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Considerations in the Development of a Digital Public Land Survey System Data Base The decision to create a digital, coordinate-based representation of the Public Land Survey System (PLSS) involves the need to examine both demand and supply considerations. Any decision to proceed with a task of this magnitude must be made within the context of explicit answers to such questions as: "Who uses the existing system? What types of activities is it used for? How would the introduction of a digital, coordinate-based representation impact on current users and potential users? What form of digital, coordinate-based representation should be selected? What should be its accuracy and its spatial coverage? How long will it take to construct the system, and what will it cost?" Responses to these questions must be examined and a decision reached within the constraints imposed by the capability of current technology to supply the desired data base and the institutional restrictions within which it would have to be created and maintained. 3.1 THE NATIONAL ROLE OF THE PUBLIC LAND SURVEY SYSTEM A fundamental question must be asked in considering the creation of a digital PLSS data base. Is the existing PLSS used? The answer is a resounding Yes! Not only has the PLSS system become imbedded in the landscape of the country, with roads following section lines and local jurisdictions often hav- ing boundaries that are descended from the PLSS, but the federal government owns some four-hundred-million acres of land in the United States, and most of these lands were surveyed under the PLSS. All transfers of public 22

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Considera-tions in the Development of a Digital PLSS Data Base 23 lands to private ownership and vice versa have come to be based on this system. It is clear that the PLSS has become, over the last century and a half, an in- herent element in the fabric of American lifein both the public and private sectors. The value of the PLSS for safeguarding private property rights is of significant importance. The aggregate level of use of the PLSS iS difficult to ascertain because of the highly disaggregate nature of this use. Nor is it pos- sible, without additional extensive investigations, effectively to distinguish use levels by different categories of users, e.g., federal versus state versus local versus private. It is clear, however, that the federal government and its operat- ing agencies are major users of the PLSS. 3.2 FEDERAL GOVERNMENT ACTIVITIES The federal government's interest in the PLSS representation of land owner- ship theoretically disappears when the lands involved are patented to non- federal interests. However, a common federal-nonfederal interest remains when the federal government owns the land on one side of a PLSS line and other government or private interests own land on the other or when federal subsurface rights extend under privately owned surface rights. Conflicts in title between federal and private interests are extensive; as many as 50,000 title claims and trespass situations exist through just one federal management entity, the National Forest System, and close to 100,000 title claims exist within the Department of the Interior (Domenici, 1981~. In addition, federal agencies often use the PLSS system to define areas within the public lands for purposes such as leasing for oil and gas, timber, grazing, and Minerals. Even after the theoretical federal interest has disappeared in transferred lands, land owners and users in the private and nonfederal public sectors continue to make use of PLSS descriptions. Although the federal government is a major user of PLSS information,its PL S S-related activities have not yet become strongly oriented to the use of a dig- ital, coordinate data base for their operation. Exceptions may be found in iso- lated cases, such as the use by the Fish and Wildlife Service of a PL S S coordinate data base within the Map Overlay Statistical System (MOSS) geographic infor- mation system, but utilization of digital approaches appears to have moved ahead much more rapidly in the nonfederal, public, and private sectors. The federal government is, of course, heavily involved as a creator of digital information pertaining to the PLSS. This information is being col- lected routinely by the National Mapping Division of the U.S. Geological Survey as part of their Digital Line Graph (DLG) representations of the non- terrain features on standard 7.5-minute maps. A digital line graph is line map information in digital form. (See Figure 3.1 for current DLG coverage.)

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~ / l ~ ~ l ~ FIGURE 3.1 The status map above shows 7.5-minute quadrangles where digital eleva- tion models have been completed. A digital elevation model is a digitized file of ground positions normally at 30-m intervals. The status map below shows digital-line-graph 7.5- minute quadrangles where at least one category, such as transportation, hydrography, or boundaries, has been digitized. (U.S. Geological Survey Activities, Fiscal Year 1981, Circular 875.) 24 1

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Considerations in the Development of a Digital PLSS Data Base 25 3.3 STATE, LOCAL, AND PRIVATE ACTIVITIES The difficulties of integrating the PLSS data with spatial data have been pointed out. How do these difficulties affect users, particularly those operat- ing in the digital domain? We now consider nonfederal government and pri- vate activities. 3.3.1 Nonfederal Government Sector The interest of several western states in digital, coordinate-based representa- tion of the PLSS iS quite clear. One example may be seen in the activities of the state of Montana (Marble, 1982), where a substantial amount of effort and attention has been given to the local creation of both township and section corner coordinate representation of the PLSS. The data were extracted, in some cases by manual measurement, from U.S. Geological Survey (USGS) maps (paper format). Accuracies are low, and the original file contained many errors that have now been generally eliminated through a series of plot-check- revise operations. This data base is used by various departments of the state in a variety of ways. The Department of Administration has utilized both the township and section data sets to produce graphic displays of the spatial pattern of land ownership (federal versus state versus local) for various study areas within the state of Montana. The Department of Revenue utilizes the system as part of its property assessment whereby recently sold parcels are identified in terms of their PLSS description from the sales document, and the digital-coordinate data set is used to translate the PLSS description into a coordinate system from which interparcei distance can be computed. These interparcel distances are then entered into a weighting equation that estimates the appraised value of parcels based, in part, on the distances and sales values of other parcels that have been sold recently and that lie in the immediate vicinity. In another instance, the state of Minnesota, through the Land Management Information Center (LM IC) ofthe Department of Energy, Planning and Devel- opment, has completed the digitizing of section corners for the state (D. Marble, State University of New York at Buffalo, personal communication, 1982; Minnesota State Planning Agency, 1982~. These corners were digitized in Uni- versal Transverse Mercator (UTM) coordinates from 7.5-minute USGS quad- rangle maps to an estimated accuracy of 30 m. The effort has resulted in an interactive digital file from which the UTM coordinates can be recalled for any section corner in the state. The system can also be used to extract any needed subset of section corners as well as to generate approximate corners for 40-acre parcels and other subdivisions of the PLSS. The effort, which took about 3 to 4 man-years to complete, now gives the statewide geographic ,

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26 MODERNIZATION OF THE PUBLIC LAND SURVEY SYSTEM information system the capability to exchange data among and between the following: 1. PLSS descriptions, 2. UTM coordinates, 3. State plane coordinates, and 4. Latitude-longitude coordinates. It is planned to replace the estimated comers in the Minnesota data base with actual, determined corners that will be obtained from the Department of Transportation or any other agency willing to provide them. The section cor- ner file will provide EMIC with a reference base for mapping, locating, and controlling data currently stored as part of the Minnesota land-management system. It completes a strong base of geocodes that now exist for Minnesota, allowing LMIC to obtain and link data from other agencies. Multipurpose cadastre activities at local government levels that involve the digitization of the PLSS corners are listed in Appendix A. 3.3.2 The Private Sector A problem encountered by many energy companies operating in the western United States is the integration of seismic, geologic, and leasing information. Data in the seismic and geologic sets are normally encoded in latitude and longitude, whereas information on the areal extent of leases and their bound- aries is normally recorded in the PLSS format. Matching of these spatial data sets (which is also carried out by government agencies such as the Minerals Management Service of the Department of the Interior) is a laborious and time-consuming manual operation. To assist in automating this procedure, several large petroleum companies began digitizing PLSS coordinates from the 7.5-minute quadrangle maps produced by the USGS. One company, Phillips Petroleum (Marble, 1982) has been working on the creation of a PLSS digital data base for nearly a decade and now has coordinates derived from USGS maps for approximately 33,000 townships and their section boundaries in the continental United States and an additional 18,000 protracted townships and sections in Alaska. (See Figure 3.2 for current extent of Phillips Petroleum coverage.) Phillips Petroleum has increased its level of activity in this area in the last three years and is rapidly moving ahead to the point where the company ex- pects within the next few years to have completed a digital coordinate-based PLSS data base for all the western states, including Alaska. In addition, Phillips is extending this data base through what they refer to as "pseudo- Jeffersonian" techniques to the state of Texas; Texas is not included in the

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28 MODERNIZATION OF THE PUBLIC LAND SURVEY SYSTEM PLSS. The techniques that they have developed for creating "pseudo-sections" in Texas will also permit them to extend this system to the eastern United States if they so desire. One measure of the demand for digital PLSS coordinate information is that, as of the end of 1981, Phillips Petroleum had sold copies of all or large portions of its digital PLSS data set to over a dozen users, most of them in the energy area. In addition, Phillips Petroleum continues to receive numerous inquiries from states and other organizations in the West about acquisition of portions of the data base. As Phillips Petroleum has made no overt attempt at marketing this data base, the situation may be taken as de facto evidence of a strong demand for a digital, coordinate-based version of the PLSS. It is inter- esting to note that, in addition to having had use of the data set themselves, Phillips Petroleum appears to have more than recovered the cost of data-base creation. 3.4 DEMAND FOR A DIGITAL PUBLIC LAND SURVEY SYSTEM DATA BASE Although the scope of this study did not include a broad investigation of de- mand for coordinates of PLSS comers in digital format,it is clear that a sub- stantial demand exists in some user classes for a coordinate-based version of the PLSS. Organizations in the private sector have been creating a digital version of the PLSS that is characterized by extensive are al coverage, relatively high density (section corner plus irregular section boundary), and relatively low accuracy. In order to link diverse natural-resource data files for purposes of spatial analysis, numerous states have devised algorithms to assign plane coordinates to records described in PLSS terms. In the state of Illinois, for example, the ILLIMAP system was created by the State Geological Survey (Swann et al., 1970~. This system contains a digital file of coordinates of all section corners in the state. It can be used for purposes of assigning coordinates to geologic and other records and for computer analysis and mapping. 3.5 ACCURACY CONSIDERATIONS The source of much of the currently digitized PLSS data is paper copies of the USGS 7.5-minute quadrangle maps. These maps are constructed to meet National Map Accuracy Standards (Appendix B); but the use of paper rather than stable-base material results in a further decline in accuracy. Without eval- uating digitizing operations such as those at Phillips Petroleum, it is not pos- :

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Considerations in the Development of a Digital PASS Data Base 29 sible to determine the level of accuracy of existing digital data sets. From a discussion of the operations at Phillips Petroleum, it is estimated that the data base as digitized would deviate significantly in terms of accuracy with respect to the data displayed on the original quadrangle maps. Similarly, from discus- sions with officials of the state of Montana it is estimated that their data set is less accurate than the one created by Phillips Petroleum. Nevertheless, these data bases are being used, and used extensively, by many nonfederal organiza- tions that are apparently satisfied with the accuracies provided by the pro- cedures that they have adopted. Based on the information in hand, it does not appear that there is a significant user demand for higher-accuracy repre- sentation of the PLSS in digital, coordinate-based form. However, it should be noted that the users identified in this chapter had minimal need to relate land-attribute information to land-parcel information. ll