Measurements versus Monuments

One of the more interesting effects of transitioning the way in which you manage your land records into the ESRI ArcGIS Parcel Editing Solution (the Parcel Fabric) is the transition from mapping parcels according to the measurements listed in the legal descriptions to mapping parcels according to the monuments called out in those legal descriptions.

Traditionally, and I am not sure whether this is the result of the limited data structures or just a misunderstanding into legal description interpretation, but, most mappers input the bearings and distances cited in the legal descriptions with little attention being paid to the accompanying monument calls.  You know.... "thence along the North right of way line" or "to the southeast corner of Lot 47" or "47.08 feet from the Northwest Section corner."

These calls, while they appear to be minor in nature, are actually more important that the actual measurements in the legal description.  For example, if a course is described as "thence North 26 degrees 15 minutes 26 seconds East, for a distance of 100 feet to the southeast corner of Lot 47", even if the actual bearing and distance to the southeast corner of Lot 47 is North 30 degrees East, a distance of 105 feet to the monument found located on the southeast corner of Lot 47, the property corner is the southeast corner of Lot 47 and the location of the monument supersedes the measurement called in the description and should be used for mapping.

These calls are commonly referred to as "monuments" and can exist in many forms: They can be natural monuments such as trees, streams, and lakes) or artificial monuments such as fences, walls, roads, streets or railroads. They can also be monuments described as the boundaries of adjacent parcels of land. The exact location of these monuments is considered "superior" to and carries a great deal of strength in determining the location of any specific corner, especially if that monument is called out in the legal description. 

Just a short note, for the monument call to be valid, it must be included in the legal description itself and cannot be inferred.

Monuments in the ArcGIS Parcel Editing Solution (the Parcel Fabric)

The Parcel Fabric is designed to enforce the manner in which the corner locations are defined, allowing the mapper to input the bearings and distances as cited in the description, but also locating the parcel corners on the existing mapped corners or "monuments".  This is accomplished using the "join" function inside the Parcel Fabric.  This "join" function allows the Parcel Fabric to locate a parcel corner so it is contiguous to and "along the right of way" or located at the "South East corner of Lot 47" or at "Section Corner" while maintaining the measurement information found in the source legal description.

This one function changes the way in which we handle the various inconsistencies contained in the legal descriptions, whether these inconsistencies are caused by differences in the precision of the measuring standards, differences in opinions of the locations of other monuments or improperly written deeds.  The "joining" functions allows us to analyze and map the "intent" of the description without being fixated on the measurements contained within. In my opinion, the one change is powerful and should be sufficient enough to convince us to begin managing our land records and parcel information in this new way.

I would be interested in hearing your views below.

Happy Holidays

We at Panda Consulting wish you and yours the brightest of Holidays and a sincere hope that we all have a Joyous, Healthful and Happy New Year.

It has been a while since I have posted anything to our blog - we have been very busy this year and I was not able to share as much as I had hoped.  To review some of the projects on which we have been able to help our Clients, please head over to our Projects page for a quick view.

Enjoy and have a Merry Christmas and a Happy New Year. 

The Parcel Data Model - The Partitioning Types

(As a result of input from Nancy Von Meyer, (see below), I have revised some of the verbiage in this posting.)

During the next two weeks we will explore the Parcel Data Model used by the ESRI Parcel Fabric Land Records maintenance solution. This week we look into the polygon types that define the partitioning of lands into smaller divisions that can be sold and next week we will look at the polygon types that are used to store and maintain information on various forms of land ownership and obligation.

The Parcel Data Model used by the ESRI Parcel Fabric is described in detail in the "GIS and Land Records" book by Nancy Von Meyer (ISBN-13: 978-1589480773). This data model provides the most comprehensive data structure focussed on Land Records we have seen to date and we highly recommend this data model to everyone involved in mapping Land Records. 

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Two Distinct Classes of Information

Overall, the data model breaks information into two distinct categories: the information that serves to partition lands (Public and Private) and the various types of land ownership (Tax Parcels, Fee Simple Ownership, Encumbrances and Separated Rights.)

The following discussion of Public and Private divisions is not intended to be exhaustive, or even thorough.  It is intended to be an introduction to the concepts and additional information can be found here.

Within each of these classes, information is split into discrete layers or "Types" of polygons.

Partitioning Classes

The Partitioning classes are divided between Federal Subdivisions of Public Lands, commonly referred to as the Public Lands Survey System (PLSS) or Rectangular Survey System and Private Subdivisions in which private landowners divide their lands for sale to individuals.

Please note that I am purposely using the word "partitioning" instead of "subdivision" because there are so many meanings and implicit baggage that comes along when we use the word "subdivision". Recognize, the word "partition" is intended to define lands that are owned by an entity that is dividing up the lands so they may sell off portions of it to other individuals.

Federal Subdivisions of Public Lands

The polygon "Types" related to the Federal Partitions directly reflect the components of the Original Surveys from which the Federal lands were sold to private citizens through patents or deeds. These "Types" also reflect in many ways the order in which the lands are identified.

Type 1 - PLSS  Townships

The PLSS Townships

The PLSS Townships

Type 1 polygons reflect the first division of Public lands from which all other divisions are derived. These "Townships", depending on the data of survey and instruction provided to the original surveyors may be anywhere from 6 miles square (the most common) to 7 miles square and several other variations. They are identified by their sequence from the Initial Point in the Principal Meridian and are referenced in a North - South direction as lying North or South of the Base Line and East or West of the Principal Meridian.

All measurements involved in the PLSS are performed using the "Gunter's Chain", generally equivalent to 66 feet in length. 

Type 2 - PLSS Sections

Section Numbering and Sequence in which section lines are surveyed to subdivide Townships

Section Numbering and Sequence in which section lines are surveyed to subdivide Townships

Type 2 polygons reflect the second division of Public lands into smaller polygons approximately one mile by one mile in size. The sections are constructed after the Township perimeter has been established and monuments have been placed at defined locations by the original surveyor. They are numbered beginning with "one" at the Northeast corner of he Township and numbered sequentially in a snake pattern alternating east to west then southerly until all sections are identified. The "normal" section of land consists of approximately 640 acres.

 

 

Type 3 - Quarter Sections

Division of sections into Quarter Sections

Division of sections into Quarter Sections

Type 3 polygons reflect the third division of Public lands into smaller polygons by connecting straight lines between the monuments established by the original surveyor at the North, South, East and West lines of the sections.  These are commonly referenced as "Quarter Sections".. The goal in establishing these quarter sections was to create as many aliquot portions of land containing as close to 160 acres of land as possible.  If, however, the sections measured longer or shorter on the northern or western tiers of sections with the Township, smaller aliquot portions are created.  These aliquot portions include the Government Lots around sovereign lands, previous land grants prior to the acquisition of the lands by the US Government or other situations. 

There is a general misunderstanding of his process within the mapping community that believes that quarter sections are derived by dividing the Section into fourths - This is incorrect.

Type 4 - Special Surveys

Special Surveys

Special Surveys

Type 4 polygons reflect the fourth division of Public lands and encompasses those situations in which special surveys were required. According to the BLM, these are non rectangular components of the PLSS including: Meandered Water, Corners and Conflicted Areas (known areas of gaps or overlaps between Townships or state boundaries). 

 

Private Partitions

The next two polygon "Types" describe the partitioning of lands by private landowners into smaller areas for sale to others. Because almost all lands in the United States are derived from the original public ownership managed by the Federal government, the exact location and definition of these private partitions (subdivisions) is completely dependent upon the quality of the location of the Public polygon types.

Type 5 - Simultaneous Conveyances (Subdivisions and Condominiums)

Private Subdivision

Private Subdivision

Type 5 polygons reflect the overall extent of privately owned lands that are partitioned into groups of smaller lots of units for sale. As with the smaller divisions of the Public lands partitions, the smaller units of the Private partitions come into existence at the moment that these partitions (Subdivision Plats or Declarations of Condominiums) are approved and recorded in the public records. In other words, the smaller divisions are "simultaneously" approved and "conveyed" into existence. Since these units are created at exactly the same time, none of these smaller units are "senior" nor "junior" to the rights of any other units in the subivision. These Type 5 polygons include those divisions commonly referred to as "Subdivisions" and "Condominiums".

Type 6 - Simultaneous Conveyance Divisions (Lots, Blocks and Units)

Subdivision Lots and Tract

Subdivision Lots and Tract

 Type 6 polygons reflect the final type within the Parcel Data Model that reflects the partitioning of land for sale and does not contain any information about land ownership. This polygon type is intended to contain the smaller divisions of land with the larger Type 5 polygons (Simultaneous Conveyances).  If the larger Type 5 polygon is a traditional Subdivision, these smaller divisions might be lots, tracts, reserved areas, road tracts or parks.  If the Type 5 polygons is a Condominium, these polygons may represent the individual unit boundaries, including its 3D space, limited common ownership, or general common ownership.

Next Week:  The Parcel Data Model - Ownership Types

Be sure to check back for our Blog Post next week in which we will continue our review of the Parcel Data Model and focus on the Ownership polygon "Types", (Tax Parcels, Owneship Parcels, Encumbrances, and Separated Rights. 

Labels or Annotation in the Parcel Fabric?

Within the Parcel Fabric, ESRI's solution for parcel maintenance, there are two distinct methods with which you can handle dimensions: labels or feature linked annotation. Each method has its benefits and limitations, as well as defenders and detractors.

Panda Consulting has historically leaned towards the labeling side when using simple feature classes for your data model.  However, the recent change in how we share our parcel data has us thinking. "Should we recommend Labels or Annotation?" recognizing there is no perfect solution.

When the primary client used for viewing GIS parcel data consisted of ArcGIS for Desktop or ArcReader, it was easy to assume that the software could read and create labels using the core labeling engine technology.  However, since the new paradigm for data publication is that any device, including smart phones and tablets, will be used to display the data, that assumption that the client has the local processing power or software to generate these labels forces us to rethink this assumption.

Storage of Dimensions in the Parcel Fabric

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Whether you decide to use labels or feature linked annotation, all dimensional information is stored in the parcel fabric within the parcel lines feature class (similar to COGO attributes on simple feature classes), where it defines the core geometry of the individual polygon. When the attribute is modified, the corresponding label or feature linked annotation is also modified.

Publication of Dimensions

If we were to keep the data within the Parcel Fabric and never share the dimensions with others, we would not need to decide between the two.  However, we now use an Extract, Transform and Load (ETL) process to take the data from the Parcel Fabric, a complex structure, into simpler lines and polygons feature classes for publication.  Since the creation of feature linked annotation requires an extra process over just storing the data in the line features and using labels. Storage and Maintenance = Advantage Labels

Feature Linked Annotation

Annotation actually has a small advantage in that, since feature linked annotation is not actually stored in the Parcel Fabric but in an annotation feature class that is related to the parcel lines, the ETL process is as simple as copying the annotation feature class into the publication database. The downside to annotation has always been that it is created for a single scale, whereas labels can be set to change size and display properties as the scale changes. If multi-scale annotation is desired, it must be created many times. In addition, annotation takes up disk space to store.

Labeling

The use of labels is complicated by the manner in which the Parcel Fabric stores those features used to produce the dimensions.  In the Parcel Fabric, all features having the same geometry type (points, lines and polygons) are aggregated into a single geometric feature class.  In addition, since lines and polygons are maintained as being either active or historic, only lines that are directly related to "active" tax parcels are used for labeling purposes.  Therefore, during the ETL process, a selection query must be used to only extract those lines that define active tax parcels.

That means, you must maintain the "Type" column in the Parcel Fabric's Parcel Lines feature class to remain correct as you modify the parcel type in the polygon feature class. ArcGIS 10.2.1 correctly sets the type attribute field during the parcel creation process, but, if you change the parcel type after creation,you must manually change the corresponding lines for the data to be correct.  While this is not difficult to do, it would be nice if ESRI would fix this problem and make the change automatic. Extraction of Dimensions = Advantage Feature Linked Annotation

Consumption of Dimensions on the Web

Once you are finished with your maintenance and  export your data for publication, there is still the issue of how the clients will consume the data. Unfortunately, ArcGIS Online does not currently support annotation features as standard feature services so, the most efficient way to currently serve out dimensions to your web maps is by creating a cached map service that sets display scales automatically for the the annotation or labels. Creation of the tiles for this cached map services is required for both feature linked annotation and labeling. Publishing and sharing on the Web = Tie

What does this mean?

So, what does this all mean?  When planning your transition to the Parcel Fabric, be sure to think about more than what you have previously decided, be sure to think about how you will be serving out your data. Understand that, no matter which method of storing dimensions, you will still probably be creating a cached web service and serving out the cache rather than the actual features.

Curves in the Parcel Fabric

There are many ways in which the Parcel Fabric differs from our current mapping concept, some of them are obvious, such as the tools we use and the way we store land records data. But there are also subtle differences that we need to understand to make the most of the Parcel Fabric - We need to especially recognize that the Parcel Fabric stores curves differently.

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Feature Class Curves

Within a standard feature class, the curve is a variation on a single line vector. Albeit, the curved line has specific properties that define the amount that the curve is "bent". In fact, when we interact with the curve using the standard editing tools, we are actually just changing those specific properties that define the curvature.  

Because of the way the curve (arc) is stored, there are many tools in ArcGIS to help you define those specific properties to cause a curve to pass through start, middle and end points (arc function) or through specific endpoints (endpoint arc function), tangent to a previous course (tangent arc) or even a bezier curve (bezier). 

A Curve as defined in the Parcel Fabric

A Curve as defined in the Parcel Fabric

 

Parcel Fabric Curves

However, within the Parcel Fabric, curves are constructed and stored as geometric definitions and the actual point of curvature (PC), point of termination (PT), the radius point (RP) and radius lines are constructed and stored. These points and lines define how the curve is drawn. Interestingly, because the actual arc of the curve is constructed within the Parcel Fabric, you cannot join or link to the actual curve - you must join or link to the defined points (Point of Curvature, Point of Termination or Radius Point.)

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The implications of this difference in the way curves are stored are important to understand. For example, if two curves are stored in the Parcel Fabric and are supposed to be concentric and there are any variations in the location or geometry stored with those curves, they will create differing points and will not be drawn as concentric. Instead, there will be two different points stored as illustrated to the right. In this example, the two curves were stored and there was a difference in the radius length stored ( this was not necessarily an error, this difference could have been simply a rounding error on the original information.) But, having these two points stored as different values will result in the curves not being drawn concentrically. 

To remedy this issue, you must force those two points to be stored as one.  To accomplish this, you use the Mean Points tool in the Parcel Editor Toolbar. 

The Mean Points tool in the Parcel Editor Toolbar

The Mean Points tool in the Parcel Editor Toolbar

Dragging a box around the points to be "meaned" (averaged) will move the points together and result in having the curves drawn as concentric, even if they have differing COGO information stored in their respective geometries.

Remember, within the Parcel Fabric, the actual joins to points and linepoints is critical to how your data is drawn.  Be sure to check to make sure the points are joined correctly and that all three points in the curve (PC, PT and RP) are correct.

What is the Parcel Fabric?

By now, most GIS professionals interested in maintaining Land Records in GIS have heard of the ESRI Parcel Fabric but, considering the adoption rate I've been witnessing, many are still reluctant to fully adopt the concept and implementation. Many of you have heard me speak in the past few years about the benefits and challenges involved in moving your data to the Parcel Fabric. Future posts will discuss options for implementing the Parcel Fabric, but in this post, I will attempt something a little less challenging and introduce the various components of the Parcel Fabric.

Conceptual Data Model - The Design

Within the Parcel Fabric, features are defined by the "type" of polygon they represent. For example there are:  Legal definitions of how land is divided such as the Public Land Survey System Townships, Sections, Quarter Section, Special Surveys, or by the legal subdivisions of the land such as subdivisions or condominiums and the interior subdivisions inside these legal entities (lots), or they are defined by "types" of interests in the land such as: taxable interests, ownership interests and other partial interests such as easements or leasehold interests. By far, this approach is the most comprehensive attempt to map all components of land interests in decades and the initial concept can be found in GIS and Land Records by Nancy von Meyers of Fairview Industries. This approach provides a logical place for all types of interests in land and tracks how the land is defined and ownership of interests in that land.

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Logical Data Model - The Way it Works in ArcGIS

The Parcel Fabric is a complete land records data model that consists of several standard feature classes in a geodatabase that are "tied together" similar to a geometric network. When I first introduce the structure to my students, I almost always describe the Parcel Fabric as a "geometric network for polygons."

As a matter of fact, the entire Parcel Fabric data model revolves around the polygon as the primary geometry with all other information being related to that polygon geometry. This is very different from the cartographic data model that many organizations are still using where everything revolves around lines.

The Parcel Fabric data model, and specifically the Local Government Information Model, provides the polygon feature class as the primary record and then has corresponding lines that define that polygon and corners that define how those lines are joined to one another.  It also includes a table to store information about the document that created the polygon (the plan table) and a feature class to store control points that define the spatial locations of the points relative to the surface of the Earth.

 Local Government Information Model (LGIM) - The Standardization

The Local Government Information Model is ESRI's attempt to develop and promote a standardized data model to allow greater consistency across organizations sharing similar needs, but also having that standardized data model adopted to allow the development of enhanced and complex maintenance functions that make Land Records maintenance more efficient.  In other words, by having everyone, or at least those organizations seeking more efficiency, adopt the LGIM, ESRI can develop sophisticated tools that allow us to perform standard maintenance tasks in fewer steps with fewer mistakes.

Tools - Let's Make it Easier

While I plan on having many posts directly relating to specifics of the Tools available for the Parcel Fabric, I can say that, with each new release of ArcGIS for Desktop, ESRI continues to improve the quality of the parcel editing tools, making them more and more efficient and making the job of the Parcel Mapper easier.

For example, some of the new tools included in the Parcel Editor functions in ArcGIS for Desktop can do things such as: take an existing parcel polygon, calculate a portion of that "parent" parcel, calculate and input all lines required for that portion and the parent parcel, store and create two new parcels and mark the existing parcel historic so we can go back and see what the parcel looked like before we performed that split. ESRI has also integrated these tools into Automated Parcel Editing Workflows that will identify and guide you through step involved in parcel maintenance.  These workflows are a great first step in making the software easier to use and, while still needing refinement, you can see the commitment of ESRI towards building a complete solution for Parcel Maintenance. 

These tools are only available once you adopt and implement the Parcel Fabric as your parcel maintenance solution. The LGIM and the enhanced parcel maintenance tools go hand in hand and make the maintenance of land records better.

Hopefully this short post helps with your understanding of the Parcel Fabric, or maybe just piqued your interest to learn more.  If you have questions or comments, please share them below.