The California Surveyor - CLSA
The California Surveyor - CLSA
The California Surveyor - CLSA
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Report To GIS<br />
Task Force<br />
Editor's Note : In 1990 <strong>CLSA</strong> introduced legislation<br />
on CIS/LIS. That legislation was<br />
modified to create a statewide Task Force to<br />
review and make recommendations on CIS<br />
standards and use within the State of <strong>California</strong>.<br />
<strong>The</strong> Task Force represents a number<br />
of state agencies, federal agencies, universities,<br />
counties, cities and interested associations.<br />
Howard Brunner was appointed by the<br />
Governor to represent <strong>CLSA</strong>. Howard recommended<br />
and Joe was appointed to the Data<br />
Development Technical Advisory Committee.<br />
<strong>The</strong> following are some excerpts from<br />
Joe's report to the Task Force. This report<br />
discusses in detail the implications of CIS<br />
systems to surveyors and recommends<br />
specific guidelines for implementation in<br />
CIS. Although this is a technical report to the<br />
Task Force, it contains valuable information<br />
for surveyors who are assisting local agencies<br />
to establish CIS or are having to provide<br />
data to agencies for use within a CIS/LIS.<br />
<strong>The</strong> lead State agency in CIS/LIS is the<br />
Teale Data Center. Although not an agency<br />
surveyors are used to dealing with or have<br />
even heard of, it is positioned to be the<br />
dominant force in CIS/LIS for the State of<br />
<strong>California</strong>. See the article "<strong>The</strong> Stephen P.<br />
Teale Data Center" on page 23 for an<br />
overview of this agency.<br />
By Joseph W. Betit, PLS<br />
Legal Considerations for<br />
Information Providers<br />
<strong>Surveyor</strong>s think in terms of reliably<br />
transmitting information related to<br />
land and property rights over long<br />
periods of time (often generations)<br />
and in terms of liability to their direct<br />
client as well as possible third<br />
parties subsequent in time to the<br />
original performance of work.<br />
<strong>The</strong> correctness of a map or data<br />
is no guarantee against misinterpretation<br />
or misapplication of data once<br />
the data is in the hands of others.<br />
GIS are dynamic systems and the<br />
inevitable changes made to the<br />
database over time create an environment<br />
for errors to occur and<br />
propagate, unlike a paper map that<br />
is a static data product. Furthermore,<br />
if data is sold in any format (paper,<br />
digital, etc.) as a product (versus<br />
delivered to a sole client for their<br />
sole use) and the data is subsequently<br />
found unsafe or dangerous<br />
for its intended use, United States<br />
product liability laws provide for<br />
strict liability for the seller of the<br />
product. <strong>The</strong>re is case law in which<br />
inaccurate graphic representation of<br />
accurate text data contained on the<br />
same chart was cause for substantial<br />
damages. In a GIS environment this<br />
would be similar to an inaccurate<br />
map composition being plotted along<br />
with the accurate data tables on the<br />
same sheet.<br />
<strong>The</strong> current approach in law today<br />
is to name anyone even remotely<br />
connected with a complaint by a<br />
plaintiff and thereby force the reluctant<br />
defendants to fight or buy their<br />
way out of litigation. This is an expensive<br />
process, especially for the<br />
innocent professional. If a licensed<br />
professional seems overly anxious<br />
about possible legal repercussions<br />
from the performance of work or<br />
delivery of product to other than the<br />
immediate client it is for good reasons<br />
based on hard experience with<br />
our legal system.<br />
Measurement Database<br />
Focus vs<br />
Coordinate Focus<br />
Coordinates change, measurements<br />
don't. Coordinates are a reflection in<br />
a desired ellipsoid or projection system<br />
of the currently selected or available<br />
reduced survey measurements<br />
contained in a database. Measurements<br />
between two points are often<br />
of variable quality (depending on<br />
measurement instrumentation used).<br />
<strong>The</strong>se differences in quality are indicated<br />
by the magnitude of the Standard<br />
Error (SE) attribute associated<br />
with each data element.<br />
<strong>The</strong> most practical way to build<br />
large survey nets for GIS systems is<br />
to use an evolutionary approach<br />
based on a measurement database<br />
that undergoes periodic least squares<br />
adjustment. <strong>The</strong> general plan for the<br />
net should include a timeline for<br />
long term remonumentation,<br />
monumentation densification and<br />
measurement upgrades.<br />
Periodic readjustments of a measurement<br />
database will result in new<br />
coordinate opinions for existing<br />
points. <strong>The</strong>refore, the following items<br />
should be recorded in the database<br />
lineage log as a part of the adjustment<br />
record; the time and date of<br />
the adjustment, the person supervising<br />
the adjustment, the type of<br />
adjustment (least squares is not the<br />
only rigorous methodology), the<br />
parameters (points held fixed,<br />
datum used, etc.).<br />
Coordinate based systems are subject<br />
to the following problem areas:<br />
1. Flexibility in representing<br />
information accurately in<br />
different coordinate systems<br />
can be difficult, especially if<br />
the desired coordinate system<br />
is not directly related to the<br />
initial coordinate datum (eg.<br />
NAD 1927 vs NAD 1983).<br />
It is much easier to regenerate<br />
new coordinates in the actual<br />
new ellipsoid model from<br />
raw measurements than it is<br />
to try and use a transformation<br />
program that uses best<br />
fit techniques.<br />
2. <strong>The</strong> coding scheme for<br />
attribute data used to track<br />
changes can become cumbersome<br />
as coordinate values<br />
are updated over time.<br />
3. Integration of coordinate data<br />
sets from diverse sources<br />
may require cumbersome<br />
translation or transformation<br />
techniques, as well as rubber<br />
sheeting in order to be<br />
integrated with the coordinate<br />
data within the existing<br />
(different) coordinate<br />
reference system.<br />
Some of the derived products of a<br />
measurement database:<br />
• Generate parcel descriptions<br />
directly.<br />
• Generate annotations typical<br />
for each type of map use<br />
automatically.<br />
20 <strong>The</strong> <strong>California</strong> <strong>Surveyor</strong> Fall 1992