In ArcGIS Pro, a least-squares adjustment can be run on a parcel fabric in the following scenarios:
- Use the Analyze Parcels By Least Squares Adjustment tool to run a consistency check on newly added data to identify potential mistakes or outlier measurements.
- Use the Analyze Parcels By Least Squares Adjustment tool to run a weighted least-squares adjustment to evaluate how newly added data affects the spatial accuracy of the parcel fabric.
- Use the Apply Parcel Least Squares Adjustment tool to apply the results of a weighted least-squares adjustment to update and improve the accuracy of parcel fabric points.
Learn more about running a least-squares adjustment
The parcel fabric is a redundant measurement network. Parcel lines connect parcel corner points to form a measurement network. Lines connect at common points and have dimensions, which define geometric distance and angle relationships with other points.
In the parcel fabric, the adjustment uses dimensions on redundant parcel lines to estimate best-fit coordinates (x,y,z) for parcel fabric points. The adjustment uses network redundancy to identify lines with potential dimension errors and lines with dimensions that do not fit with the rest of the network (outliers).
To summarize, a least-squares adjustment works on a parcel fabric as follows:
- The adjustment uses direction and distance dimensions on both current and historic parcel boundary lines.
- Points connected to boundary or connection lines are also used as measurements in the adjustment.
- Line dimensions and point coordinates can be weighted in the adjustment. Coordinates and dimensions with higher accuracy are given higher weights; that is, they are given less allowance for change. They will have a greater influence on the outcome of the overall adjustment results by staying closer to their original position or dimension.
Adjustment types
Different types of adjustments can be run on the parcel fabric depending on whether you are evaluating or improving spatial accuracy.
- Free network adjustment—The measurement network is not constrained by control points, and measurements are checked for mistakes.
- Weighted/Constrained adjustment—Two or more control points are included in the adjustment to constrain the measurement network and compute updated coordinates of free points.
Consistency check using a free network adjustment
A consistency check runs a free network adjustment on the input parcels to ensure that parcel lines do not contain mistakes in their dimensions. For example, a consistency check can be run after new parcels have been manually entered from a new record.
A consistency check evaluates the dimensions of the input lines, and dimensions that do not fit with the solution are identified as outliers or possible blunders.
Weighted least-squares adjustment
A weighted least-squares adjustment is a constrained adjustment that uses control points and line dimensions to estimate updated, more spatially accurate coordinates for parcel fabric points. A weighted least-squares adjustment can be run to evaluate and improve the overall spatial accuracy of the parcel fabric. Control points are points with known x,y,z coordinates. Control points constrain the adjustment and are used to compute updated coordinates for free (unconstrained) points.
In a weighted least-squares adjustment, line dimensions and control points can be weighted based on their accuracies. Control point accuracies are known and weights can range from completely constrained (highest accuracy and x,y,z does not change) to lower weights (lower accuracies) that allow more movement. Dimension accuracies are generally based on the legal parcel record. Parcel dimension from more recent parcel records generally have higher accuracies and thus higher weights in the least-squares adjustment. Lines and control points with higher weights have a greater influence on the outcome of the least-squares adjustment.
A weighted least-squares adjustment also can be used to update the coordinates of lower-weighted control points and identify areas in the parcel network where more control is needed.