Underground Facility Mapping - Benefits Of GPS
Case Studies
Huntimzton Beach, CA
Psomas assisted Huntington Beach in the development of an enterprise wide GIS
that included water, sewer, and storm utility features. The city acquired a parcel
landbase from Orange County that was compiled using GPS and COGO methods.
This base is ideal for GPS survey integration due to its high accuracy. Huntington
Beach did not want facilities pre-marked due to traffic disruption and citizen
complaints that arose when surrounding cities used pre-marking. Additionally,
they wanted a 100 percent inventory of facilities to form a foundation for automated
facilities management maintenance systems.
Psomas located facilities throughout the 28 square mile (67-square-kilometer) urban
area using RTK GPS surveying and other survey methods. Busy street areas were
surveyed with two person crews using conventional surveying methods due to
safety concerns. A total of 29,000 facilities were located by the field surveyors
locating every point they identified while walking the streets. This method resulted
in the capture of approximately 5,000 more points than were shown on existing atlas
maps. This 100% inventory approach demonstrated where the existing atlas maps
needed updating.
Huntington Beach’s stringent needs proved GPS to be the most appropriate
collection method for locating facilities.
- Required 100% inventory of facilities
- Positional accuracy of points had to meet or exceed 0.5 feet
- An existing landbase supported high accuracy GPS facility mapping
- The city-wide GIS required spatial consistency among all layers. Map sources had
various spatial quality and were not suited for digitizing.
Los Arweles, CA
Psomas was selected to map the City of Los Angeles sanitary sewer system. The city
has one of the worlds largest system and is mapped on 5,800 sewer atlas sheets. An
automated Sewer Information Management Maintenance System (SIMMS) is used
to manage an inventory of all facilities and track maintenance activities. The
Bureau of Engineering required development of a GIS to improve the management
of the sewer atlas system.
Digitizing of the sewer atlas maps was chosen as the most appropriate method for
facility conversion for several reasons.
- Existing atlas maps are reasonably complete and uniformly spatially accurate
- The atlas maps align with the existing digital landbase
- Digitizing from atlas maps is the lowest cost method
- Many people can concurrently work on the conversion to minimize conversion
time
- High traffic volumes in Los Angeles make in-street GPS surveying dangerous
and increases costs due to safety precautions
Capistrano Vallev Water District, CA
Capistrano Valley Water District is developing a GIS system that will improve the
mapping quality of the District and support other District activities such as capacity
modeling and facility management maintenance activities. Psomas performed
engineering drawing conversion for the pilot project. The District does not have a
map atlas. The facility records are represented by more than 2,000 engineering
drawings at 1“ = 40’. These plans vary in age from the early 1920’s to current. The
content and physical quality of the source documents are also variable. A significant
number of the drawings supersede other drawings. The District has acquired a high
accuracy landbase from Orange County and wishes to map all facilities onto this base
and create a uniform atlas map.
A COGO methodology was selected for compiling the atlas due to the following
reasons.
- The high accuracy landbase provided an excellent framework to locate water
facilities based on offsets from centerlines and record distances.
- Costs had to be kept to a minimum while still achieving a spatial accuracy that
would meet the District’s needs for the foreseeable future.
- GPS surveying would be complicated due to the lack of a map atlas that could
guide the surveyors throughout the District.
Selecting A Methodology
The methods of facility conversion discussed are each appropriate for specific
circumstances. Utilities planning a facility conversion must begin with a definition
of requirements and an assessment of existing record quality prior to selecting a
conversion methodology.
Digitizing is appropriate when existing atlas maps are accurate and complete.
COGO is an excellent method for defining the infrastructure when no atlas map is
available. A high quality land base with street centerlines is necessary. The large
number of engineering drawings that must be managed may drive costs up. COGO
is an effective maintenance tool when new tract maps are created.
Photomammetrv is appropriate for building a planimetric basemap and orthophoto
base. Collection of utility locations is problematic due to the need to pre-mark
utility locations. Updates using photogrammetry are not cost effective.
QS provides an effective means of collecting any number of points. It works well
for maintenance updates as well as initial map conversion. Conventional survey
techniques may be needed to supplement the GPS.
As mapping technology advances, the demand for higher accuracy data will rise.
The use of field data collection devices connected to GPS receivers allows for the
rapid and direct integration with GIS data. Long term integration of GIS with other
applications, and the impact of data maintenance should be a major factor when
deciding on a utility conversion approach. No method is best for all cases. Careful
consideration of needs at the beginning of a GIS conversion project will help assure
the investment in data will be useful for decades.
About The Author
Mr. Craig H. Gooch works for Psomas and Associates in California. He is a Senior
GIS Consultant specializing in business process re-engineering, data and process
design, requirements analysis, and GIS implementation management.
