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. Plan Characteristics
Atlas maps represent the utility infrastructure as a continuous map composed of tiled map sheets. The atlas is intended to represent the current state of the network. The maps generally do not represent utility features redundantly except at map sheet edges where sheet-to-sheet overlap may be shown. The mapping symbology and information content is usually consistent across the entire service area. Unique facility feature identifiers may be shown on the maps to associate tabular maintenance information. The underlying basemap of the atlas maps maybe accurately mapped using photogrammetric methods or precision coordinate geometry (COGO) methods. Often however, the basemap may not be spatially accurate, having been developed with hand drafting techniques not designed to result in a precision map. Atlas Characteristics
Digitizing Map digitizing is usually the lowest cost method of inputting the utility infrastructure into a GIS format U the source maps are sufficiently accurate to meet the needs of the GIS. Digitizing directly from source documents will replicate existing inaccuracies. Often the sources are not accurate enough and require re-drafting onto an accurate base prior to digitizing. The re-drafting process is tedious, time consuming, and may result in data of inconsistent and unknown positional accuracy. The re-drafting process requires interpretation of utility locations onto a new map base through scaling that is prone to judgment and process errors. The re-drafting process is relatively fast, can be performed by low skilled technicians, and much of it can be performed without a workstation. The resulting positional accuracy confidence is moderate but often adequate for many needs. COGO COGO is a high precision method for inputting the location of facilities based on drawing measurements. Facility location definitions found on record drawings often reference street centerlines, easements, or other land base features. Because of this, effective COGO development of a facility network requires a highly accurate landbase with street centerlines, rights of way, and easements. The results achieved from calculating the facility locations can be excellent when an accurate land base is available. Costs associated with COGO input are high compared to other techniques. The skill level of the data entry operator must be high and a computer workstation is needed for all phases of the work. Working from many engineering plans requires a significant amount of document handling when trying to determine where each sheet connects to other sheets. The engineering documents may only reflect the network at the time of initial design or installation. Subsequent updates may not be shown on engineering drawings, and if shown on atlas maps, they may lack sufficient information to calculate the location of features with precision. Older utility districts that have been developed and maintained over a long time frame may have source documents that replace earlier documents. Careful sorting and evaluation of documents prior to conversion adds to the effort further. Photogrammetry Photogrammetric mapping is an effective method of developing land based maps of known accuracy. Orthophoto mapping or linear mapping of planimetric features provide an excellent backdrop for many utility applications. Mapping underground 257.utility features with photogrammetric methods requires pre-marking surface facility locations so they are visible on the aerial photography. Lines connecting the surface features (valves, maintenance holes, hydrants, etc.) can be drawn quickly once the features have been accurately located. This method canresult in very accurate mapping, however there are several significant drawbacks. Pre-marking of surface features is costly, time consuming, becomes “street graffiti”, and irritates the public. Any facility components obscured by trees, vehicles or other obstruction cannot be located. Aside from these significant problems, the mapping can result in a highly accurate map of facilities. Information consistency between the land base and utility may be excellent if both are developed from the same photography. Photogrammetry and GPS techniques provide methods of precise positioning of features visible on the ground surface. Other features such as bends and crosses must be mapped from as-builts or atlas drawings. The orientation of pipes and the manner in which they connect to valves and hydrants must also be defined by the atlas or as-built drawings. GPS/Digitize GPS provides a rapid means of field surveying utility locations. Real time kinematic (RTK) positioning GPS technique enables centimeter-level positioning instantly at the time of observation. This process of field surveying can replace photogrammetric utility positioning and cost effectively locate every utility when coupled with conventional survey techniques. Moreover, maintaining consistent positioning of added or relocated facility components can easily be performed to a consistent accuracy specification. For initial data collection during facility conversion, the project must be laid out and a plan developed for visiting every utility feature visible at ground surface. A GPS base station is established, then mobile units carried in backpacks by surveyors are carried to each valve, maintenance hole and other utility feature. The surveyor sets up a prism pole with a GF’S antenna over each utility feature. Centimeter level positioning can be achieved during one minute of GPS observation data. During this time, the surveyor associates feature codes identifying the feature. Daily production rates vary depending on the density of features. The time of point occupation is considerably less than the time to pre-mark for aerial photography. Facility positioning can be accomplished over any period of time with a consistent accuracy of measurement. If cars are parked over a maintenance hole or valve, the surveyor simply returns to the point at another time. By walking the field, all 258.visible utility features are measured. In many cases, utilities not shown on atlas maps are located due to the 100% enumeration of facility features. Conventional surveying must be used where structures obscure satellite visibility. This may be a limiting factor in highly urbanized downtown areas that have tall buildings. The RTK method requires continuous radio transmission between the GPS base station and field unit. At times, interference or loss of signal can cause complications and delays. Mapping of the facility network is similar to the photogrammetric approach once the surface features are surveyed. Digitizing is used to connect pipes between the surveyed features guided by plans and atlas sheets. The reference land base must be highly accurate so that the surveyed locations remain positioned relative to road centerlines and other basemap features. 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.
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.
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 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. | ||
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