The California Surveyor - CLSA

• Base maps of project areas
can be extracted for proposed
project area and the SE
analyzed to determine if
existing survey data is of
sufficient quality to justify
avoiding resurvey.
• Indices to adjacent survey
and title data can be generated
• Parcel history can be compiled
using tabular & graphic outputs.
• Evolutionary survey
measurement and polygon
database production.
Some other uses of a GIS survey
database advocated by the Bureau
of Land Management would be researching
original survey work, correlating
patterns and quality of work
accomplishment, reliability estimates
of positional and measurement data,
and providing supporting control
network and graphic representation.
Finally, as pointed out by BLM
writers, any measuring system must
be tightly coupled to supporting
documentation (original material and
summaries) to maintain the integrity
of the measurements and provide
future generations with a basis to
judge the quality and value of the
work.
Such things as the character of the
found or set monument, the procedures
and type of equipment used,
who performed the work and their
qualifications are all important data
that needs to be associated with each
point measurement.
Need for GIS to
Distinguish between
Measurements vs
"Psuedomeasurements"
A GIS has the ability to create,
manipulate and query polygons.
Parcel boundaries, easements and
other property descriptions are actually
derived mathematically
(pseudomeasurements). That is, parcel
dimensions are derived from
survey field measurement ties made
to found or set survey boundary
monuments (angle points in parcel
polygon). These field survey ties are
made from the control points of the
survey control network.
Lines are very seldom run directly
between monuments using today's
field procedures. Instead, inverses
are later computed in the office between
the appropriate survey point
coordinate data. These inverses then
become the sides of the closed
polygons (legal descriptions) constituting
legal parcels.
During the subdivision design
process further fragmentation
(streets, lots, etc.) of the primary
polygons are created, and these
created parcel lines are also
pseudomeasurements. Furthermore,
a boundary survey can have multiple
opinions (all valid), depending
on the found evidence held and the
particular application sequence and
weight assigned to the rules of legislative
law and court case law that
are applied. It is therefore critical
that any supporting control survey
layer be 'read only' and subject to
change or modification only with the
full knowledge and participation of
the land surveyors managing
and/or providing the information to
the system.
At the local level, the individual
land parcels and descriptions of
rights in land impacting those parcels
are of primary concern.
Interestingly, the configuration of the
many types of polygons and the
attributes attached to the polygons
are derived from the primary
pseudomeasurements (primary parcel
boundaries), which are directly
linked to the survey control layer.
Many of these subsequent polygons
are a result of map analysis
using predetermined criteria or a
result of the application of rules
concerning legally defined spheres
of influence. These subsequent polygons
are subject to legislative or ministerial
change outside of the normal
land transfer or subdivision process.
Therefore a hierarchy of polygon
data needs to be established in a GIS
with the land ownership polygon
system being the primary polygon
base and the most protected from
unauthorized change.
The integrity of the spatial database
link to the information base,
that is the extent to which any real
world feature is actually located at
the absolute position represented on
a paper map or computer graphics
equivalent, may affect the ability of
the jurisdiction to rely on spatial
queries and analysis for decision
making processes.
Audit Trails for Survey
Control Layer of Gl
Consider using a keyboard and input
device command capture buffer
to log operator commands to the
system as a way to monitor changes
made to original data sets. The current
practice of simply backing up a
changed data set does not reflect how
and where and when changes were
made and requires large amounts of
redundant storage space. Storing
operator commands would then require
that only three files be saved.
The original data sets (archived), the
command log and the current state
of the database. All other intermediate
states can be regenerated if necessary
by automated reprocessing of
the operator commands to the point
in time that is of interest.
Small changes to a GIS are often
difficult to retrace or even find, especially
when polygons have been
dissolved and rebuilt. A change log
would be much easier to decipher
than the GIS database itself.
Need for Use of True
Geodetic Coordinate
Systems in a GIS
Current GIS software treats coordinate
data as 'flat world', or plane
coordinates and this is inappropriate
for larger GIS such as for regional,
and statewide systems (Office
of Planning and Research,
CalTrans). The failure to use true
geodetic coordinates for large areas
will result in poor correlation between
the survey point coordinates
and related physical features due to
scaling distortions.
The Public Resources code needs
to be changed to require the use of
true geodetic coordinates (latitude
and longitude) for GIS that encompass
areas larger than one county, or
that incorporate areas of multiple
counties or that adjoin other states
(Oregon, Nevada, Arizona) or other
countries (Mexico).
If State Plane Coordinates or
Universal Transverse Mercator
Fall 1992 The California Surveyor 21