Measurements, coordinates, and accuracy in the parcel fabric
This topic applies to ArcEditor and ArcInfo only.
Parcel boundaries in the parcel fabric are defined by bearing and distance dimensions typically derived from raw survey measurements. Often, these bearing and distance dimensions do not accurately close the parcel polygon. That is, if you compute around the parcel boundary using the dimensions, you will not return to the exact coordinates of the starting point. The difference between the starting and closing coordinates of the parcel is referred to as the parcel misclosure and is a measure of the accuracy of the parcel dimensions. The smaller the misclosure, the more accurate the parcel dimensions. Larger misclosures indicate blunders in the original survey measurements or data entry. Smaller, acceptable misclosures usually occur due to the following:
- Scale discrepancies between overlapping surveys
- Differences in accuracies between overlapping surveys done in different time periods
- Human error and inconsistency
- Measurement error
Acceptable tolerances for parcel misclosure are typically specified by government agencies, the survey profession, or other agencies governing the conduct of surveys in a jurisdiction.
Parcel misclosure provides a first-level quality check when entering parcels into the parcel fabric. Erroneous measurements and data entry errors can be caught before the parcel is joined. Joining parcels into the parcel fabric provides a second-level quality check through a display of transformation residuals, which are derived from transforming the parcel from its local coordinate system to the coordinate system of the parcel fabric.
A least-squares adjustment of all joined parcels provides the final and most comprehensive level quality check.
In the parcel fabric, parcels are entered using a traverse. A parcel traverse is a sequence of dimensioned lines that start and end at the same point and define new points after each line sequence. When entering a parcel traverse in the parcel fabric editor, the traverse misclosure is always displayed.
Parcel dimensions are derived from raw survey measurements observed in the field. Raw survey measurements always have an associated accuracy, which is a reflection of the quality of the measurement. The closer the measurement is to its true value (the more correct it is), the more accurate the measurement. Because parcel dimensions are derived from raw survey measurements, parcel dimensions can have an associated accuracy as well.
In the parcel fabric, accuracies are assigned to parcels by date of survey. This is because in general, survey accuracy has improved with survey technology and survey practice advancements. Accuracies are grouped by categories, and each category corresponds to a date of survey.
The accuracy category assigned to a parcel line in the parcel fabric is significant for the least-squares adjustment. Accuracy categories provide a weighting system in the parcel network. For example, a high-accuracy category is considered a high weight in the least-squares adjustment. Boundary lines with high accuracies (weights) adjust less than boundary lines with low accuracies.
Parcel point coordinates
The physical location of a parcel corner is defined by a parcel point in the parcel fabric. The parcel point has x,y,z coordinates, which model the current "best fit" representation of that point. Parcel point coordinates are initially generated during the parcel joining process or during the data migration process when migrating existing parcel datasets into the parcel fabric.
When joining a parcel to the parcel fabric, coordinates of the unjoined parcel in local coordinate space are transformed to the spatial reference of the parcel fabric. When the parcel fabric is adjusted using least squares, the parcel dimensions are used together with the control points to improve the accuracy of these initial coordinates. As more accurate parcel data is added to the parcel fabric, more accurate coordinates result from the adjustment.
Only an adjustment of the parcel fabric can alter the coordinates of a parcel point. Coordinates are derived quantities that are held as transient attributes of the point rather than a definition of the point.