Tuesday, April 20, 2010

Cadastral/Parcel Mapping Quality Components

We are often asked “how good is that parcel mapping?” or “how accurate does my parcel mapping need to be?” Unlike data sets that are built from a single source, such as aerial imagery or topographic maps or lidar, the answer to the questions of how good or how accurate for parcel mapping is a case by case, parcel by parcel response. Certainly the results of an analysis could be rolled up to a statement about the overall data set but the quality of parcel data is much more than just spatial accuracy. What are the quality components for parcel or cadastral mapping?

Currency – How current is the parcel map?

The currency can be stated as simply as this parcel map includes all land transactions as of last Friday, or within one month or as of yesterday. The currency statement for the mapping might be different than the currency of the associated attributes or attributes that the map may be tied to. For example the assessment value information might be from last year. A parcel split might be current as of yesterday but the assessment values for the new parcels might be from the prior year until the year-end assessment information is brought current. The mapping currency should be stated separately from the related attributes.

Completeness – How complete is the parcel map?

At first glance most organizations might reply 100% complete, we map everything. But consider if the parcel map includes federally managed lands, are these mapped and indicated as federally managed land and does it reconcile with the federal agencies mapping of the same parcel? Are state managed lands included? Are right of ways included? Is this parcel mapping taxable lands? If it is taxable lands are all taxable lands on the roll represented on the map? Typically an ongoing parcel program will have 100% coverage for the parcels they are mapping, but it is always worth an extra look to be sure.

Lineage – Does every parcel polygon have a link to its source document?

An important part of the parcel mapping process is recognizing that it is a representation of the descriptions and content of the legal document from which it was generated. These documents can be survey plats, deeds, right of way plats or any number of types of conveyance documents. Providing a link to the source document that was used to define the geometry is an important and essential component of the parcel mapping. Note that this is the link to the document that defined the geometry and may not be the same as the document that contains information on the current owner.

Relative Positioning or Closure – Does every polygon have an indicator of the relative accuracy of the legal description?

For organizations that run coordinate geometry on their parcel it is possible to get a record of the linear error of closure (LEC) of the legal description, however not every organization uses coordinate geometry for parcel mapping and some legal descriptions are just not suited for this type of analysis. For example a parcel legal description may call out a river centerline or a physical feature such as a tree or road and the measurements to execute a linear error of closure analysis are not easily derived. To solve this it may be possible to develop a simple coding scheme that indicates the general quality of the associated legal description for the parcel polygon. For example the relative positioning codes could be something like the following.

1 – parcel closed, LEC at least 1 part in 1,000.

2 – parcel closed but the LEC was less than 1 part in 1,000 or one line of the parcel was a physical feature call that matched the imagery closely.

3 – parcel is drawn entirely from reference to imagery and physical references and these calls match the imagery.

4 – parcel metes and bounds description did not close and had to be best fit with surrounding parcel or imagery evidence.

5 – parcel closure or relative accuracy could not be ascertained or assigned.

6 – no attempt was made to determine the parcel relative accuracy.

These are just an example of some the things to consider in a relative accuracy coding. Another alternative might be to assign the LEC as the relative accuracy and indicate if no LEC was determined.

Absolute Positioning – How well odes the parcel legal description match control point location of the boundary?

This is the most traditional indication of “accuracy”. This is an indication of how well the parcel fits with other parcels and how well it ties to control measurements. For example if the corners of a subdivision have been located with global positioning system (GPS) how close is the corner location on the parcel map to that GPS location? Similarly if an organization has jurisdiction-wide orthoimagery, how close are the physical calls in the legal descriptions to the visual evidence on the imagery? F a parcel description runs to the middle of the road does the parcel mapping show the parcel line on the middle of the road, assuming the road on the imagery is the same road called for the legal description. Beware of the difference between the center of the right of way and the center of the road. These are often different. Even with the nuances of legal descriptions and visible evidence it is often possible to use the imagery to get a sense of the absolute positioning of the parcel maps.

Attributes Present – Are all of the attributes for the parcel map included?

This is a check of the parcel map database to assure that all of the parcel polygons have the attributes they are supposed to have. If the parcel has a tax identifier, is it entered into the parcel map database and is it correct? If the parcel map database uses annotation, are the annotation fields complete and correct? This evaluation would also apply to systems that use line types or any other codes for cartographic display or parcel classification.

The following two components are indications of the overall parcel mapping program in an organization. These components might be termed best practice as opposed to quality components, but these two components contribute to the overall quality of the parcel mapping.

Archived Data – Does the parcel mapping program include tracking historically mapped parcels?

Archiving data might be as simple as taking an annual snap shot of the parcel mapping database and archiving with the backup data. It could also be snap shots of the data periodically such as monthly or quarterly. In a transaction based parcel mapping system each parcel change will result in archiving the changed parcel to an historical parcel file or something similar so the parcel map can be used to “roll back” through time and visual observe changes. Parcel data that is archived in this manner typically keep the parcel number and linkages to source documents with the historical geometry.

Maintenance Process – Is there a documented and sustainable maintenance process in place?

Because parcel and cadastral data are so dynamic, potentially changing on a daily basis, it is important to have a defined and sustained maintenance process. A parcel map may be perfect at the moment it is delivered from conversion process, but it will soon be out of date if there is not a sustainable maintenance process. This means the updates and maintenance have to be consistently and on a regular basis. Even if that basis is once a year, it is a known maintenance cycle.

The reason organizations invest in parcel mapping is to use the resulting with related data such as the tax roll or deed recording or permitting or zoning or a plethora of other applications. The last three components of parcel mapping data quality categorize the data distribution or data publication. This is separate from the evaluation of the production data.

Publishing Schedule - Is the parcel map published on a regular schedule?

Do the customers for the parcel data know when to expect parcel mapping updates? Are updates provided or notification of updates made available to those who depend on the data. Notification might be that the most current information is pushed to a data service and when users add the data to their canvas the updates are reflected on their project without the need for additional notification.

Metadata – Does your published parcel data have essential metadata?

Metadata does not have to be onerous. If an organization focuses on establishing essential metadata for published data, then the metadata should be relatively stable and can be published with the data set as an XML file or as an html file or embedded in the distributed or published data set. With limited resources there is less demand for metadata for production data, data that is internal to the organization than there is for published data that will be likely be consumed by people who have little or no local knowledge. The FGDC Cadastral Subcommittee has developed a limited set of essential metadata and tools for creating the published version of this data, that can be found at this link http://www.nationalcad.org/showdocs.asp?docid=1153&navsrc=Standards&navsrc2=

Attributes Standardized – Are the attributes in the published data translated to standard naming conventions and content?

The FGDC Cadastral Subcommittee has developed a publication standard for core parcel data. This is a minimum set of information that meets a large audience of data requests. Some organization have added to this core level information but the goal is to encourage all published parcel data to be in or have a version of the published data that matches this standard. Added attributes are a bonus. The publication guideline can be found at this link.


So how good is your parcel map? Applying some or all of these quality measures and then developing an evaluation rating might be one way to answer that question. It should also be a way to measure the increasing quality of the parcel map as the maintenance progresses.

Sunday, April 4, 2010

Authoritative Cadastral Data

Unlike other spatial data (hydrography, topography, orthoimagery, etc), cadastral data defines rights and interests in land. Cadastral data are also unique because it is created and maintained by over 4,000 separate entities across the country.

The vision for the National Spatial Data Infrastructure (NSDI) and the National Cadastre within the NSDI is to have a single source of authoritative cadastral data that is controlled and managed by designated data stewards. Access to this data is facilitated by compiling and integrating the data into trusted data sources at state or regional levels. This will reduce duplication of effort and assure that the best available information is used in decision-making.

Cadastral data includes assessment records that support the real estate property tax system, recorded land documents such as deeds and mortgages, indices and summaries of this data such as a grantor-grantee and tract indexes, and survey information captured in plats and surveys.

Authoritative data comes directly from the creator or authoritative source. It is the most current and accurate and has been vetted according to official rules and policy. The data has a known accuracy and lineage and can be verified and certified by data stewards in the authoritative source. In some terminology this is termed the “primary” data source.

Trusted data describes data sets that are published by someone other than the authoritative source and is often the compilation of multiple sources of authoritative data. It is “trusted” because there is an “official process” for compiling the data from authoritative sources and the limitations, currency and attributes are known. The data are often formatted into a standardized form and linkages to the originating source are provided with the data. This trusted source is recognized by the authoritative source as an “official” publisher of this subset. Typically a trusted source is established to integrate data from multiple jurisdictions and to compile it into a standard format. This trusted data is adequate, convenient and cost effective for users who need a regional view and have to deal with multiple sources of data, but there is an understanding of the necessity when final decisions are being made, particularly about rights and interests of specific properties, that the user must go to an authoritative source and acquire authoritative data directly to ensure that they have the most current an accurate data.

Data stewards have the responsibility to organize, collect, maintain and provide data. Data stewards are those closest to the data creation, they have recognized expertise in the field and follow professional standards.

When there is not a single source for data, such as nationwide assessment data which has over 4,000 local government sources, and the effort to acquire authoritative data from individual authoritative sources is impractical, then it is reasonable to acquire trusted data from a trusted source.

All cadastral data collections that do not come from authoritative or trusted sources are “unofficial” or “shadow” copies whose value degrades over time relative to the rate of updates to the authoritative data. Unofficial data is often duplicative and creates redundancy by re-publishing data that are already available from a trusted source. Unofficial sources create confusion among the general consuming public by providing un-maintained duplicative data and “unofficial” parcel-like data sets that can unexpectedly harm or damage property rights with inaccurate out of date information.

Recognizing the importance of authoritative sources for authoritative cadastral data that may be provided through a trusted source will be essential to protect individual land rights, to support local governments and other parcel producers in their authorized role of data stewards and to ensure that the user community has the best available and most current cadastral information.

The FGDC Subcommittee for Cadastral Data, following the directives in OMB Circular A-16, has developed a series of documents over the past ten years that describe the concepts and polices related to the creation, use and publication of cadastral data. There is more information on authoritative and trusted sources for cadastral and data stewards for various components of cadastral data at the FGDC Subcommittee’s publication site (http://www.nationalcad.org)

Saturday, March 13, 2010

Detroit Vacancy Survey – Making Parcel Data Relevant to the Discussion

In February the Detroit Free Press Published an article describing the results of a recently published survey of residential housing in Detroit. The survey was completed by researchers who drove all the streets and completed windshield surveys on the condition of single family residential housing. Quoting from the Detroit Free Press Article (http://www.freep.com/article/20100220/BUSINESS04/2200371/1318/Survey-finds-third-of-Detroit-lots-vacant) “A little more than 35% of the city's 343,849 residential parcels are either vacant lots or abandoned shells of buildings. … But the survey also found surprisingly upbeat results in Detroit's most vital districts. The survey found that more than 90% of the city's occupied residential units are in good or fair condition”

The article goes on to discuss the value of this parcel-by-parcel data “In a world where investment decisions and government aid are driven by hard data, the survey offers an unblinking, parcel-by-parcel look of Detroit's condition. Kurt Metzger, a demographer and director of the nonprofit agency Data Driven Detroit, which helped manage the survey, said the parcel data eventually could be merged with other databases -- foreclosures, health and educational statistics, crime patterns -- to allow the most precise imaging ever of a city and its challenges.”

These results are intriguing for Detroit and the parcel-by-parcel information is exactly what was called for in the Land Parcel Data for the Mortgage Crisis (add link to report) report from the May 2009 stakeholders meeting in Washington DC. But beyond the findings that parcel data are valuable to decision makers and an important part of the strategic planning for recovery, these results also raise an interesting question about why was a drive by inventory needed to get this information? The Detroit Free Press Article states “And, in the future, the agency hopes to add advanced mapping software and other tools to its Web site to make the survey results more accessible and useful.” Isn’t this exactly the type of service local government should be providing?

If local governments have automated parcel data linked to assessment rolls, why couldn’t the vacancy and housing conditions be assembled from the accurate and certified assessment data? Why couldn’t this information as well as commercial and industrial property as well as multiple family housing be pulled from assessment files and tied to parcel maps? Given the importance of these findings and the continued property value and mortgage issues facing local economies shouldn’t this data be available everywhere there is parcel mapping?

The other interesting aspect of the results of this survey was that the findings published on the web were generalized to the block level to prevent criminals from targeting vacant houses. But if Google and Microsoft, to name two, are providing street level views of these areas, and criminals can search the web and find this study and determine the block level data, couldn’t they also pull up the street view and look or maybe even just drive by themselves, after all the search radius has been reduced? It is certainly a concern that vacant housing attracts a criminal element but isn’t there an accountability and transparency issue in publishing the locations of the vacant structures? Isn’t this a way for citizens to hold their elected officials accountable? Doesn’t tracking changes in vacant structures (rehab or gentrification of vacant structures), improving vacant lots and tracking condemnation and demolition actions create a visible measureable for public policies?

The enthusiasm and interest expressed by public officials to the results of this survey seem to indicate that providing a web service or even a mobile service on vacancy, condition and tax use classification from an authoritative source that can also show trends and be combined with other parcel based authoritative data like tax delinquency and assessed value is something we all should be considering for our parcel data. It’s one more way GIS and parcel data can be part of the game and part of the discussion.

Land Surveyors, Parcels and GIS

In the March 2010 Professional Surveyor Magazine Craig Dylan published an article titled “A Long Survey” (add link). Mr. Dylan describes a GPS survey along the Delta-Mendota Canal in California completed by BLM Land Surveyors. The article on the use of the GPS is interesting especially the use of the GPS, GNSS and California Surveying Virtual Surveying Network (CSVSN). But as almost an aside he quotes the BLM Land Surveyor Tim Jackson on the time savings with CSVSN and efficiencies the BLM has gained in just five short years. The article also describes the benefits of capturing PLSS, property and right of way monumentation as part of these projects. The surveys found original monuments, identifying where property boundaries may need further survey work and even errors in places where reference monuments were used rather than the actual corners.

Through the canal boundary survey the GPS measurements found trends in subsidence, identifying areas at risk for further subsidence and compared past elevation observations to the new results. This information benefits the canal managers by providing information on where future infrastructure repairs maybe needed and where to watch for particular types of damage arising from subsidence.

Although not mentioned in the article, taking the results of this survey and combining it with other BLM and private land surveys to create a standardized representation of the PLSS that could be used by GIS staff and other land surveyors extends the benefits found by Mr. Jackson.

What if the results of this detailed and well research survey project could be rolled into county parcel mapping easily? The benefits would be extended beyond the source agency to the other agencies. Couldn’t this type of project be studied from a benefits perspective to create a financial case for the role of land surveys in improving decision making, improving the quality of public data sets and improving the public’s perception of public data sets? Isn’t the role of land surveying more than establishing boundaries and improving individual data set quality? Extending the results to support future aerial photography control and supporting infrastructure management decisions puts the land survey directly into the day to day business of decision makers.

Thursday, January 14, 2010

Parcel Mapping and Subdivisions

Subdivisions and Condominiums are the most common simultaneous conveyances for most local tax mapping agencies. In many systems these important features are lost in the tax parcels. Why should we care about simultaneous conveyances and go to the effort of tracking and mapping them in tax parcel systems?

What are Simultaneous Conveyances?

Simultaneous conveyances are created when one or more parcels of land are created at the same time, i.e. simultaneously.

When several parcels of land are created at the same moment in time, such as lots in a subdivision, several parcels in a will, or sections in a township, all parcels have equal legal standing; they were all created at the moment of the filing of the subdivision of the plat, at the moment of the death of the testator, or in the case of a U.S. PLSS Township at the time the plat was approved. (Brown's boundary control and legal principles By Walter George Robillard, Curtis Maitland Brown, Donald A. Wilson, section 11.3 p 301

This is important to legal boundary analysis because this means that the boundaries within the simultaneous conveyance all have equal standing and in most cases this means there will not be junior or senior rights within the bounds of the simultaneous conveyance. The implication for tax parcels is that the interior boundaries should not be differentially adjusted or moved but should be treated with equal weight.

The contrary to a simultaneous conveyance is a sequence conveyance. These are parcels of land that are created. “Sequential conveyances are those written in deeds in which junior and senior rights between adjoining parcels” (Brown's boundary control and legal principles By Walter George Robillard, Curtis Maitland Brown, Donald A. Wilson, section 11.2 p 301). For tax parcel editing the term remainder parcel, the portion of a parcel that remain after a new parcel is created, is often used to describe the sequence conveyance situation. The implication for tax parcels is that in a sequence conveyance the parent parcel is senior, it existed first and must get it’s land first, the second parcel is subservient to the parent parcel. If there is an overlap situation, the parent parcel land is protected first.

There are many other evidence and legal description nuances that need to be considered in resolving discrepancies that should be interpreted by a land surveyor but it is important to recognize the basic differences in these descriptions in the tax parcel mapping environment.

Another characteristic of Simultaneous Conveyances is that they do not overlap. With each new simultaneous conveyance a new system for describing land is created. If there are appparent stacked subdivisions the latest subdivision area supersedes the prior subdivision, essentially vacating the prior subdivision. There are also nuances to this rule, such as with public right of ways and whether or not an official vacation of plat needs to be filed, and state law may create some variability but this is a general operating principle. The implication for the tax parcel mapping is that the most current subdivision should be the one used to control the geometry of the tax parcels. As new subdivisions are recorded and stacked on top of prior subdivisions, the area previous described should be changed to reflect the new survey.

Tax Parcel Mapping Versus Title Search

In most tax parcel mapping systems, it is sufficient to map the currently active simultaneous conveyances. The tax map may need to be able to identify or base legal descriptions on the underlying subdivision lots or condominium units but it may not be necessary to show the title history or evolution of subdivisions that existed prior to the current plats.

However, if an application is a mapped representation of the title history then it will be necessary to address stacking simultaneous conveyances. In some jurisdictions the office that tracks legal document recording may use a tract index to index the documents. A tract index summarizes real property transactions and is typically available in the offices of Recorder of Deeds. Only a few states require their recording offices to maintain this type of index. Among these states are Nebraska, North Dakota, Oklahoma, South Dakota, Utah and Wyoming. In addition, some other states permit recording offices to maintain tract indexes (for example, Kansas, Minnesota, Ohio, and Wisconsin). A mapped history of the simultaneous conveyances can provide the basis of these tract indexes.

In addition to document indexing, land surveyors need simultaneous conveyance history to complete boundary evidence and title boundary research. Having these records automated can increase the efficiency of historical document search for land surveyors and abstractors.

Understanding the intended uses and requirements of applications that will use the record of historical simultaneous conveyances will be important in the application and data design used to automate these records.

Sunday, January 10, 2010

Tax Parcel Numbers – Limitations and Advantages

There has been some recent discussion about adding parcel numbers to the Home Mortgage Disclosure Act data sets (HMDA). This discusses some limitations and advantages of using tax parcel numbers outside the real estate tax system.

Typically tax parcel numbers are real estate tax system record identifiers that may also be used to relate tax parcel polygons and recorded documents to assessment and taxation records. They have evolved out of record based systems designed to track current and historical tax valuations, relate valuation to tax bills and manage tax credits and payments. As automated tax parcel mapping evolved the tax parcel number was a natural to include with the maps, providing a linkage to tax records.

Tax Parcel Number Limitations

One of the primary limitations of the tax parcel number is it often excludes tax exempt lands such as government managed land, churches, right of ways and public building sites. In some cases all of the exempt land is assigned the same code and in other cases no parcel number is assigned to exempt properties. There are exceptions of course and some jurisdictions do assign a tax parcel number to all parcels. Other idiosyncrasies with parcel numbers are

1) They may not be unique over time

2) They may change with annexations and tax district changes, and

3) Local formats and structure are highly variable.

Parcel Numbers Not Unique - Many local government have a practice of re-using parcel numbers when a parcel is split. If a portion of an existing parcel is conveyed the parent or remainder parcel often keeps the original tax parcel number that was originally assigned to the entire parcel. The geometry and legal description of the original parcel has changed with the conveyance or split but keeping the original parcel number allows many systems to create a linkage to the original parcel and be able to track the lineage. This practice is most common when the owner of the parent parcel did not change in the conveyance. Some property tax systems keep a parent-child table for tax numbers that explicitly stores the lineage of tax parcels, at least the lineage of tax parcel numbers, which can be used to trace the tax parcel history. If these tables exist and are used, then it is less of a problem to change the tax parcel number of the remainder parcel in a parcel split situation.

This practice of re-using parcel numbers is changing slowly with advances in automation of tax and assessment records and the migration to newer systems, especially systems linked to GIS. The practice of re-assigning tax parcel numbers is still fairly common throughout the U.S.

Affects of Annexation and Tax District Changes – Annexation changes a parcel’s tax district which can also change the primary real estate tax authority, for example if an unincorporated area is managed by the county, annexation to a city can transfer the primary real estate tax authority to the city. In some cases parcel numbers are changed with annexation. In most cases the history of the tax parcel number is tracked through parent-child tables.

Local Formatting – The variability of the structure and format of the local tax parcel number is legion. Spacing, punctuation, alphanumeric characters, length, rules and ordering are all subject to variation. Even in states with specified content individual county formats may have different spacing and punctuation in the tax parcel number. There are even counties where the tax number is formatted differently in the mapping software than in the tax assessor databases and this is surprisingly more common than expected. For the most part the software that manages the tax data dictates the format of the tax parcel number. There are many creative ways these software packages and local assessors embed location and parcel characteristics into the tax parcel number.

Tax Parcel Number Advantages

Even with all these caveats and variations locally assigned and managed tax parcel numbers are an important and essential data elements for many applications and downstream users of the data.

Identifies vacant land – Unlike site addresses tax parcel numbers provide a link to all taxable lands. Many jurisdictions do not assign site addresses to vacant land but is the land is taxable it will have a tax parcel number.

Connect to recorded and official documents – Many local governments, sometimes mandated by state law, recorded document numbers are included with the tax parcel number in the tax roll. These can include deeds, mortgages, mortgage releases, liens and many other documents such as permit records, inspections well and septic records. This can be very helpful when searching landownership even if the content of the linked documents may not be included in databases.

Identifies the entire parcel – The tax parcel number identifies and can be used to locate the entire parcel of land. Site addresses typically relate to just the structures on the land while the tax parcel identifier has information about the entire parcel including all structures in the tax parcel records connected by the tax parcel identifier.

Current and past records – The tax parcel identifier links to current and historical tax records, typically at least three years of past records. This makes it possible to see trends in tax bill payment history, trends in value and trends in use and tax rates.

As with any single data element, the tax parcel number is not a panacea, but it has enough value and application to be a useful addition to link to local tax and property records.