Tutorial: Running a parcel fabric adjustment

Available with Standard or Advanced license.

Complexity:

Advanced

Data Requirement:

ArcGIS Tutorial Data for Desktop

Data Path:

\ArcGIS\ArcTutor\Parcel Editing

Goal:

Successfully complete a fabric least-squares adjustment

This tutorial describes a general workflow for a parcel fabric adjustment. The adjustment is a least-squares adjustment that uses the stored bearings and distances on the parcel lines—together with the control point coordinates—to obtain a best-fit solution of all the points in the selected network. In this tutorial, the control points have already been entered and linked to their corresponding fabric points. When a parcel fabric adjustment is run, an adjustment summary is displayed. The adjustment summary displays information about how well line dimensions (COGO attributes) have adjusted to the best-fit solution. The summary also enables you to identify any problems or errors with the parcel data that may need to be corrected. Once errors have been corrected, the adjustment can be applied to parcel fabric points in the selected network. The adjustment is run repeatedly until the maximum displacement in the northings and eastings are close to zero, meaning that the adjustment has converged onto an optimal solution.

1. Open ArcMap and load the Parcel Editing map document (ParcelEditing.mxd) from your \ArcTutor\Parcel Editing\ folder.
2. Zoom to the fabric adjustment area shown below or click the Bookmarks menu and click Fabric Adjustment Area.



Fabric adjustment area

3. Click the Parcel Editor arrow and click Start Editing.
Before running a fabric adjustment, you will do a check fit of the control network. The check fit process checks how well the parcel network fits with the control network.

Note:

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.

4. Click the Maintain Control Points tool on the Parcel Editor toolbar.
All visible control points in the map extent are listed on the Control dialog box.
5. Click the Check Fit button.
Check fit residuals are displayed next to each control point.



Check fit residuals

Since all residuals are 0, the parcel network fits very well with the control network. Check fit residuals don't need to be 0 to run a fabric adjustment but should be within an acceptable range. In this case, residuals of 2 feet and above would be investigated before an adjustment is run.
6. Close the Control dialog box.
7. Click the Select Parcel Features tool on the Parcel Editor toolbar and drag a box to select all the parcels in the fabric adjustment area.



Select adjustment parcels

Tip:

You don't have to select the control points being used in the adjustment. Control points are automatically detected on selected parcels because they are linked to fabric points.

Note:

Parcel fabric line selection has been toggled off for this map document. To toggle on parcel line selection, click List By Selection on the Table Of Contents window to change the selectability of layers in the map document.
8. Click the Parcel Editor arrow and click Adjust.
9. On the Adjust Coordinates dialog box, type a value of 0.100 U.S. Survey Feet for the Distances residuals tolerance.
In this example, you start with the assumption that the data is very good, and you expect the adjustment to succeed with all residuals less than 0.3 feet (three times the distances residuals tolerance).

Note:

A parcel fabric adjustment fails if any residual is greater than three times the bearings or distances residual tolerances.

10. Check 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. 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.

11. Leave all other options unchecked. Post-processing options are used once adjustment shifts have converged to 0.00.
12. Click Run to run the adjustment.
The Least Squares Adjustment Summary dialog box appears and reports that the adjustment has failed.

Adjust Coordinates dialog box

When first running an adjustment on a parcel network, the initial step is to use the adjustment summary displayed in the Least Squares Adjustment Summary dialog box to understand how good the line dimensions (COGO attributes) are and to understand if there are any potential problems in the data that need to be fixed. The adjustment process will compute coordinates that are the best-fit position for all the fabric points; this computation uses all the bearing and distance values on the lines.

Note:

The adjustment computation does not alter the original COGO attributes on lines.

The adjustment summary compares the new positions of the parcel lines with the original observed or recorded COGO dimensions and reports the differences between the computed and observed values. These numbers are called residuals. Any residuals that are large compared with the residual tolerances that you specify on the Adjust Coordinates dialog box are outliers, meaning that they fall outside your expected range. This means that residual tolerances you specify defines your tolerance for outliers in the data. If you choose higher residual tolerances, it means that you are willing to accept larger differences between the adjusted end points of lines and the associated COGO values. The report will only show residuals that are greater than the distance or bearing residual tolerances that you specify. Until you get to know the data, you will not know what to use as residual tolerances. The approach described in this tutorial shows how to interpret results in the adjustment summary, allowing you to get to know your parcel data and choose appropriate residual tolerances for the parcels that you are adjusting. The distance residuals tolerance is the best tolerance to use when getting to know the parcel record data.

Learn more about residual tolerances on the Adjust Coordinates dialog box

13. Scroll down to locate the Parcel lines residuals report - Difference between computed and recorded values heading. There are 5 outlier lines reported under this heading.



Parcel lines report

Outlier lines are indicated in the report with a ## symbol. The last line of the report explains that the problem with these lines is that the difference between their recorded distance values and the new computed position exceeds three times the residuals tolerances that you specified. In other words, the differences between the computed distances and the recorded distances for these five lines are all greater than 0.30 feet.

Note:

The residuals are determined by a preliminary adjustment; none of the points have had their positions changed in the database at this stage.

These five outliers in the lines occur in two parcels: LS 269 / LS 269 and ROS 6692/ ROS 6692. (These parcels have been stored to represent the plan boundaries/subdivisions, so the plan name and parcel name are the same. The convention for naming in the report is [parcel name]/[plan name].)

The largest outlier of these five lines is 0.622 feet. This means that for the adjustment to succeed, the Distances residuals tolerance should be at least one-third of this value—in other words, after rounding to two decimals, at least 0.21.

One simple approach to have the adjustment succeed is to increase the residuals tolerances. However, if the residuals tolerances required for the adjustment to be successful is very large (10 feet or more), then this is a strong indication of blunders in the data. In these cases, it is important to check that the values recorded for these lines match the original record values, as found on the legal description for the parcel.

Notice that there are also two points with the object IDs 876 and 881 that are very close to each other. These two points are listed under Close points reported. Close points are parcel fabric points that lie closer than the specified Close points within tolerance and do not have a line between them.



Close points reported

Close points indicate that it is very likely these points should be the same point, that there is a topological inconsistency, and that these points should be merged. It is important to merge close points, because, by doing so, you are increasing the redundancy in the network; this is good for helping the adjustment succeed. For example, if there are two points close together and each have two lines defining their positions, you can merge the points and increase the redundancy by having four lines defining the position of just one point.

The least-squares adjustment summary has provided you with information about the actions needed to make this adjustment successful: first, to increase the residuals tolerances and second, to merge some close points. It is important to note that increasing the distances residuals tolerance to 0.21 feet will make this adjustment succeed for this particular set of parcel data. However, after the points are merged, the data will have been changed, and different adjustment summary results will be expected. A good rule of thumb—if the previous adjustment was not successful—is to use three times the previously used value. In this case, use 0.3 feet. First, however, the close points need to be merged.

The parcel fabric is not editable when the Least Squares Adjustment Summary dialog box is open; however, the information in the adjustment summary is also available in the results file.

14. Click Results File to access the results file of the adjustment summary.
The default text editor on your system opens the complete adjustment report for this adjustment.
15. Click Cancel to close the Least Squares Adjustment Summary dialog box.
16. Click Cancel to close the Adjust Coordinates dialog box.
17. Using the Select Parcel Features tool , drag an empty box on the map display to clear the selection of the parcels that were adjusted.
You will now merge the close points. To locate the points, use the Point Report tool located on the Parcel Editor toolbar.
18. Click the Point Report tool to open the Point Report dialog box.
19. If necessary, move the Point Report dialog box so that all the adjustment parcels are visible.
20. On the Point Report dialog box, click the one point button and type 876 in the Point ID text box.
21. Click the Report button.
Coordinate information, point scale, and convergence are displayed for the point.



Point reporting on one point

Learn more about the point reporting tool
22. Click Map Location to pan to the location of the point.

    The point location flashes briefly in the map.

23. Zoom in to the location of the point. You may need to click Map Location several times to help you locate and zoom in to the point.
24. In the Table Of Contents window, right-click the points sublayer and click Label Features to turn on point ID labels for the points.



Close points displayed in the map

25. Close the Point Report dialog box.
26. Click the Merges Unconnected Points Inside a Given Rectangle tool located on the Parcel Editor toolbar.
27. Drag a box around the close points.



Merge points

28. Click OK on the Mean Points dialog box to accept the default mean point tolerance.
The points are meaned (averaged) and merged into a single point.



Merged point

29. Close the adjustment results file.
30. Zoom back to the extent of the adjustment parcels. Click the Bookmarks menu and click Fabric Adjustment Area.
31. Open the job book. Click the Parcel Editor arrow and click Job Book.
Notice that there is a job with a mean points description. Each edit is tracked automatically in the job book, and the job book provides an audit of edits on the fabric.
32. Close the job book.
33. Click the Select Parcel Features tool on the Parcel Editor toolbar and drag a box to select all the parcels in the fabric adjustment area as you did in step 7.
34. Click Parcel Editor > Adjust.
Since the adjustment data has been edited by the merging of two points, you should expect the behavior and results for the adjustment summary to be slightly different. It was previously determined that a distance tolerance of 0.21 feet should be used for the adjustment to succeed. To be assured that the adjustment succeeds, you will use three times the previously used number, and use 0.3 as the tolerance.
35. Type 0.3 for the Distances check tolerance on the Adjust Coordinates dialog box.
36. Click Run.
The adjustment completes successfully. Notice that there is now information displayed about the maximum shift of the adjustment.

Maximum shift, which is the largest coordinate shift in the adjustment, is displayed above the Accept button for quick access. The same maximum shift information is displayed together with a point ID further down in the adjustment summary.

In the parcel fabric adjustment, the goal is rerun or iterate the adjustment until the points are no longer changing and the maximum shift becomes 0.00 feet for both easting and northing. The adjustment needs to be manually iterated—iteration is not automatic.
37. Click Accept on the Least Squares Adjustment Summary dialog box.
38. On the Adjust Coordinates dialog box, click Run to perform the second iteration of the adjustment.
Notice that the maximum shift is now at point ID 1018, and it has moved less than one hundredth of a foot.
Since the shift value approached closer to zero after the second adjustment, the fabric adjustment is converging. If the shift values become higher after the first iteration, this indicates that the adjustment is diverging. A diverging solution happens when one of two things has occurred:
1. There is a large blunder in the dataset, and no solution is possible until the blunder is corrected.
2. The best solution for the given dataset has already been reached, and the solution converges only to diverge on the next iteration. This occurs when 0.00 easting and 0.00 northing convergence is too stringent for the quality and precision of the dataset.
If the data is good quality and there are no major blunders, the adjustment will usually converge after three iterations.
39. Click Accept and run the adjustment for the third iteration.
The maximum shift of 0.000 easting and 0.000 northing shows that the adjustment has converged.
40. Click Accept for the third time on the Least Squares Adjustment Summary dialog box.
Now that the adjustment has converged, post-processing options can be enforced. The adjustment is run one final time with post-processing options checked. The Force line points onto lines if within option is checked to force any line points that have drifted off their adjacent lines during the adjustment process back onto their adjacent lines. The Straighten road frontages option is checked to enforce collinearity of lines categorized as road frontages. If the bearings and distances of the road frontage lines differ by less than the tolerances specified, the lines are forced to be collinear.
41. Choose the Force line points onto lines if within option and leave the default tolerance of 0.328 feet.
42. Click Run to run the adjustment one more time.
43. Click Accept and click OK on the Adjust Coordinates dialog box to apply and complete the adjustment.

Tip:

After an adjustment has been applied, you can still click Undo on the Standard toolbar to undo the adjustment.

44. Click Parcel Editor > Save Edits to save the edit session and complete the tutorial.