GIS Applications for Water, Wastewater, and Stormwater Systems - Chapter 8 ppsx

• Advantages of GIS maps• GIS mapping steps • Mapping case studies LIST OF CHAPTER ACRONYMS AM/FM Automated Mapping/Facilities Management AML Arc Macro Language DRG Digital Raster Graphi

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CHAPTER 8 Mapping

GIS provides powerful and cost-effective tools for creating intelligent maps for water, wastewater, and stormwater systems

A sewer system map created by GIS (Borough of Ramsey, New Jersey).

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• Advantages of GIS maps

• GIS mapping steps

• Mapping case studies

LIST OF CHAPTER ACRONYMS AM/FM Automated Mapping/Facilities Management

AML Arc Macro Language

DRG Digital Raster Graphics (USGS topographic maps)

NAD-27 North American Datum of 1927

NAD-83 North American Datum of 1983

QA/QC Quality Assurance/Quality Control

SPC State Plane Coordinate (Map Projection System)

TIGER Topologically Integrated Geographic Encoding and Referencing System (U.S Census Bureau Mapping System)

UTM Universal Transverse Mercator (Map Projection System)

VBA Visual Basic for Applications

This book focuses on the four main applications of GIS, which are mapping, ing, maintenance, and modeling and are referred to as the “4M applications.” In this chapter

monitor-we will learn how to implement the first m (mapping).

LOS ANGELES COUNTY’S SEWER MAPPING PROGRAM

In the 1980s, the Sanitation Districts of Los Angeles County, California, envisioned

a computerized maintenance management system that would provide decision makerswith essential information about the condition of the collection system A sewer andmanhole database was subsequently developed, but investments in GIS technologywere deferred until the early 1990s when desktop PCs became powerful enough torun sophisticated GIS applications In 1993, a GIS needs analysis study was performed,which recommended implementation of a large-scale enterprise-wide GIS An in-house effort was started to implement the recommendations of the study Severalsections in the Districts formed a project committee to pilot test GIS technology thatcould be duplicated in all 25 districts The pilot project developed a mapping appli-cation for a small district that acted as a front-end to the large database of sewerage

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facilities developed in the 1980s and 1990s At this point, much of the informationwas nonspatial, including multiple databases in a variety of formats and paper maps.Converting this information into GIS proved to be the most time consuming and costlyoperation Creation of the layers for sewers and manholes was the most laborious ofall the layers that had to be created Manhole data were represented by approximately24,000 points or nodes digitized from the paper maps, using a base map The GISlayers were created in CAD software and linked to the legacy databases Once linked,detailed data such as sewer pipe and manhole construction material, size, condition,flow, capacity, and inspection data were available for query and analysis through anintuitive map-driven interface By 2003, the pilot project had grown to become thefirst enterprise-wide solution deployed by the Districts Called “Sewerage FacilitiesGIS,” the system allowed users to access and view data from the legacy databases byselecting sewers and manholes on a map or using standard queries The mappingapplication also provided sewer tracing functionality that proved helpful in delineatingstudy area boundaries for design projects, annotating sewers with flow direction, andtracking potential discharge violations (Christian and Yoshida, 2003).

of uniform size cells called pixels, each with an associated data value Many complexspatial analyses, such as automatic land-use change detection, require raster maps.Raster maps are also commonly used as base maps (described later in this chapter).Existing paper maps that are used to create GIS maps are called source maps.

Topology

Topology is defined as a mathematical procedure for explicitly defining spatialrelationships between features Spatial relationships between connecting or adjacentfeatures, such as a sewer tributary to an outfall or the pipes connected to a valve,which are so obvious to the human eye, must be explicitly defined to make the maps

“intelligent.” A topological GIS can determine conditions of adjacency (what is next

to what), containment (what is enclosed by what), and proximity (how near thing is to something else) Topological relationships allow spatial analysis functions,such as network tracing, that can be used to facilitate development of hydraulicmodels for water and sewer systems

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Map Projections and Coordinate Systems

Because the Earth is round and maps are flat, transferring locations from a curvedsurface to a flat surface requires some coordinate conversion A map projection is amathematical model that transforms (or projects) locations from the curved surface ofthe Earth onto a flat sheet or 2D surface in accordance with certain rules Mercator,Robinson, and Azimuthal are some commonly used projection systems Small-scale(1:24,000 to 1:250,000) GIS data intended for use at the state or national level areprojected using a projection system appropriate for large areas, such as the UniversalTransverse Mercator (UTM) projection The UTM system divides the globe into 60zones, each spanning 6˚ of longitude The origin of each zone is the equator and itscentral meridian X and Y coordinates are stored in meters Large-scale local GIS dataare usually projected using a State Plane Coordinate (SPC) projection in the United States

A datum is a set of parameters defining a coordinate system and a set of controlpoints with geometric properties known either through measurement or calculation.Every datum is based on a spheroid that approximates the shape of Earth The NorthAmerican Datum of 1927 (NAD27) uses the Clarke spheroid of 1866 to representthe shape of the Earth Many technological advances, such as the global positioningsystem (GPS), revealed problems in NAD27, and the North American Datum of

1983 (NAD83) was created to correct those deficiencies NAD83 is based on theGRS80 spheroid, whose origin is located at the Earth’s center of mass The NAD27and NAD83 datum control points can be up to 500 ft apart

Coordinates are used to represent locations on the Earth’s surface relative toother locations A coordinate system is a reference system used to measure hori-zontal and vertical distances on a map A coordinate system is usually defined by

a map projection The GIS and mapping industries use either latitude/longitude- orgeodetic-based coordinate grid projections Because much of the information in aGIS comes from existing maps, a GIS must transform the information gatheredfrom sources with different projections to produce a common projection

Map Scale

Map design addresses two fundamental map characteristics: accuracy anddepicted feature types Both characteristics vary with map scale Generally, largerscale maps are more accurate and depict more detailed feature types Smaller scalemaps, such as U.S Geographical Survey (USGS) quadrangle maps, generally showonly selected or generalized features Table 8.1 summarizes the relationships amongmap scale, accuracy, and feature detail

of the GIS application results Use of inappropriate data in a GIS map may lead to

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misleading results and erroneous decisions, which may erode public confidence orcreate liability.

Data Errors

There are two types of data errors: inherent errors embedded in the source ofdata and operational errors introduced by users during data input, storage, analysis,and output Inherent errors can be avoided by using the right kind of data Operationalerrors can be prevented by quality control and training

A data conversion team should be aware of sources and magnitudes of data error.For example, spatial information in USGS 1:24,000-scale (7.5-min) topographicmaps is certified to have 90% of its features within 50 ft (15 m) of their correctlocation 50 ft is large enough to underestimate the runoff from a new developmentand undersize a detention pond for adequate stormwater management

Map Accuracy

A primary factor in the cost of data conversion is the level of positional accuracy.Required map accuracy and resolution depend on the application in which the mapswill be used A 2000 survey conducted by the Geospatial Information and Technol-ogy Association (GITA) indicated that the water utilities were seeking more landbaseaccuracy of 5-ft compared with other utilities, such as the 50-ft accuracy sought bythe gas companies (Engelhardt, 2001; GITA, 2001) The same survey for 2002indicated that the water industry required the highest accuracy in their GIS projects.Among the water organizations, 27% were using a 6-in landbase accuracy compared

to electric (12%), gas (17%), pipeline (17%), and telecom (0%) organizations (GITA,2003) These data reveal that a trend toward increasing accuracy may be emerging

in the water industry

Engineering applications usually require ±1 to 2 ft accuracy For planning andregional analysis applications, ±5 to 10 ft accuracy is generally appropriate (Cannistra,1999) Sometimes relative accuracy (e.g., ±1 ft from the right-of-way line) is moreimportant than an absolute level of accuracy (e.g., ±1 ft from the correct location) Forthe applications where positional accuracy is less important, supposedly low-resolutiondata, such as USGS digital orthophoto quadrangles (DOQs), may be acceptable Inother applications where features must be positioned within a foot of their actualposition, even the presumably high-resolution data, such as 1-m IKONOS imagery,

Table 8.1 Relationships among Map Scale, Accuracy, and Feature Detail

Map Scale

Minimum Horizontal Accuracy, per National Map Accuracy Standards

Examples of Smallest Features Depicted

1 in = 50 ft ± 1.25 ft Manholes, catch basins

1 in = 100 ft ± 2.50 ft Utility poles, fence lines

1 in = 200 ft ± 5.00 ft Buildings, edge of pavement

1 in = 2000 ft ± 40.00 ft Transportation, developed areas,

watersheds

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may not be accurate enough As a rule of thumb, a database built from a map will havepositional inaccuracies of about 0.5 mm at the scale of the map because this is thetypical line width of the drawing instrument This can cause inaccuracies of up to 12

m in a database built from 1:24,000 mapping, such as USGS DRGs (Goodchild, 1998).Precision and accuracy are two entirely different measures of data quality andshould not be confused A GIS can determine the location of a point feature precisely

as coordinates with several significant decimal places However, many decimal places

in coordinates do not necessarily mean that the feature location is accurate to a 100th

or 1000th of a distance unit Once map data are converted into a GIS environment, thedata are no longer scaled, as the data can be scaled as desired to create any output mapscale However, the spatial data can never be any more accurate than the original sourcefrom which the data were acquired GIS data are typically less accurate than the source,depending on the method of data conversion Therefore, if data were captured from asource map scale of 1 in = 2000 ft, and a map was created at 1 in = 100 ft, the mapaccuracy of features shown would still be 1 in = 2000 ft (PaMAGIC, 2001)

be overlayed, analyzed, and plotted together Because the base map serves as thereference layer for other layers, its accuracy can affect the accuracy of other layers.This is especially true if the base map is used to create other layers by on-screen(heads-up) digitization

Selection of an appropriate base-map scale is largely determined by the earlierchoice of GIS applications Each application inherently requires a certain minimumbase-map accuracy and certain map features For engineering and public-worksapplications, the required map accuracy is in the range of ±1 ft, as dictated by theneed to accurately locate specific physical features, such as manholes and catchbasins Planning applications, which most often deal with areawide themes, do notgenerally require precise positioning Accuracies of ±5 ft, or perhaps as much as

±40 ft, are often acceptable Less detailed maps, showing nothing smaller than roadsand buildings, for example, may be adequate for many planning applications.Whatever the range of mapping requirements, the base map must be accurate anddetailed enough to support applications with the most demanding map accuracies ofbetter than ± 2 ft.Utility asset location also requires mapping that depicts specificsmall features such as manholes and catch basins As shown in Table 8.1, theserequirements are met by a map scale of 1 in = 50 ft

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There are three common types of base maps: digital orthophotos, planimetricmaps, and small-scale maps.

Digital Orthophotos

For laypersons, digital orthophotos (or orthophotographs) are scanned aerialphotos For GIS professionals, they are orthorectified raster images of diapositivetransparencies of aerial photographs Creation of a digital orthophoto requires morethan a photo and a scanner, and includes surveyed ground control points, stereoplotters, and a digital elevation model In fact, the digital orthophoto creation processinvolves many steps, which are listed below:

• Establish ground control

• Conduct aerial photography

• Perform analytical aerotriangulation

• Set stereo models in stereo plotters

• Capture digital elevation models

• Scan aerial photographs

• Digitally rectify the scanned photographs to an orthographic projection

• Produce digital orthophotos

Digital orthophotos are popularly used as base maps that lie beneath other GISlayers and provide real-life perspectives of terrain and surroundings that are notavailable in the vector GIS layers Typical vector data do not show vegetation.The vector layers can show the manhole location but may not include the vegetationhiding the manhole High-resolution orthophotos with submeter accuracy canguide the public-works crews directly to a manhole hidden behind bushes Know-ing the land-cover characteristics before leaving for an emergency repair of abroken water main will allow the crews to bring the appropriate tools and equip-ment Knowing whether the job will be on a busy intersection or in somebody’sbackyard will determine the kind of equipment, material, and personnel requiredfor the job Figure 8.1 shows a water system map overlayed on a digital orthophotobase map with an accuracy of ±1.25 ft Typical digital orthophotos cost $800 to

$1600 per mi2

Planimetric Maps

Like digital orthophotos, planimetric base maps are also created from aerialphotographs However, instead of scanning the aerial photos, the features are digi-tized from them Thus, whereas digital orthophotographs are raster files, planimetricmaps are vector files Planimetric maps generally show building footprints, pavementedges, railroads, and hydrography Parcels digitized from existing maps are oftenadded to the mix Figure 8.2 shows a sample planimetric map for the Borough ofMunhall, Pennsylvania, extracted from the Allegheny County land base The bor-ough’s sewer lines and manholes are overlayed on the planimetric base map Thecost of planimetric maps depends on the level of detail and, therefore, varies signif-icantly from project to project The typical cost range is $2,500 to $10,000 per mi2

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Small-Scale Maps

Small or rural systems often use small-scale street maps or topographic maps

as base maps Street maps can be created by digitizing the existing maps, obtainedfrom a government agency (e.g., U.S Census Bureau, USGS, or state department

of transportation) or purchased from commercial data vendors In the U.S.,1:24,000-scale raster topographic map layers called digital raster graphics (DRG)are provided by USGS Shamsi (2002) provides detailed information about thesources of small-scale maps Users should be aware of the scale, resolution, accu-racy, quality, and intended use of small-scale base maps before using them Mostmaps at scales of 1:100,000 and smaller are not detailed enough to be used as sitemaps or engineering drawings, but they can be used for preliminary studies andplanning projects Figure 8.3 shows interceptor sewers and pumping stations forthe Kiski Valley Water Pollution Control Authority in Pennsylvania, on a base map

Figure 8.1 A water distribution system overlayed on a digital orthophoto base map.

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of streets The 1:100,000-scale solid roads are from the U.S Census Bureau’s 1990Topologically Integrated Geographic Encoding and Referencing System (TIGER)data The 1:24,000-scale dashed roads are from the Pennsylvania Department ofTransportation Unlike double-line pavement edges shown on the planimetric maps,these road layers show the single-line street center lines The difference in theposition of the roads in the two layers can be attributed to the resolution, scale,and accuracy of the two layers.

ADVANTAGES OF GIS MAPS

The most challenging part of a GIS application project is to obtain the right kind

of maps in the right format at the right time Therefore, maps are the most importantcomponent of a GIS Without maps, you simply have a computer program, not a GIS

Figure 8.2 A sewer system overlayed on a planimetric base map.

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In many water and wastewater systems, there is a backlog of revisions that arenot shown on the maps and the critical information is recorded only in the memories

of employees However, there is no longer any excuse to procrastinate because based mapping is easy and affordable

GIS-In the past, users have selected computer-aided drafting (CAD) and automatedmapping and facilities management (AM/FM) systems to map their water and sewersystems, CAD being the most common method Although a map printed from CAD

or AM/FM might look like a GIS map on paper, it does not have the intelligence of

a GIS map GIS maps are intelligent because they have attributes and topology Mostconventional CAD maps do not have attributes; they simply print data as labels orannotations For example, the mapmaker must manually write the pipe diameter next

to a pipe or must manually change the pipe color or line type to create a legend forpipe size This is a very cumbersome process On the other hand, GIS stores theattributes in a database and links them to each feature on the map This capabilityallows automatic creation of labels and legends at the click of a mouse button InGIS, map labels and legends are changed automatically if an attribute changes InCAD, the mapmaker must manually delete the old label and retype the new label.Only a GIS map knows the spatial relationships among its features Called topology,this capability makes the GIS maps intelligent For example, a GIS map is intelligentenough to know which watershed is adjacent to which Although both the CAD andAM/FM offer map layers to store different types of objects, only a GIS map has thecapability to relate data across layers The spatial relations among layers allow spatial

Figure 8.3 A sewer system overlayed on a streets base map.

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analysis operations such as identifying the gate valves that must be closed to isolate

a broken water main for repair

A commercial map atlas company may use a CAD system because its tions are primarily for cartographic products A telephone company will use anAM/FM system to support its telephone system operations and maintenance, because

applica-it must be able to quickly trace a cable network and retrieve applica-its attributes (Korte,1994) For a water or sewer system, a GIS map is most suitable because the systemmust conduct many types of spatial analyses, asking questions such as how manycustomers by type (residential, commercial, industrial) are located within 1000 ft of

a proposed water or sewer line

In addition to water and sewer system management, GIS-based maps can alsosupport other needs of the municipality For example, a planning department cangenerate 200-ft notification lists as part of its plan review process A public worksdepartment can conduct maintenance tracking and scheduling A public-safetydepartment can perform crime location analysis GIS-based mapping is thereforethe most appropriate mapping technology to meet all the mapping needs of amunicipality

GIS MAPPING STEPS

GIS mapping consists of five typical steps:

produc-Needs Analysis

Mapping work should begin with needs analysis as described in Chapter 2 Theneeds analysis study describes the features that should be captured during the dataconversion step, mapping specifications (accuracy, resolution, scale, etc.), and sourcedocuments For example, the needs analysis determines whether or not the customermeters should be mapped If the answer is yes, then how and to what accuracyshould they be mapped Are precise coordinates needed or can they be drawn at theend of service line? For large systems (populations greater than 50,000), a pilotproject should be conducted as described in Chapter 2 (Needs Analysis) The pilotproject allows potential fine-tuning of mapping specifications in a two- to four-sheetarea before starting the map production phase

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Data Collection

When existing maps do not exist or are inadequate, mapping data should becollected using a field survey with or without GPS Required data are usuallyscattered among a multitude of different organizations and agencies For instance,

it is claimed that 800 worker-years would be needed to convert England’s sewersystem to digital format, including field verification work (Bernhardsen, 1999) Fielddata collection using mobile GIS and GPS technology that employs handheld devicesand tablet PCs is becoming common for collecting attributes (Chapter 7, MobileGIS) The question of quantity should be evaluated carefully before the data con-version is started Too little data limits the mapping applications Too much datatypical of a “data-driven” approach might be wasteful

Captured data are stored in layers also referred to as coverages or themes Forexample, manholes are captured as a point layer, sewer pipes are captured as a linelayer, and sewersheds are captured as a polygon layer The layers that supportdevelopment of other layers should be given a higher development priority Forexample, geodetic control and base-map layers should be developed first becausemost other layers depend on them

Data conversion includes capturing both the graphic (geometry and coordinates

of features) and nongraphic (attributes) data Graphic and nongraphic data may beentered simultaneously or separately

Capturing Attributes

The annotations (labels) shown on the source map are the most common source

of attributes The source document ID (e.g., drawing number) is one of the mostimportant attributes that should always be captured during data conversion Featuressuch as valves and hydrants have unique IDs, which can be easily captured fromlabels (annotations) on source maps The features without IDs should be assignednew IDs during data conversion Some data conversion application software allowautomatic ID creation during data conversion

Most source maps show pipe sizes as labels or legends, which can also be easilycaptured during data conversion Geometric properties like length, area, and perim-eter of features can be internally calculated by GIS and entered as an attribute Forexample, if pipe length is labeled, it can be entered as a Length_Map (source map)

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