"MUNICIPAL PARCEL BASE MAPPING AND DATA CONVERSION METHODS"

The following sections provide brief summaries, in outline form, of the main points presented by the listed speakers at the March 3, 1998 meeting of the Massachusetts Geographic Information Council (MGIC). 

Moderator - Shaun Hayes of the Pioneer Valley Planning Commission

Overview of the topic:
Parcel databases have become the foundation for many land based decisions including: proprty tax assessment, land use and zoning maps and studies, economic development analysis, site selection and evaluation.  The March user meeting featured a discussion of commonly used methods of compiling parcel base maps and methods of converting these data into automated GIS/mapping systems.  Presentations focused on municipal mapping activities including research, data development, conversion, application and maintenance of parcel maps and related data. 

The March user meeting was an expanded session featuring exhibits by GIS product and service providers in addition to the regular agenda of presentations by GIS professionals.  Due to user requests, MGIC invited vendors representing software, consulting, photogrammetry, data conversion, hardware and other related businesses to send representatives who were available to provide information about their companies, discuss users' needs and answer questions. 

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Presentation by Linda Petrella, Director of Planning, City of Springfield, MA
"The Springfield GIS Experience"

Introduction to Springfield’s GIS

• GIS Team established to implement the technology
• Secured local bond funds to do aerial photography and other tasks for the creation of tax maps
• Prepared RFP for a firm to conduct a needs assessment and the overall GIS development

Outcome of the Needs Analysis

• An evaluation of the quality of our existing tabular database and the accuracy level of the Assessor’s block plans
• An assessment of the adaptability of aerial photography taken in 1993 and corresponding digital data
• Consultant was thoroughly aware of how we used tabular data and maps and used that information to customize our GIS

Importance of Parcel Databases & Maps

• Strong reliance on use of Assessor’s block plans and database
• 90% of the map data is on block plans prepared in the 1940’s
• 800+ maps at a scale of 1”=50’
• Remaining 10% on Atlas maps with a scale of 1”=400’
• Tabular database has 43,000 parcels with ownership, building and site characteristics
• Each parcel is assigned a unique 9 digit identifier

Who Uses and Maintains the Data?

• Principle users and maintainers of current map data
• Principle users and maintainers of current tabular data
• Need to establish standards and formal procedures for digital data

Advantages of Implementing an Automated Parcel-Based Map

• Integration of tabular and map data
• Efficiency in using and maintaining the tabular   and map data
• Improved analytical tool
• Upfront cost is substantial, howvever, long-term   benefit outweighs the cost

Data Conversion Process

• RFP for Aerial Photography
• RFP for Data Conversion
• Database Design
• Scrub the base maps
• Establish work program
• Choose pilot area to test and evaluate your methodology
• Develop a manageable system to deliver and receive data
• Determine how quality control reporting will be done for digital and hard copy data

Now that we have GIS data…….

• How do we maintain the accuracy of the data in light of financial constraints?
• How do we widely disseminate data and still charge for the compilation of data under the Public Information Act?
• How do we continue to build on what we have?

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Contact Information:
City of Springfield
Planning Department 

Phone: (413) 787-6021
 

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Presentation by Wayne Feiden, Planner, City of Northampton
"Northampton GIS"

Needs Analysis - 1989

• Never had a GIS budget - costs were part of the general office operating money
• 1 day a week technician and interns
• Now have a full-featured GIS that serves the city's needs
• Wayne did the needs assessment - met with all department heads who used geographic data in any form
• Goal - to build full-featured GIS somewhere down the line

Findings

• Absoulutley critical that parcel data be the base map - every project needs parcel mapping
• Knew they needed to maintain accuracy of maps - even though they didn't know what that accuracy was
• Wanted a system that would let them improve parcel accuracy over time
• Started looking for software - knew they wouldn't have full-time staff
• Prime goal - easy to use: chose Atlas GIS (most friendly in 1989) but knew that down the line would probably change and so looked at conversion packages -- in 1989 this was a big issue
• Initial Goal - not creating data for surveyors but to support individual projects. Started with goal of 10-meter accuracy - wanted to see if their data would match this; knew this was lousy accuracy and needed to improve over time; didn't want to duplicate digitizing - tried to get best accuracy possible.

Pilot Study (1990)

• Northampton State Hospital - had good control in the area and the deeds ranged in detail
• There was a lot of planning around this area at the time, which helped them bury the costs
• Digitized the parcels from 2 sheets (town has about 150 sheets)
• compared the accuracy of these maps with the known accuracy
• found the accuracy here was within accuracy - felt that digitizing would be an acceptable approach
• prime control points were Mass Highway Department control points (points the city set up in 1950s); also could compre with rivers from orthophotos.

Methodology (1990-1991)

• Bought digitizing tablet, computer and software
• How to digitize rest of city?
• in-house vs. outsource? -- Looked at cost of vendors - decided to do it in-house
• hand digitize vs. scan? -- Decided that scanning in 1990 had too much error ro use
• Hired interns from UMass
• did checkplots for every sheet
• printed everything on vellum, overlaid on originals
• Digitized geographic boundaries
• parcel lines
• assessor IDs
• Original maps tied into Northampton coordinate system  - converted to Massachusetts State Plane NAD27
• Edge matching issues
• the sheets didn't match up perfectly
• did some rubbersheeting, sometimes split the difference, sometimes went back to original deeds
• urban area in 1964 -- 1" = 100'
• rural area in 1964 -- 1" = 200'
• By late 1991 had all the parcels with most errors corrected

GIS Replaces Analog (1991)

• Combined assessors' and zoning maps - (threw out mylar zoning maps) - now showed zoning lines and parcels on all maps
• Added property line dimensions (interns)
• Readability issues - new maps weren't as readable as old maps (e.g. - digital text was all horizontal, old maps' text was placed along line)
• Conversion issues - wanted to make sure all data put in was going to be convertable - didn't want fancy-placed text - left everything as convertable as possible

Maintenance

• Always tweaking a bit - e.g. moving annotation to make each sheet more readable
• Yearly updates from new surveys - survey line filed with the City for a property change
• DPW checks for accuracy of survey lines and determines how to put them in
• Office of Planning and Development adds to GIS database, reprints (now require surveys to be hooked into Mass. State Plane and give them highly accurate data or pay additional fee and not hook into MSP, which was the initial approach; the problem was that these standards were a bit lax - didn't know how good the control points were, etc.)
• Maps rubbersheeted when some more accurate data available
• Logical map identification system - e.g. system for control which works for all projects, etc.
• Looking at how to improve the accuracy of existing maps; conducted a project to assess accuracy of existing maps. Also looking at control and looking at ways to add to control system (City owns GPS)

Improving Accuracy

• GPS used for data accuracy
• As for Jan 1 - new subdivisions -- all submissions were required to submit in NAD83 with highly accurate control, published control points with documentation of accuracy.
• There are approval-not-required subdivisions -- if within 500' of an approved control point, need to tie in to control system, if not, not required
• Any city surveys are now adding GPS control points (ways to add to monument network and bury cost)
• Switching to ArcView GIS and Data Automation Kit
• Will be switching fully to NAD83 this year
• Careful about documenting carefully where all data came form and the accuracy of it
• Looked at different conversion packages and how their NAD27 to NAD83 conversion would impact accuracy
• Assessed various packages and found large diferences in their accuracies

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Contact Information:
Wayne Feiden
City of Northampton
Office of Planning and Development 

Phone: (413) 587-1266
Website: http://city.northampton.ma.us/
 

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Presentation by Patrick O’Brien, Senior GIS Analyst, DesLauriers & Associates, Inc.
"Issues in GIS Parcel Mapping"

High End Parcel Conversion

• Aerial Photography
• Coordinate Geometry (CoGo)
• Both are expensive

What are the alternatives?

• Digitize existing maps utilizing less expensive control sources
• Lower quality but much less expensive
• May pave the way for higher end GIS in the future

Why is control so important?

• What is the purpose of a control source?
• Majority of tax maps were drafted as a general representation of property boundaries
• Accuracy of source documents not precise
• Maps may not be drafted using precision drafting practices

Alternative Control Sources

• Conduct GPS or traditional survey for points that can be recognized on maps
• Use GPS to create road centerline network
• Boston Edison or other utility data
• MassGIS orthophotography
• Other sources...

Building the Composite Base

• Convert maps by CoGo, digitizer or scanning
• Decide database model beforehand
• Maps most often do not align correctly
• “Rubbersheeting” and other methods employed to resolve discrepancies

Map Issues

• Discrepancies which require research
• “Paper” roads & subdivisions
• Overlooked lot splits/combinations
• Areas of ambiguity to town
• Map updates out of sync with database update

Assigning GIS Parcel IDs

• Enables GIS to link to appraisal database
• Map/block/lot concatenation is safest
• ID should be entered the same way as in appraisal database
• Standardize n

Linking to Appraisal Database

• Link to non-Windows system achieved by:
• Special software drivers
• ASCII download
• Map/blocks/lots values must match
• Expect that a number of records won’t link between GIS and CAMA
• Will require inter-departmental cooperationaming conventions

Integrating Other Databases

• Databases, spreadsheets, etc.
• Databases can be imported or linked
• Common data field is necessary
• Common values must be explicit to link
• Data “scrubbing” usually required
• Use standardized databases where possible

High End Parcel Development

• High reliance on data integrity
• Layers will overlay precisely
• Can perform higher end geographic analysis
• Rubbersheeting of “external” data still required

High End Applications

• Topographic analysis
• Utility mapping
• Wetlands proximity analysis
• Facilitating of engineering design
• Availability of many more data layers

Lower End Implementation

• Appropriate for trend analysis, general query and analysis
• Recurring problems with overlays
• Be careful how geographic mapping and analysis is applied!
• Use “hotlinks” or other references to precise data sources
• Still have many applications

Sample Low End Applications

• General querying
• School enrollment forecasting
• Block grant applications
• Septic system failure analysis
• General sewer/water infrastructure management
• Pavement management
• Routing

Some Recommendations

• Be specific when contracting for services
• Use common databases
• Develop new layers from a common base
• It’s OK to start small
• Build interdepartmental understanding/support
• Make the GIS generate revenue
• Try to sell GIS expansion on easy projects

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Contact Information:
Patrick O'Brien
DesLauriers & Associates, Inc.
130 West Street
Walpole, MA 02081 

Phone: (508) 668-5010
Fax: (508) 668-4512

Last Updated 4/19/2000
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