About control points in the parcel fabric

This topic applies to ArcGIS for Desktop Standard and ArcGIS for Desktop Advanced only.

Control points define and publish accurate, surveyed x,y,z coordinates for physical features on the surface of the earth.

Control points are added to the parcel fabric so that parcels can be adjusted to the control point network in a fabric least-squares adjustment. The result is parcels that are accurately georeferenced on the surface of the earth. While parcel dimensions accurately define parcel boundaries in relation to each other, control points, when used in a least-squares adjustment, result in accurately defined spatial locations for parcel corner points.

Control points in the parcel fabric are defined by x,y coordinates on a projected grid (a 2-D plane) and, separately, by a z-value representing height above sea-level. Parcel fabric control points do not represent coordinates in a three-dimensional Cartesian coordinate system that defines an actual ground position. The distance computed between two control points in the parcel fabric (using x and y only) is a grid distance.

Distance attributes in the parcel fabric are considered to be ground distances. The parcel fabric least-squares adjustment uses the z coordinate values on the control points to compute coordinates on the projected grid. Thus all parcel fabric coordinates (including control) are projected coordinates on a grid (2-D plane) and are computed from record distances measured/stored as ground.

Note:

Control point coordinate values are held fixed in a parcel fabric least-squares adjustment.

Adding control points to the parcel fabric

Control points are added to the fabric as stand-alone control points that are not connected to the parcel fabric, or they can be added and linked to fabric points. Control points can only participate in a fabric least-squares adjustment if they are linked to an existing fabric point. Any point in the fabric can be linked to a control point. This is done when a control point location can be correctly identified with a fabric point location.

Control points are located on parcel corner points or at the endpoints of connection lines. When control points are added and linked to an existing fabric point, the point might not necessarily match the location of the control point. This is because the control point location is more accurate than that of the fabric point location. Even though the control point may not sit exactly on top of the fabric point, it is still linked to the fabric point.

Often, control points do not lie on parcel corner points; for example, they are located on street centerlines. In this case, connection lines can be added from parcel points to the control point locations. You can also use an unclosed parcel to model street centerlines and have the control located on the corner points of the unclosed parcel.

Unclosed parcel

Control point on an unclosed parcel representing a street centerline

When creating connection lines that connect to a control point, the dimensions of connection can be implied or calculated from the record of survey or plan.

Connection line to control point

Control points can be manually added, where the coordinates are typed in, or control points can be imported into the parcel fabric. When adding control points manually and individually, the link between a fabric point and a control point can be immediately established. When importing a set of control points, a tolerance or search radius can be specified where the control point is linked to the first fabric point found within the search radius. If no fabric point is found in the search radius, a control point is imported as a stand-alone control point and can be manually linked to a fabric point at any time. Manually linking control points to fabric points is useful when control points are added first and parcels are added later, for example, adding parcels to a new, empty parcel fabric.

Linking control points to fabric points

Control points can be linked and relinked to fabric points when the Control dialog box is open. The Control dialog box can be opened by clicking the Maintain Control Points tool on the Parcel Editor toolbar. Control points are linked to fabric points by clicking to snap on the control point, then clicking to snap on the fabric point. The Network Point ID attribute on the Control dialog box is immediately updated with the fabric point ID.

Fabric point ID of control point

Sometimes, the point ID to which a control point is linked can change. This can happen when a fabric point is merged with another point; for example, the endpoint of a newly added connection line is the same location as another point in the fabric and needs to be merged with that fabric point into a single location.

Active control points

There are two types of control points in the parcel fabric: active and inactive control points. Active control points are linked to a fabric point and are used in the fabric least-squares adjustment. Inactive control points are either stand-alone control points that are not linked to any fabric point or have been deactivated from participating in a fabric least-squares adjustment. In some cases, not all of the control points need to be active in an adjustment, and deactivating control points is a way to detect and remove potential blunders in the data that may cause an adjustment to fail. The fabric least-squares adjustment requires a minimum of two control points; this is called a minimally constrained adjustment. Doing a minimally constrained adjustment helps to detect blunders in the dimensions because the influence of the control point coordinates is minimized. For large adjustment areas, a network of well-distributed control points is recommended.

Active and inactive control points

Checking the fit of control points

When there are more than two active control points in the map display, you can run a check fit of the control points. The check fit function checks how well the parcel network fits with the control network. Check fit is also useful for evaluating control points for use in a fabric least-squares adjustment. Control points that appear to be outliers can be deactivated from participating in the adjustment.

The check fit process computes transformation parameters between the linked fabric point coordinates and the coordinates of the control points. The transformation used is a Helmert transformation. The computed parameters (rotation, shift in x, shift in y, and scale) are then applied to the linked fabric point coordinates to compute temporary new values for the fabric point coordinates. The differences between the newly computed fabric point values and the original control point values are reported as residuals for each active control point. The residuals (dx, dy values) show how fabric point coordinates, when transformed to their control point network, fit with that network. Residuals are reported next to each control point on the Control dialog box.

Check fit residuals

Check fit residuals and the fabric adjustment

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.

Large, inconsistent check fit residuals indicate that the parcel network does not fit well with the control network. It is not recommended that a least-squares adjustment be run on parcels that have large check fit residuals on their associated control points. The fabric adjustment will most likely fail because parcel lines will need to adjust using large tolerances to fit with the control network. Least-squares adjustment tolerances should not be large for the adjustment to succeed. This will result in poor, unreliable results. The parcel network below (slightly exaggerated) does not fit well with the control network and this is confirmed by the large, inconsistent check fit residuals.

If you know that the parcel fabric has fairly good measurement precision and is devoid of blunders, any erroneous or incorrectly identified control points will show large residuals when running a check fit. If you know your control point coordinates are sound, large residuals will most likely indicate a blunder surrounding the associated fabric point coordinate. If the source of the higher residual is not immediately apparent, you can choose to remove the control point from the least-squares adjustment. If there is a blunder in the parcel data surrounding the associated fabric point, the blunder will be exposed in the least-squares adjustment.

If a check fit is run after a least-squares adjustment, residuals for active control points will be zero because all the linked fabric points have been transformed into the control point coordinate system during the adjustment.