Basemap digitization: everything old is new again

A quality assurance-oriented approach helped San Francisco complete a basemap development project, normally a time-consuming process, in less than a year.

Prior to computer basemap digitization, surveying and mapping of parcels, streets and highways, sewer lines, bridges and buildings meant many hours of legwork and mounds of paperwork.

Today, with orthophotos and cadastral and graphic references, visual detailing of city and county data takes on a whole new look. And, this technology means that anticipating and documenting discrepancies between paper-based maps and digital tiles is easier and can ensure that the project will be completed on time — if not early — with fewer errors and headaches.

San Francisco completed the digitization phase of an ambitious GIS basemap development project in August 1996. The digitization and file conversion work, normally an incredibly time-consuming process, was accomplished in less than a year. City administrators credit the project’s rapid turnaround to a partnership concept and a quality-assurance-oriented approach that anticipated discrepancies among the city’s data sources during the digital conversion process and established predefined guidelines for rectifying problems. The San Francisco basemap project is also noteworthy for the variety of digital map products that will be available to city and county departments.

For example, San Francisco will use the basemap to create a GIS Library that contains cadastral information, planimetric features, street centerlines, orthophoto images and associated non-graphic data stored in the end users’ choice of AutoCAD, ARC/INFO, MapInfo and DIME files. The centralized library structure with its multiple file types and formats will enable participating departments to withdraw single or multiple layers of information to create customized maps that are compatible with their particular software environments. “The basemap is the cornerstone upon which we will build a huge foundation of applications,” says Deborah Vincent dames, staff director in the San Francisco Chief Administrator’s Office. “The GIS Library will provide information accessibility that supports long-range development, city planning and emergency 9-1-1 response for numerous city agencies.” Creating a basemap with 170,000 parcels and related records in four file formats to accommodate more than two dozen city and county departments has been no small task.

Accomplishing the conversion involved a partnership of vendors working closely with personnel from several city and county agencies, all of whom participated in in-depth planning sessions to ensure that the project specifications were spelled out in detail and all of whom were given well-defined responsibilities.

PROJECT DEVELOPMENT

Data digitization and file conversion comprised the third phase of San Francisco’s four-phase basemap development plan initiated in 1992 by the Public Works Department. At that time, a city-wide Electronic Information Processing Steering Committee endorsed plans for a centralized GIS basemap that would allow departments to share map resources, develop city-wide map and database standards and coordinate public projects among multiple city and county agencies such as street and underground construction projects and emergency 9-l-1 dispatch.

Maintaining departmental autonomy was critical. San Francisco has a decentralized organizational structure with more than 50 departments. This necessitated development of the multi-format GIS Library, allowing departments the flexibility to create their own application maps.

“This GIS Library structure also supports the needs of the [departmental] users and can evolve in an intelligent fashion,” says John Applegarth, manager of information systems for San Francisco’s paramedics division. “Users can feed information back into the library to keep the basemap applications updated.” To meet the potential mapping requirements of so many departments, the Steering Committee realized that the accuracy and resolution of the basemap should match or exceed the highest specifications of any one department. Therefore, the committee decided on a three-inch pixel resolution to accommodate mapping of city property and a two-and-a-half-foot accuracy for emergency vehicle navigation. Digital orthophotography was the only basemapping source that could meet those requirements within the city’s budget. During the project’s first and second phases, ground survey work was conducted, aerial photography was collected and 252 digital orthophotographs were produced. A technical project management team was selected during phase one to help develop a detailed work plan for the project.

PLANNING THE DIGITIZATION

In determining the major file types that would comprise the GIS basemap, the project participants considered specific mapping needs, such as tax assessment, emergency vehicle routing, municipal property maintenance and general urban planning of the various end users. Four basemap file types were chosen based on map features commonly used together in the stated applications:

* Cadastral, which includes block lines, lot lines, shoreline, street names, lot numbers, addresses and political boundaries. This file also includes an associated database of block, lot and address information;

* Planimetric, which includes curbs, edges of pavement, parks, military roads, freeways, bridges, major buildings and city landmarks;

* Centerline Map, which contains the street centerlines and intersection nodes. Each centerline and node is linked via a unique identification number to a tabular database of street information; and

* Orthophotos, stored as raster images, which give end users the ability to identify and extract physical features of interest to them, such as buildings, paved areas, trees and swimming pools.

Tiling structure, which refers to the way map files are divided, was a critical consideration in developing these layers. Tiling was usually necessary to keep digital file sizes manageable for ease of use in GIS applications. Because the cadastral and feature files contain large volumes of data, they were divided into 73 tiles. The much denser orthophotos were tiled into 252 files, corresponding to the city’s standard 2,000-foot-by-3,000-foot map sheet. The centerline file for the entire city area was digitized as a single tile because it contained a much lower density of data.

Four documents–the digital orthophotos, the assessor’s block and lot paper maps and database file, the Bureau of Streets’ grade maps and the Pavement Management and Mapping System (PMMS) database–were used during the digitization phase to create the cadastral, feature and centerline files. The block and lot maps contain the property boundary information, and the grade maps include the right-of-way data. The PMMS includes some of the information necessary to trace the centerlines.

PARTNERSHIP AND QUALITY

A team that included an overall project coordinator, a GIS consultant-ARC/INFO expert and a digitization vendor for cadastral, feature and centerline files handled the Phase Three digitization and conversion.

Phase Three best exemplifies the importance of San Francisco’s partnership approach to Quality Assurance (QA) and overall project management.

Rather than foster a work environment that stressed work completion according to a predetermined schedule, the public works department devised a concept that placed a priority on delivery of a quality product.

Central to this concept were production process flowcharts that specified the responsibilities of each vendor and participating city departments in the digitization and QA procedure. Although QA and work flowcharts are not unique to this type of project, the development of an “exception management process” made the San Francisco project stand out.

A critical assumption in the exception management process–and a factor often overlooked in many digitization projects–is that technical problems will be encountered, especially discrepancies among multiple sources of map information. From the very outset, the department formed an Exception Management Team and established guidelines to address and rectify each problem or exception.

The speed with which the digitization was accomplished owes largely to this exception management process. In digitization, technical problems and discrepancies tend to repeat themselves. So by deciding how to solve specific problems the first time they are encountered, the Exception Management Team eliminated the need to redigitize numerous files to correct repeated errors.

THE EXCEPTION MANAGEMENT PROCESS

The exception management process was an integral part of the digitization work flow and QA process. Its full description, therefore, should be included within the context of the digitization process.

Digitization was conducted using AutoCAD software running on Pentium 150 workstations. In the first step of the process, paper block and lot and grade maps were scanned. The digital orthophotographs were then loaded into AutoCAD as background reference images upon which the digitization would be conducted.

The scanned maps were overlaid on top of the ortho images and displayed on the workstation screen. The operator used customized AutoLisp routines along with the standard AutoCAD drawing functions to trace streets, sidewalks, parcels, property boundaries and other features that would be included in the cadastral and feature vector maps, and the associated parcel database was converted into AutoCAD format.

This automated routine linked each database record with the appropriate parcel in the cadastral file. Once the cadastral files were completed, they were overlaid back on the orthophotos for creation of the centerline files. Again using AutoCAD, the technicians traced street centerlines down the middles of roads and highways using the orthophotos and cadastral files as graphic references.

Property boundary markers, such as fence lines that were visible in the orthophotos, occasionally did not match parcel lines on the block and lot maps.

In some cases, problems were more severe; streets that existed on the orthophotos had no corresponding record in the city files and vice versa. Frequently, the same street would show two different right-of-way widths in separate source maps.

As each discrepancy was encountered, it was digitally recorded in the Excel spreadsheet and red-lined in a “special exceptions” file within AutoCAD.

Each week these files were delivered to the Exception Management Team, which included representatives from the assessor’s office, public works, pavement management and mapping and the Bureau of Street Use and Mapping.

Because of potential legal ramifications, questions regarding parcel boundaries and locations were addressed with a blanket policy stating that information on the grade map would always take precedence over other data.

Location of physical features, such as roads, was dealt with differently since these features’ exact positions are critical to their use in emergency vehicle navigation. In this case, legal descriptions had to take a back seat to geographic reality and defer to the highly accurate orthophotos.

Other questions unrelated to feature locations or legal descriptions, such as road names or parcel identification numbers, were simply referred to the appropriate city department for guidance. After each weekly meeting, the exception management solutions–there were ultimately more than 1,000–were incorporated into the regular digitization procedure.

The city plans to maintain these exceptions in a database file that will remain associated with the GIS basemap in case questions arise during future application projects.

QUALITY ASSURANCE

The exception procedure is the first step in the QA process. Following adjustment of the digitization in accordance with the exceptions, the cadastral/features/centerline files were submitted to the city and to the technical project manager/QA overseer for quality control work.

The technical project manager conducted most of the QA checking in AutoCAD.

The first check was a partially automated process performed by an operator at an AutoCAD workstation. A single tile was displayed on the screen while the operator referred to the paper original of the block and lot or grade map.

The operator moved through each tile, zooming in on small areas of the tile from left to right and then panning down a screen and continuing across. The operator made a visual check to ensure that every item on the digital tile was on the paper map and vice versa.

Distance commands easily measured the widths of rights-of-ways and centerlines. The operator ensured that every parcel edge matched correctly and was in its proper relative position. The quality control of the cadastral database files was an automated process.

An automated routine was used that checked the parcel identifiers and street name information in the database against the graphics files to ensure a one-to-one relationship.

This procedure took less than one hour per tile. In some instances, only small errors were discovered, and San Francisco approved the work and accepted delivery.

In other cases, the tile was returned to the digitization team with conditional approval, indicating which quality assurance items needed to be corrected for final approval.

BASEMAP CONVERSION STATUS

Digitization of the cadastral/features files and the QA process was completed in August 1996. Centerline files had also been completed and submitted to the city.

As these AutoCAD files were accepted by the city, they were passed to the GIS consultant-ARC/INFO expert for conversion into ARC/ INFO, MapInfo and DIME formats. ARC/INFO and Maplnfo each have built-in AutoCAD data conversion functions that were customized for the San Francisco basemap conversion project. These conversion functions will be turned over to the city for future maintenance.

The public works department staff performed quality control checks on these converted files. An automated routine to check the ARC/INFO conversions against the original AutoCAD files was written, and a manual QA process was used for the Maplnfo and DIME files.

The department is now preparing to proceed with the fourth phase of the project–development of the GIS library and preparation of a strategy to keep the basemap updated, delivering the completed files to a central repository managed by the city’s Information Systems Department.

The public works department credits the early and exhaustive planning with this project’s swift progress.

Nearly two full months were spent with the vendors before initiation of the digitization phase to ensure that each party knew its responsibilities.

Planning and implementation were aided immeasurably by other city and county departments that invested not only capital but also staff time to actively assist the vendors in every phase of the project.

Creating a team-oriented environment in which city employees and mapping and surveying vendors focused their unified efforts on delivering a high-quality product helped the process, and the combination of detailed planning, quality assurance and teamwork helped San Francisco accomplish each conversion phase of its digitization objectives on schedule and with minimal discrepancies.

George White is program director for the San Francisco Basemap Development Project; Anil Ranadive is marketing director for ValueCAD, Berkeley, Calif.; and Kevin Corbley is a freelance writer in Denver, Colo.