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4
Technical and Economic
C onsiderations
"Why is it necessary to establish a coordinate system for the Public Land Sur-
vey System?" The answer is that the Public Land Survey System (PLSS) iS in-
consistent and cumbersome for use with modern spatially oriented technology.
The PLSS records a position on the surface of the land, for example, the
Southwest corner of Section 6, Township 4 North, Range 3 East, Fourth Prin-
cipal Meridian. These locations were surveyed and marked as part of an Ordi-
nance of 1785 and subsequent legislation and regulations. Surveyors hired by
the General Land Office and later the Bureau of Land Management performed
the surveys with the instruments available. There are similarities to the na-
tional geodetic network developed by the National Geodetic Survey and its
predecessors in placing monuments for horizontal geodetic control. These
. . . .
slm1 arltles are
1. Monumenting points on the surface of the land,
2. Determining positions for those points, and
3. Performing the positioning with the available technology
.
It must be remembered that in the PLSS the lines are not numbered; it is the
land between the lines that is designated. For example, Township 6 North
(commonly marked T 6 N) is not a line but a 6-mile space in a north-south
direction, between T 5 N and T 7 N. Also, Section 20, for example, refers to
an area of approximately 1 square mile, not a line. To refer to a line one must
say, "the line between Sections 20 and 21."
In performing the survey, the original instructions called for all corrections
30
OCR for page 31
Technical and Economic Considerations
31
to be accumulated in the northernmost row and westernmost column of sec-
tions in each township (the proper identification of the area of one township
by one range). These Sections are 1, 2, 3, 4, 5, 6, 7, 18, 19, 30, and 31. It
would be extremely rare to find the corners of these sections one mile apart
or the section equaling precisely 640 acres. Corrections for the Earth's curva-
ture were made by establishing standard parallels generally at intervals of 24
miles (Figure 2.4~.
From the preceding it is obvious that the PLSS is not a uniform mathe-
matical grid nor is its designation of a point directly adaptable to modern
computer technology. It is simple to say the Southwest corner of Section 4,
Township 8 North, Range 7 West in oral or written communications; it is
anything but simple to encode this for computer processing.
A report of the Committee on Geodesy (1982) states:
. . . if the control surveys are to have multiple-purpose applications, then accuracies at
least attendant to the federal classification of third order, Class I, or second order, Class
II, should be met.
The relative accuracy for Third Order, Class I, is 1 :10,000 and for Second
Order, Class II, it is 1:20,000 (Federal Geodetic Control Committee, 1974,
1976~. At half-mile spacing, this requires that, to meet Third Order, Class I,
specifications, the position of each corner should be accurate to a standard
deviation of +0.19 ft and to meet Second Order, Class II, to +0.09 ft.
That report also suggests the density of the control needed, namely,
With respect to spacing, monumented points of known location . . . should be so dis-
tributed throughout the area concerned as to permit their ready use in the collection of
both cadastral and earth-science data.
Typical recommendations range from 0.2 to 0.5 mile between monuments in
urban areas to 1 to 2 miles in rural areas.
This chapter discusses the economic and technical consideration for plac-
ing coordinates on the PUSS corners. The techniques will range from manually
measuring and estimating the coordinates from existing maps showing the
PLSS corners to the most advanced observational techniques, including the
Global Positioning System (GPS). Included between these two extremes will
be a discussion of the use of existing survey traverse data, photogrammetric
methods, and standard geodetic surveying.
The costs per section corner vary from less than $1 per point for coordi-
nates scaled or measured from a map to more than $1000 per point for coor-
dinates obtained for monumented corners by using modern observational
technology. In general, the higher the accuracy, the higher the cost. A graph
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Technical and Economic Considerations
33
illustrating cost versus accuracy for various techniques is presented in Figure
4.1.
The discussion that follows pertains only to obtaining coordinates for a
PLSS position; this chapter does not address the separate and unique problem
of determining whether the position of the PLSS point being coordinated is
legally acceptable (see Section 2.3) as being the best position of the original
survey monument.
4.1 ACQUISITION OF COORDINATES FROM DATA ON FILE
4.1.1 Acquisition of Coordinates from Me USGS 1:24,000 Maps
The most prevalent source data for digitizing the PLSS are the 7~-minute
topographic maps produced by the U.S. Geological Survey. Section corners
as well as section lines are shown on these maps. There are three categories of
section corners: found corners, indicated corners, and theoretical corners
(U.S. Geological Survey, 19694. Found corners can be plotted at national map
accuracy standards. There are greater uncertainties in the plotted positions of
indicated and theoretical corners. The locations of these corners may be ex-
pressed in one of the three reference systems shown on the maps: latitude-
longitude, Universal Transverse Mercator, or State Plane Coordinates. Mea-
surements are made either in a manual mode utilizing only a scale or, much
more commonly, by using one of the many computer-assisted digitizing sys-
tems. An inherent problem with manual digitizing is the incidence of gross
operator error that must be detected and corrected.
Since the accuracy of these maps is +40 feet for 90 percent of the defin-
able points, accuracies higher than this level cannot be assured. However, it is
quite possible to degrade the accuracy through inaccurate and careless mea-
-
surements. Using stable base source materials, higher accuracies are attainable.
However, it should be noted that the precision of modern digitizing equip-
ment far exceeds the precision inherent in the cartographic source data.
The capture of point information, such as section corners, can be done
rapidly and inexpensively as compared with line or "stream" digitizing, which
is slow and expensive.
Automatic or mass digitizing is becoming increasingly used for the capture
of large quantities of cartographic data. The cost and speed of this process is
extremely sensitive to the nature and quality of the originals; with high-
quality originals, processing times for an entire map sheet may be as low as
20 minutes.
In the present state of the art, mass digitization does not represent a
l
OCR for page 34
34 MODERNIZATION OF THE PUBLIC LAND SURVEY SYSTEM
cost-effective method of capturing PLSS coordinate information. The section
corners represent a relatively sparse set of points with respect to a given 7~/~-
minute map, and the unit cost of coordinate capture for these points via mass
digitization (which involves substantial setup times and extremely high capital
equipment costs) would be large when compared with the unit cost of manual
digitizing.
Precise cost information could not be obtained, as most of the agencies
and organizations engaged in capturing digital information pertaining to the
PLSS from maps do not maintain adequate cost-accounting records. Informa-
tion supplied by the National Mapping Division of the USGS indicates costs
of $50 to $350 and times of 4 to 12 hours per map. Phillips Petroleum esti-
mates that its costs are nearly an order of magnitude below those of the
National Mapping Division. However, the relative accuracies of these two data
sets have not been evaluated (see Section 3.4~.
4.1.2 Compilation of Coordinates Directly from the Source Documents
Used for Compiling 1: 24,000 Mapping
A potential source of coordinates for some PLSS corners is the information
used for compiling the 1:24,000 scale maps. Information on the proportion
of coordinates from various sources was not available. Some were positioned
by National Mapping Division (NMD) field staff, and some were positioned
on NMD stereoplotters using targeted corners or the intersection of the sec-
tion lines as the method for locating the corner. The committee did not
explore the status of the source material for the compilations. It is to be
expected, however, that retrieval would be a time-consuming and costly pro-
cess. The costs of retrieval ($2-10 per corner) are estimates (see Section 4.2.2
for field costs).
4.1.3 Acquisition of Coordinates from Archival Survey Data in the
Bureau of Land Management and Other Repositories
"Standard procedures and specifications" for performing PLSS surveys have
existed for over a century. As the surveyors of the PLSS were establishing a
legal framework for the ownership of land, it was required that the complete
record of the survey be archived. Because the original survey defines the
boundary, the first step in retracing aPLSS sectionlineis the retrievalof the
original survey notes; the surveyor doing the retracement must then "walk in
the steps" of the surveyor who set out the line.
Starting in 1910, one corner, the southeast corner of each township, had to
have latitude and longitude coordinates determined for it. In view of the fact
OCR for page 35
Technical and Economic Considerations
35
that the network of the U.S. Coast and Geodetic Survey (now the National
Geodetic Survey or NGS) had not in general preceded the survey, coordinates
were usually determined by astronomic means and occasionally by some
other improvised method. Where geodetic control was available, it was used
for establishing the coordinates of the comer. Subsequently, many comers
have, for various reasons, been tied to geodetic control; these ties, along with
the data from the original surveys could be used to compute coordinates for
each corner. There are two ways in which these data could be processed.
They could be treated as traverse data with closures being made between
available geodetic control points, or they could be adjusted using an accept-
able method.
4.1.3.1 Computation of Coordinates from Archival Data Using Traverse
Reduction Methods
Every modern survey office has one or more computerized traverse reduction
routines. The Cadastral Division of,the Bureau of Land Management (BEM)
uses the National Geodetic Survey's TRAV 10 program, which performs a
least-squares adjustment of all horizontal observations in a traverse and com-
putes adjusted coordinates (latitude and longitude) for all traverse stations.
Although the main application of these routines is for the reduction of cur-
rent surveys, it would be possible to use them to compute coordinates from
retrieved archival data for all the PLSS corners that have been surveyed.
As much of the older data are no longer in BLM custody, it would be
necessary to go to state archives or, in some cases, to the National Archives
in Washington, D.C., for photocopies of the original data. For a typical town-
ship there would be some 208 pieces of information that would have to be
tabulated in a form suitable for keypunching, then checked, keypunched,
and processed. Although the Cadastral Division (Wilcox, 1981) has estimated
that the direct cost of doing this would run from $50 to $90 per township
($1.00 to $1.80 per corner), the committee is of the opinion that the cost
would be much higher.
Without considerable analysis, the accuracy of these data is unknown.
However, as some of the older data were acquired when specifications re-
quired a closing accuracy of 1:1,280, it is certain that some of these data
would be very inaccurate. In fact, it is highly probable that some of the
coordinates would have an uncertainty as large as that of the digitized data
discussed in Section 4.1.1.
In view of the uncertain quality of coordinates computed from archival
data using traverse reduction methods, we do not recommend proceeding
with a program of computing the data in this way without any additional
survey or processing or computing effort.
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OCR for page 38
38 MODERNIZATION OF THE PUBLIC LAND SURVEY SYSTEM
4.1.3.2 Computation of Coordinates from Archival Data Using Network
Adjustment Procedures
If archival data were retrieved as described in the preceding section, signifi-
cant improvements would ensue from the use of a network adjustment pro-
cedure rather than the simple traverse reduction described. The amount of
improvement would, to quite an extent, depend on the number and the
quality of ties to the geodetic framework that were available. Thus, while
there would be an improvement of perhaps 20 or 30 percent at some points,
the major shortcoming of this method is that in regions where there are no
ties to the geodetic network there would not be any significant improvement.
In addition, costs for coding, keying, validating survey data, and network
adjustment would be appreciable. There is no more apparent reason to rec-
ommend retrieving and adjusting the old traverse data than there would be to
recommend retrieving and computing by the traverse method discussed.
Although at this stage there is no recommendation to retrieve and process
all the archival data, the possibility of retrieving selected sections of them is
discussed in Section 4.4.
4.1.4 Summary and Cntique of the Methods for Acquiring Coordinates
from Data on File
The range of accuracies and approximations to the costs for collecting data by
each of the methods discussed above is summarized in Table 4.1(a) and pre-
sented graphically in Figure 4.1. From the "comments" section of the table
it is apparent that none of these methods of using existing data will provide
coordinates of high quality for any significant number of points. Thus it
would appear that the existing data can only support spatial analysis at re-
gional scales; therefore, it is recommended that the least costly method be
used. The method of digitizing PLSS corners from 1:24,000 maps seems to
provide the accuracy needed for most spatial analytical purposes. However,
as these methods do not satisfy multipurpose cadastre standards (Committee
on Geodesy, 1980), there would appear to be little need to apply them on a
regional basis for this purpose.
4.2 ACQUISITION OF COORDINATES FROM ONGOING SURVEYS
In the preceding sections various methods of acquiring coordinates for PLSS
corners from existing files in either the NMD or the BLM or other archives
were discussed, and it was concluded that consistent accurate coordinates could
not be obtained from any of these sources. In this section the various methods
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Technical and Economic Considerations
39
of obtaining coordinates with an accuracy suitable for a multipurpose cadastre
are discussed. With minor exceptions the discussion in this section applies to
methods for use in the future, and only in special situations will acquisition
of coordinates from archival data be considered.
4.2.1 Prerequisites: Primary Geodetic Control, Monumentation of Corners,
and Clearing Line of Sight
Although there has been extensive densif~cation of the primary geodetic net-
work within the PLSS states, it cannot be assumed that a sufficiently dense
secondary network is available, and, in general, it must be assumed that only
the primary geodetic framework is available. Thus, the procedures and the
costs discussed pertain to the densification considered necessary to provide
adequate control for the method of positioning the section corners.
In view of the legal status of section corners, it is implicit that only quali-
f~ed land surveyors can decide whether the monument at a corner is in its
original location and hence useful and usable as a boundary marker. In the
subsequent discussion it will be assumed that each corner to be coordinated
will be a firmly emplaced monument at the proper point such as to make it
legal as a boundary marker. It will also be assumed that in wooded areas
sufficient clearing will have been done around the station to give line-of-
sight access where necessary. It is recognized that clearing, retracement, and
monumentation certified as being a correct resurvey may cost well in excess
of $1000 per corner. Nevertheless, it will be assumed that this, if necessary,
has been done prior to the establishment of coordinates for the corner.
The term "mixed technology" is used to describe those procedures where-
by some geodetic densification must be done along with some other method
of tying in the individual section corners.
4.2.2 Mixed Technologies
4.2.2.1 Mixed Technology I: Geodetic and Land-Survey Methods
With mixed geodetic and land-survey technology, the geodetic ties to the out-
side corners of each township are made by geodetic methods and the internal
ties to the section corners are made using land-survey methods. In other
words, the ties between township corners are those that would be done in
retracement or resurvey of the section lines for boundary definition purposes.
Hendrix (1981) discusses three different methods of making the ties from
primary networks to township corners; his results are summarized in Table
4.2. Here it is noteworthy that, though the cost per point is on the order of
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40 MODERNIZATION OF THE PUBLIC LAND SURVEY SYSTEM
TABLE 4.2 Recapitulation of Cost Dataa
Cost per Point
Method
Canon City Georgetown Average
$1333 $1024 $1178
n.a. 839 839(?)
Conventional
Electronic traverse using
Zeiss Elta 2
Satellite Doppler point positioning 885 628 756
using Magnavox MX1502
aFrom Hendrix, Table IV.
$1000, Hendrix states that there is no problem meeting the specifications of a
3-m position on the SE corner of a project using these methods. Although the
report does not give any definition for the 3-m position, we will assume that
this means a standard deviation of 3 m; for points 6 miles apart this is approx-
imately 1 :2000, which is well below the accuracy specified for Third Order,
Class II, control densification. On the assumption that these points should be
adequate for controlling large-scale maps, be they topographic or cadastral,
i.e., as a foundation for a multipurpose cadastre, an accuracy of at least
1:10,000 would be required. Thus, presumably some increase in cost would
be involved, although the general state of the art is such that any one of the
following methods is capable of giving closures of 1:10,000 as required for
Third Order, Class I: conventional, electronic traverse using the Zeiss Elta 2
satellite Doppler point positioning using Magnavox MX1502, and inertial.
As the Cadastral Survey Division of the BLM works to survey specifica-
tions, i.e., to "closure requirements," they are not able to assess the accuracy
of the points that they establish.
We recommend that the Cadastral Survey Division of the Bureau of Land
Management express results, especially ties to township comers, in terms of
point position accuracies as well as closures.
4.2.2.2 Mixed Technology II: Geodetic and Photogrammetric Methods
In an effort to address the requirements of densification for a multipurpose
cadastre the National Geodetic Survey (NGS) undertook an extensive test
project in Ada County, Idaho, in 1978 and 1979. As for most mixed tech-
nologies, this was done in two stages. First, the geodetic control was densified
by conventional geodetic methods, and then the positioning of the section
corners was done by photogrammetric methods. Using data from Gergen
(1981), Hamilton (1981), Lucas (1981), and Perry (1981), costs are estimated
at $1150 per section corner. Approximately one third ($380) was the pro-
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Technical and Economic Considerations
41
rated cost for geodetic control densification and the balance ($770) was for
the photogrammetric determination of the coordinates. [See Table 4.1(b).]
This can best be described as a tour de force in which photogrammetric tech-
nology was pushed to its ultimate limit of accuracy. It did meet the require-
ments for second-order control, but as shown in Table 4.1(b), the costs were
high.
This demanding experiment, however, does not preclude the use of more
conventional aerotriangulation procedures for positioning section corners
(Swann, 1981) and property corners in sparsely settled areas.
4.2.2.3 Mined Technology III: Geodetic and Inertial Technology
Doppler satellite positioning of township corners and inertial technology
positioning of intervening section corners have been used in many places with
satisfactory results (Doxey, 1977~. It is a method that is comparable with the
methods discussed above.
4.2.3 New Technology
4.2.3.1 Inert~alPositioning
An inertial positioning system is a self-contained, automated, and independent
inertial measurement system capable of instantly acquiring geodetic data
(Cole, 1980~. Various accuracies have been quoted (Babbage, 1977; Hadfield,
1977~; they normally range from 20 to 50 cm over a distance of 40 km. Cost
factors depend on the terrain being surveyed, the vehicle being utilized to
transport the system, and the density of control. Costs range from $350 to
$1000 per point; but these costs are drastically reduced with increased density
(Wickham, 1977~.
4.2.3.2 GlobalPositioning System (Interferometr~cMethodJ
Researchers at the Massachusetts Institute of Technology (Counselman et al.,
1982) and, independently, a group at the Jet Propulsion Laboratory (Mac-
Doran et al., 1982) report the development of instruments based on the inter-
ferometric principle using GPS satellites, which give accuracies to 0.1 ft
rapidly and at a projected modest cost. A merit of this technology is that it
would be suitable for occupying section corners without any further densifi-
cation of the primary geodetic network being required. Although developers
of new technology invariably are overoptimistic about both accuracy and
costs, nevertheless, there is every indication that the principle is sound and
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42 MODERNIZATION OF THE PUBLIC LAND SURVEY SYSTEM
that the potential to achieve what they are claiming is realistic. Eliminating
the requirement for geodetic densif~cation gives this methodology a distinct
advantage.
Although costs of approximately $400 per point are claimed, it is more
realistic to recognize that there are certain irreducible transportation and
manpower costs and that, regardless of the efficiency of the equipment, these
irreducible costs will persist. Thus, costs in the range of 5700 to $1000 are
suggested in Table 4.1(b).
4.2.4 Summary and Critique
It is implicit in this section that most of the coordinate determinations would
be done as part of an ongoing operational program whether by the NGS,
NMD, BLM, or other government agency or private firm. This aspect is dis-
cussed further in the scenario in Section 4.4.
In this section, the data are summarized in Table 4.1 (a), (b), and (c) and
presented graphically in Figure 4.1. From the table and the figure it is ap-
parent that accuracy varies from less than 0.1 ft to greater than 100 ft. i.e.,
about 3 orders of magnitude; similarly, costs vary from less than $1.00 to
more than $1000 per corner—again, more than 3 orders of magnitude.
It is recommended that all agencies and firms assess and maintain records
on the accuracy and unit cost of the coordinates they derive, including over-
head costs.
4.3 COMPARISON OF COSTS AND ACCURACIES
The multipurpose cadastre requirements (Committee on Geodesy, 1982) are
for accuracies better than +0.35 ft for urban and suburban areas. For rural
areas this requirement may be relaxed to 1-2 ft; however, we recognize that
this requirement needs further refinement. In Section 3.5 it is indicated that
for spatial analysis at regional scales map accuracies (+40 ft) are meeting
current user requirements. From this it is apparent that there are two emerg-
ing user groups, widely separated on the necessary accuracy with respect to
PLSS coordinates. This study did not identify a significant user group for
PLSS coordinate data between the multipurpose cadastre requirements and
the 40-ft accuracy range.
We conclude that, unless some agency orgroup has a specific cost-effective
requirement for PLSS coordinate accuracy in the range from + 0.35 It to +40
ft. there is no merit in using resources to get PLSS coordinates within this
range.
.~
OCR for page 43
Technical and Economic Considerations
4.4 A REALISTIC SCENARIO AND RECOMMENDATIONS
43
If all 2.8 million PLSS corners were to be monumented and coordinated to an
accuracy suitable for the multipurpose cadastre, the cost would be in excess
of $2 billion in 1980 dollars. Although it is not realistic to advocate a crash
program costing $2 billion, it is realistic to develop a strategy that will in time
meet the requirements of the multipurpose cadastre.
By a program of ongoing integrated surveying, i.e., by putting appropriate
coordinates on PLSS corners whenever possible and by establishing a national
data base for PLSS coordinates, In the long term the goal will be achieved.
For example, from time to time the NMD has occasion to establish additional
control for mapping; if they put as much of this control as possible on PLSS
monuments, they will be contributing to the multipurpose cadastre and
hence, over the long term, contributing to their own requirements for coordi-
nates of many features being mapped. Similarly, the NGS strengthens and
densities the geodetic network from time to time. If they, too, use existing
PLSS monuments wherever possible, they will be contributing to the multi-
purpose cadastre. Both of the above activities are, of course, dependent on a
secure monument's being in place at a section corner. Therefore, the BLM
should be placing good quality monuments at every section corner that Is
reoccupied. This activity, in itself, will go a long way toward maintaining this
valuable national resource—the Public Land Survey System.
Noting that the Bureau of Land Management (ELMS is using potentially
precise instrumentation to establish the positions of PLSS corners, it is
recommended that BLM determine these positions with sufficient accuracy
to serve as control for the multipurpose cadastre.
Many other agencies, industries, and private practitioners determine coord-
inates for PLSS corners and all are encouraged to establish control of quality
suitable for multipurpose use. Municipal governments and counties frequently
density control and, fortunately, usually do include section corners in their
networks.
As an encouragement for these activities, it is highly desirable to establish
a national policy on the responsibility for a data base of geodetic-quality
coordinates on PLSS corners. Initially it is suggested that the Cadastral Sur-
vey Division of BEM take this responsibility, recognizing that In the the
responsibility for part of it may devolve to state, county, or municipal govern-
ment agencies. We therefore recommend the following:
1. The National Mapping Division, National Geodetic Survey, Bureau of
Land Management, and all other agencies having responsibility or need for
densified geodetic control should undertake to put control on PLSS corner
monuments wherever and whenever possible. It is also recommended that the
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44 MODERNIZATION OF THE PUBLIC LAND SURVEY SYSTEM
accuracy of the control so placed meet the minimum standards to serve the
multipurpose cadastre.
2. The Bureau of Land Management and other agencies and property own-
ers responsible for property abuttingsection lines should ensure that all section
corners and quarter-section comers on their boundary be monumented with
monumentation equivalent to or better than that specified for Third-Order
Control monumentation.
Representative terms from entire chapter:
multipurpose cadastre
