Running a parcel fabric least-squares adjustment
This topic applies to ArcGIS for Desktop Standard and ArcGIS for Desktop Advanced only.
A parcel fabric least-squares adjustment is run on a selection of parcels.. The least-squares adjustment engine uses dimensions on parcel lines together with control points to determine the statistically most likely coordinate location for every parcel point in the network.
The least-squares adjustment process determines a more accurate location and a more accurate representation of the line geometry for each parcel line. The original parcel line dimensions (attributes) are not altered. The geometric and spatial representation—the parcel line shape—of the dimensions is updated from the newly adjusted coordinates.
A minimum of two active control points are required to run a fabric least-squares adjustment. Control points can be imported into the parcel fabric or manually inserted in an edit session.
It is always good practice to run a check fit on your control points before running a fabric least-squares adjustment. The check fit residuals indicate how well the parcel network will fit with its control network. Parcel lines are going to have to adjust at least by the amount of the check fit residuals in the least-squares adjustment. Therefore, check fit residuals can be used as a guideline for setting adjustment tolerances on the Adjust Coordinates dialog box. For example, the control network may be offset by 20 feet from the parcel network, but 20-foot adjustment tolerances are not necessary for the adjustment to succeed.
Adjustment tolerances
Before running a fabric least-squares adjustment , you need to specify adjustment tolerances on the Adjust Coordinates dialog box.
Adjustment tolerances determine the amount of movement (adjustment) that will be allowed in the parcel fabric during the least-squares adjustment. Generally, the more accurate and reliable the data is, the less the data will need to adjust and tolerances should therefore be small.
The Bearings residuals and Distance residuals tolerances on the Adjust Coordinates dialog box determine whether or not the adjustment will succeed. The remaining tolerances are set for informational purposes.
Bearings residuals
The maximum allowable difference (residual) between the bearing computed from the adjusted line shape and the original recorded line bearing. If a line is found with a bearing residual greater than the tolerance specified, then line is listed in the adjustment summary. If the bearing residual is three times greater than the specified tolerance, then the adjustment fails.
Distances residuals
The maximum allowable difference (residual) between the length computed from the adjusted line shape and the original recorded line distance. If a line is found with a distance residual greater than the tolerance specified, then line is listed in the adjustment summary. If the distance residual is three times greater than the specified tolerance, then the adjustment fails.
Line points offset
The maximum allowable distance between a line point and its adjacent parcel line. If a line point is found that lies further from its adjacent parcel line than the tolerance specified, then the line point is listed in the adjustment summary.
Close points within
The maximum allowable distance between two points that are not connected to each other, that is, they don't have a line between them. If two points are found that lie closer to each other than the tolerance specified, the points are listed in the adjustment summary.
This tolerance is useful for identifying overlapping points that should be merged into a single point.
Use line points
When this option is checked, the position of the line point influences the outcome of the least-squares adjustment. Equations are generated from the angular difference between lines connected to a line point and adjacent straight lines and are used in the adjustment.
It is recommended that this option be checked to detect erroneous line points, that is, lines that are adjusting outside of tolerances because of line points. You would leave this option unchecked for analysis purposes. If this option is not checked, line points will not be constrained to lie on their adjacent parcel lines and slivers and gaps may result. Furthermore, if this option is unchecked, the Force line points onto lines if within option is unavailable, so any line points that have drifted off their adjacent lines during the adjustment process cannot be forced back onto the lines.
If during joining, a straight line was bent or flexed to fit the location of a line point, the line will be straightened back to its original dimensions. The line point (and lines connected to the line point) will be adjusted to the straight line provided that the adjustment of the lines is within tolerances. You can choose to keep lines bent to line points by checking the Bend lines to fit line points that are offset beyond option. Lines that lie further from their line points than the tolerance specified for bending lines will be bent to their line points.
Use dependent lines in adjustment
Dependent lines are often used to represent parcel line types that are dependent on parcel boundary lines, such as easement lines. In most cases, dependent lines should follow, and be dependent upon, the boundary lines of a parcel. If the this option is unchecked, dependent line dimensions will not participate in and influence the adjustment process. After the adjustment is completed, any dependent lines will receive the same adjustments that were applied to the parcels. If the option is checked, dependent line dimensions will participate in and influence the outcome of the adjustment.
Adjustment postprocessing
After the least-squares adjustment is completed, a few postprocessing procedures can be applied to enforce geometric constraints. These include enforcing line points and straight lines.
Force line points onto lines if within
During the adjustment process, lines may be adjusted such that line points shift off their adjacent parcel lines. When a line point lies within the force line points tolerance, it is forced back onto its parcel line after the adjustment process has completed. If it is outside the tolerance, a warning message is written to the adjustment report.
Straighten road frontages
Often, a series of adjacent lots in a subdivision plan requires that front and/or back lot lines have the same bearing, meaning that the individual lot lines are intended to be collinear. If a series of lines are categorized as road frontages (category 5), then this tolerance can be used to ensure that these lines are collinear. Road frontage lines are made collinear if the bearings differ by less than the specified tolerances (in length and in seconds).
Running a least-squares adjustment
A fabric least-squares adjustment should be run repeatedly until the maximum and average coordinate shifts become 0.0 or do not change.
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Maximum and average coordinate shift of the least-squares adjustment |
Each time an adjustment is rerun, an adjustment iteration has been performed. You are iterating an adjustment if you click Run on the Adjust Coordinates dialog box or Accept on the Least Squares Adjustment Summary dialog box, then click Run again on the Adjust Coordinates dialog box to run another adjustment iteration. Once OK is clicked on the Adjust Coordinates dialog box, the adjustment is completed with no more iterations. Once the maximum and average coordinate shifts become 0.0 or do not change, the adjustment has converged to an optimal solution.
Tip:Adjustment post processing should be enforced only after adjustment convergence is reached. The force line points and straighten road frontages options should remain unchecked until the adjustment has converged. Once the adjustment has converged, run the adjustment one more time with these options checked.
To run a least-squares adjustment, follow these steps:
- Select the parcels you want to adjust in an edit session and click Parcel Editor > Adjust to open the Adjust Coordinates dialog box.
- Enter the appropriate tolerances and click Run. Leave the post processing options unchecked.
- If you are happy with the adjustment summary, click Accept on the Least Squares Adjustment Summary dialog box.
- If the adjustment has not converged, run the adjustment again and click Accept to iterate the adjustment.
- Once the adjustment has converged, run the adjustment one more time and specify post-processing tolerances.
- Click OK on the Adjust Coordinates dialog box to apply the adjustment.
Tip:A minimum of two control points are required to run a fabric least-squares adjustment. You do not need to select control points because active control points are linked to fabric points. Control points are automatically detected on the points of selected parcels.
Tip:All parcels extracted and opened in a parcel fabric job will be adjusted by a least-squares adjustment.