Underground Facility Mapping - Benefits Of GPS
Craig H. Gooch
Psomas and Associates 1700 Iowa Avenue, Suite 160
Riverside, California 92507
(909) 787-8421
E-Mail: cgooch@psomas.com
Abstract
Underground facility mapping and basemap conversion into Geographic
Information Systems (GIS) and Computer Aided Drafting (CAD) formats have been
performed predominantly using photogrammetry or map atlas digitizing methods.
Advances in Global Positioning System (GPS) technology during the last few years
have allowed new mapping methods that result in comparable costs, shortened
duration, and very high quality data collection. This paper presents three
underground utility conversion methodologies and contrasts the implications of
each approach. Case studies are used to illustrate different situations, methods,
benefits, and implications.
Photogrammetry, design drawing source conversion, and advanced GPS facility
locating methodologies will be summarized. Guidelines for methodology selection
will be discussed with implications on conversion costs, system integration,
positional accuracy, and ability to achieve overall project goals. A detailed review of
GPS field locating and data collection processes will be presented to clarify the
project conditions where a GPS solution is most favorable.
Introduction
This paper presents alternative mapping methods and the associated implications in
an effort to provide a reference framework for conversion method selection The
long term uses of the resulting utility map must be anticipated and documented
during the needs assessment process and within the database design. Organizations
facing digital map conversion at the beginning of GIS projects must make key
decisions but are least prepared to do so because of their unfamiliarity with
conversion method consequences. Selection of a mapping method for underground
utilities will affect project costs, duration, and resulting database quality.
Utility map record sources are reviewed for their appropriateness for alternate map
conversion methods. The methods will be analyzed and guidelines for selecting an
appropriate method presented. Project case studies are cited to illustrate conversion
method appropriateness.
Utility Map Record Sources
Utility map records consist of maps showing the location and component
definitions of utility networks. These maps or drawings contain tabular
information that define the characteristics of features. The locational and tabular
data is organized, cleaned, and input into the GIS during the conversion process.
Utility record characteristics vary greatly depending on prior mapping practices,
organization of map records, and management of map maintenance information.
Utility atlas sheets may have been developed as a graphic inventory of the utility
network. In other cases, individual engineering plans and as-built sheets may be the
only map source. Engineering plans and atlas maps have different characteristics
that affect map conversions.
When planning the conversion, source documents need to be inventoried and
evaluated. Source document organization, condition, content, and content quality
should greatly influence the mapping method and the resulting quality. Record
drawings provide a very detailed and complete representation of the utility at the
time of development for the project area. However, the large number of drawings,
inconsistent drawing standards, feature redundancy, and probable lack of updated
facility information complicates the conversion effort.
Atlas maps provide a continuous and complete representation of the utility
network similar to how it will be represented within the GIS. However, the
positional information on atlas maps may not be adequate for the GIS and direct
map digitizing may not be appropriate.
Engineering Plans
Engineering plans are developed prior to the construction of new utilities. The area
covered by a set of plans varies depending on the project. They may cover a land
subdivision or a small area representing an upgrade to the existing infrastructure.
Plans often are drawn on multiple sheets that are not drawn as a continuous map.
As a result, the plans form a mosaic of sheets that overlap one another in space and
time, making it difficult to build a continuous GIS Network. Agencies who rely on
engineering plans as their map base spend a significant amount of time searching
for information because of the fragmented nature of the plans.
