I get a lot of questions about the new Cadastral Editor being released soon (9.2 SP3), so I thought I'd post some general information about it that was published in the
UC 2007 Q&A. Sorry for the length of the post, but I thought I'd just put it all out here for review:
Q: What is the cadastral fabric dataset?
A cadastral fabric is a new type of dataset in the geodatabase used to represent and hold data for a continuous surface of connected parcels. Parcel boundary line dimensions in the cadastral fabric match the dimensions on the survey record. Dimensions in the cadastral fabric are edited when corrections need to be made to match the survey record. A parcel split adds two new parcels to the fabric and maintains the original parcel as historic. In fact, whenever parcels are edited or replaced by new survey records, the existing survey information is retained as historic thus always preserving the original survey record.
- A cadastral fabric dataset in the geodatabase is made up of these key feature layers:
Parcel lines, which store and preserve recorded boundary dimensions
- Parcel points, which store X, Y, Z coordinates. The X and Y vertices are derived from a Least Squares Adjustment.
- Parcel polygons, defined by parcel lines
- Line points, which are parcel corner points that lie on the boundaries of adjacent parcels
- Control points, which have accurate, published coordinates for a physical location
In addition, tabular information used in the cadastral editing workflow is also maintained as part of the cadastral fabric dataset. This includes:
- Plans, which store information about the record of survey
- Fabric jobs, which track edits to the cadastral fabric
- Accuracies, which are used to derive weights that are used in Least Squares Adjustments
- Adjustment vectors used to store displacement vectors derived from Least Squares Adjustments that are used to rubber sheet and update other related feature layers that map onto the cadastral fabric (such as buildings, utilities, and so forth)
In the geodatabase, the parcel-based topology of the cadastral fabric dataset defines the arrangement for how parcels, boundary lines, corner points, and other features share coincident geometry.
Parcel polygons are defined by a series of boundary lines, which store recorded dimensions as attributes in the lines table. Specialized topological behavior supports multiple survey records for adjacent parcel boundaries whose dimensions are specific for each parcel even though the boundaries are shared. Parcel polygons are also linked to each other by connection lines -- for example, connection lines that cross roads.
Because each parcel is either linked or connected, a seamless network of connected parcel boundaries (i.e., the continuous cadastral fabric) is formed. Parcel lines have endpoints, which are the parcel corners. Parcel corner points are common between adjacent parcel boundaries, establishing connectivity, and maintaining topological integrity in the fabric.
Q: How is spatial accuracy maintained in a cadastral fabric?
Spatial accuracy in the cadastral fabric is incrementally improved and maintained through time using adjustment by least squares, which may be done either as each new plan is entered or on an as-needed basis. Control points are processed together with recorded dimensions to derive new, more accurate coordinates for parcel corners. Parcel corners locate parcels on the surface of the earth, resulting in an accurate coordinate-based cadastral system that improves as new surveys are entered through time.
ArcGIS Survey Analyst is intended to support the incremental creation and management of coordinate-based cadastres.
There are different interpretations of what a coordinated cadastre is. Within the U.S., Canada, Australia, and many other locales, the coordinates are improved by the surveyor's approach of
least squares adjustment. The least squares approach applies a best-fit solution of all the existing historical survey record information on lines together with newer control point positions. Since these point positions can be continuously updated through time, the coordinates are not intended to provide the true legal representation of parcel corners, but are rather the best representation given all the historical legal record information available within the system.
The Cadastral Editor technology supports a coordinated cadastre with the goal to improve and establish digital representation of coordinates at the corners of parcels represented in a Land Information System (LIS).
Traditionally, surveyors recorded parcel boundaries by the use of bearings and distance dimensions. Cadastral boundary networks were defined with no accurate reference to real-world locations.
With the advent of GIS and high accuracy GPS surveying, it has become significantly easier to use coordinates to geographically define parcel locations. Accurate coordinates give the closest estimate of the true location of a point on the ground, and through least squares, can provide error information indicating the reliability of the coordinates.
Traditional survey methods used for relocating property boundary corners may be interpreted in different ways. When different surveyors use different data to re-establish the location of a boundary, boundary location disputes can arise. A coordinate provides a unique and unambiguous record of a point and can be quickly and accurately relocated with the use of GPS. To gain maximum benefit from the use of coordinates, a system needs to be in place within the cadastre that provides a measure of the reliability and accuracy of coordinates in a parcel boundary network. The more accurate and reliable the coordinate, the higher its weight and influence would be in determining the location of the boundary in the network.
Q: Why are the Cadastral Editor and Cadastral Fabric important?
Existing methods for parcel data management have accuracy limitations. Traditional parcel data management has focused on entry of individual parcel and subdivision plans that use coordinate geometry (COGO) to enter high accuracy metes (bearings and distances) and bounds (neighboring lands) descriptions. Here is a metes example: “Commencing at the point of beginning then North 45° East 320.00 feet then Northwest 26° 200.00 feet…"
In these workflows, each individual parcel or subdivision plan is entered independently of all other survey plans. This workflow is adequate for management of individual land records. However, a continuous depiction of the parcel fabric across the whole jurisdiction is also required.
Governments and land management agencies now require greater accuracy than in the past. As new photogrammetric platforms are released and levels of resolution for orthoimagery, GPS, and Lidar continue to improve, the accurate capture of the continuous cadastral information is key to large-scale GIS use in urban settings. Cadastral systems will need a continuous Cadastral Fabric to be managed and referenced to real-world coordinates using a comprehensive geospatial framework such as ArcGIS.
In most GIS applications today, the continuous fabric is created by rubber sheeting and fitting together individual plans. This work is typically performed as a cartographic adjustment operation that uses a small number of control points resulting in a low accuracy result. In most cases, the original record data and survey information is maintained independently of the cadastral fabric and the connection between the cadastral fabric and the original record of survey is broken once the adjustments take place.
In many forward-thinking cities, counties, states, and provinces, agencies are searching for solutions that can improve the accuracy of their continuous parcel fabrics as each new subdivision plan is entered and integrated into the fabric.
In addition, the feature geometry of many GIS data layers fit onto and are coincident with the cadastral fabric. These layers can also be adjusted using the same least squares adjustment properties used to build the integrated, continuous cadastral fabric.
The result is a highly accurate representation of the continuous cadastral fabric that meets the goals of surveyors as well as GIS professionals and can support GIS applications that need increasingly more accurate data representations.
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