P - CLSA

Developing Efficient Survey Techniques
James K. Crossfield, Ph.D
Associatie Professor, Department
of Civil and Surveyng Engineering,
California State University, Fresno,
Fresno, CA 93740
(Continued from Spring Issue)
The Real Property Connection
Perhaps the most significant driving
force behind the burgeoning
number of computerized land information
systems is the lure of
avoiding duplication and waste by
having one department or agency,
rather than several, create and
maintain the maps and related land
records information. Efforts to keep
costs down in such new systems
often lead to development of quickfix,
spatially imprecise systems that
satisfy only some of the system requirements.
More precise spatial requirements
for future large scale
mapping, land ownership records,
or engineering design requirements
are often not attainable in this
scenario. Thus, a system that cannot
avoid redesign may not have
avoided duplication.
The term "spatial imprecision,"
as used here, is a measure of the
distance between the location of a
point as recorded in the computer
and the location of that point on the
ground. Unfortunately, most computerized
land records information
systems disregard spatial precision
in their mapping applications by using
one or more of the following
techniques:
1. "Rubber sheeting" to stretch
and/or distort map data;
2. Arbitrary elimination of apparent
overlaps and gaps that
are fully documented as existent
in written legal records;
3. Digitizing maps of unknown
reliability to create computer
map data having an even
greater lack of spatial precision.
Typical land records information
systems create digitized boundary
files from USGS IV2 minute
topographic map sheets and existing
assessors' maps. The positional
uncertainty of any USGS
map point is typically about 40',
while the typical assessor's map
contains about 10' of spatial im-
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precision at every map point. Public
decisions about private propertj^^
rights should not be made when th^^^
spatial uncertainty of the system
may be larger than some owners'
properties. A sample survey conducted
in Spring 1985 by surveying
students at California State University,
Fresno, found that property
owners were tolerant of no more
than a one-half foot error in the location
of their boundary lines.
The following list summarizes
several important facts associated
with the increasingly popular computerization
of land records information
systems:
1. Modern computerized land
records information systems
are necessary so that society
can correctly and quickly
answer fundamental questions
about public and private
rights and responsibilities to
land.
2. The common thread linking all
types of environmental,
resource, and ownership
aspects of land is the
geographical location of thesj^^
items on the ground. Thos^^
who deal with ground coordinates
on a daily basis are the
individuals who must provide
guidance for the development
of spatially reliable computerized
land records information
systems.
3. Modernized land records information
systems must be
designed from the ground up
with positional data of known
quality used to create each
new overlay of information.
4. The spatial reliabihty of all
data sets must be known at all
times so that land ownership
rights and responsibilities are
not incorrectly determined
because of imprecise information.
Fundamentally, then, the
cadastral overlay portion of
multipurpose land information
systems must be designed to include
the spatial and written components
of the entire collection of
deed, abstract, and land survei^
records relative to each parcel c^B
land. To do less would run counter
to long-estabUshed common law
(Continued on page 8)
Page 6 The California Surveyor — Summer, 1986