The California Surveyor - CLSA

coordinates are used for a GIS, the
software must handle the ellipsoid
rigorously. This means carrying
adequate terms beyond the first
term in the computation equations
and using adequate floating point
settings in computer hardware
and software to ensure accurate
computations.
True geodetic coordinates should
be based on the North America
Datum 1983 (NAD 83) which is in
turn based on the World Geodetic
System 1984 (WGS 84).
Other reasons for shifting to latitude
and longitude have to do with
the end users of GIS, the public,
and the coming dominance of GPS
as the positioning system of choice
for both the public and surveyors.
1. The concept of latitude and
longitude is easily explained
to layman and decision
makers, the state plane
coordinate projection
systems are not.
2. The concept of latitude and
longitude is not associated
with coordinate geometry by
the layman. They associate it
with their geography and
history classes, unlike coordinate
geometry which is
associated with their high
school mathematics classes.
Why build in resistance to
GIS because of the use of
certain types of esoteric
coordinate systems when the
whole problem can be avoided.
3. The layman will be quite
familiar with latitude and
longitude from using GPS in
their cars, boats and airplanes,
in fact the operating range
of these common vehicles will
necessitate using a latitudelongitude
system because state
plane coordinate systems are
designed for relatively limited
geographic areas.
4. Latitude and longitude are
associated with exploration
and adventure, a GIS in
latitude and longitude will
relate to the entire globe rather
than a limited island on it.
5. Modern computers make
rigorous latitude and longitude
computations an easy task to
perform. The reason for the
development of state plane
coordinate systems in the 1940's
was due to the lack of computation
machines capable of
handling geodetic latitude and
longitude. The state plane
systems have served us well
for almost fifty years, but it is
time to leave them behind for
mapping. Surveyors will
continue to use the system for
smaller projects because the
units relate to construction and
local boundary solutions more
directly (feet or meters rather
than degrees, minutes and
seconds), but the conversion
back and forth is easily
accomplished by surveyors and
is of little concern to the
layman.
The North American Vertical Datum
of 1988 (NAVD 88) should be
adopted as quickly as possible, however,
only 25% of NGS control in
California has been upgraded from
the 1929 adjustment. Very little of
the USGS and none of the Corps of
Engr. vertical control has been included.
This is an area that must be
approached with caution since there
is roughly a meter difference between
the two datums in many parts of
California.
All coordinate sets must clearly
state the datum and date of readjustment
as a permanent part of the
record.
To properly fit measurements into
a three dimensional geodetic coordinate
system (ellipsoid), especially elevations,
control station heights,
heights of instruments, heights of
targets, prisms or antennas must be
included as part of the measurement
database or be derived from
processing measurements in the
database.
GIS project internal primary survey
control points should at least
meet 1st or 2nd order FGCC distance
precision ratios (depending on
the distance between points which is
a function of the degree of densification
the survey control net has
undergone). However, the exterior
of the project polygon should have
its primary stations tied to the GPS
High Precision Geodetic Network
(HPGN) to ensure accurate matching
(edge matching) to adjoining GIS
projects. The HPGN is many degrees
of precision beyond 1st order (a standard
developed for conventional
survey instruments). Tieing to the
HPGN will allow the blocking out of
geographic areas by coordinate definition
as spheres of influence for
each project with assurance that later
attempts to merge data will have a
minimum of fit problems.
Applying FGCC and Other
National Agency Standards
Great care must be used in phrasing
referral in legislation to the FGCC
accuracy classification standards and
specifications. Few surveyors and/
or laymen or decision makers are
aware that these standards and specifications
encompass far more than
simple precision ratios (1:50,000 etc.).
The FGCC standards and specifications
are based on over a hundred
years experience building and maintaining
a vast continental survey
control network. It is an industrial
grade document meant to produce
control measurements and control
stations that will endure for generations
and as such it is far beyond the
needs of the average GIS in its
totality.
The FGCC accuracy standard encompasses
minimum distance precision
ratios between any two points,
it is intended primarily for survey
networks (extensive cross connections
between points) not traverses,
sets forth minimum distances between
any two points according to
which accuracy class is specified,
specifies monument standards for
accuracy classes, indicates the form
of station descriptions and specifications
for reference monuments, specifies
minimum procedures for each
type of measurement instrumentation,
specifies data formats and the
method of data reduction and adjustment
including the parameters
that must be included during geodetic
least squares adjustment of
conventional or GPS data.
Even a third order point is an
22 The California Surveyor Fall 1992