or standard (aliquot quarter-quarter section or further subdivision) parcel created from the raw data. Due to intersections of lines, it is entirely possible that a special parcel from raw data may be made up of several subdivision parcels. The aliquots are easily identified from their point ids.

Merging subdivision parcels with LLD (Legal Land Description)

What is left to identify are the specific lot numbers for the identified lots, lots which replace an aliquot identification because it borders a closing line, and specifically what the special parcels represent. Prior to the creation of GCDB, BLM was responsible for the creation of LLD from sources such as master title plats. These files indicate which aliquots contain lots and define their lot numbers. This file also indicates which aliquots contain special surveys and what these special surveys are (mineral, meander, grant, etc.). LLD also contains plat acreages for lots and special surveys.

GMM finally assigns lot numbers to all resolved parcels which are in "outsides" or are replacements of what initially was thought of as an aliquot (but is not as it is adjacent to a closing line). Acreage as computed from coordinates is compared to plat acreage. Subdivision parcels identified within "insides" can now be associated with its legal land description as exists in LLD.

GCDB and LLD were two independently created sources of U.S.P.L.S. information. While a variety of checks existed in their creation, un-found blunders and misinterpretations will occur. Mismatches of GCDB to LLD indicate problems exist in one or both of the systems. These problems need to be resolved before validity of the two systems is truly achieved. The graphical visualization of the parcel and their identifiers, with special graphical effects for mismatches, is the final component of the process.

Conclusions

GMM's technologies as applied to township subdivision, parcel boundary creation, identification of parcel boundary intersections, creation of parcels before and after subdivision, identification of lots and specials, and finally match-up to the legal land description library have been presented. The result is the boundary information for BLM's Land Information System.

Acknowledgements

This work has been partially funded through the cooperative agreement between USDI-BLM Eastern States and the University of Maine. The authors especially thank cooperative assistance representative Corky Rodine of BLM Eastern States Cadastral Survey.

References

Blanchard, B.M. (1991), "Utility Program Development: A Digitally Integrated Measurement Management System for the U.S. Public Land Survey System," masters thesis, University of Maine, 86 p.

Gilson, H. (1992), "GCDB - What, Where, Why?," Presented at the ACSM-ASPRS GIS/LIS Fall Meeting, 8 p.

Hintz, R.J., Blackham W.J., Dana, B.M., and J.M. Kang, (1988), "Least Squares Analysis in Temporal Coordinate and Measurement Management," Surveying and Mapping, Vol.48, No.3, September, pp.173-183.

Hintz, R.J. and C.J. Rodine (1990), "Automation and Precision In a Cadastral Surveying Environment," Proceedings of the ACSM-ASPRS Annual Convention, Vol.1, pp.124-133.

Hintz, R.J. , J.L. Wahl, K.B. Wurm, and D. McKay (1993), "The Use of Public Land Survey Records in Establishment of a Geodetic Framework for Land Information," ACSM/ASPRS 93 annual meeting proceedings, Vol. 1, pp. 52-62.

Hintz, R.J. and J.L. Wahl, "Specialized Automated Functions for Subdivision and Parcel Recognition within the U.S.P.L.S." (1994), ACSM/ASPRS 94 annual meeting proceedings, Vol. 1, pp. 123-134.

Rodine, C.J., J.L. Wahl, B. Parker, B.M. Blanchard, R.J. Hintz (1991), "Progress Report on the Development of an Integrated PLSS Cadastral Measurement Management and Retracement Survey Software System," Proceedings of the ACSM-ASPRS Annual Convention, Vol. 1, pp. 146-155.

U.S. Department of the Interior Bureau of Land Management (1973), Manual of Instructions for the Survey of the Public Lands of the United States 1973, U.S. Government Printing Office, 333 p.

U.S. Department of the Interior Bureau of Land Management (1991), "Cadastral Survey Measurement Management Version 1.0 Documentation," U.S. Department of the Interior Bureau of Land Management Division of Cadastral Survey, 152 p.

U.S. Department of the Interior Bureau of Land Management (1994), "Geographic Measurement Management Version 2.0 Documentation," U.S. Department of the Interior Bureau of Land Management Division of Cadastral Survey, 118 p.