About control points in the parcel fabric
This topic applies to ArcEditor and ArcInfo only.
Control points define and publish accurate, surveyed x,y,z coordinates for physical features on the surface of the earth. A control point network is 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.
In a fabric least-squares adjustment, the horizontal and vertical coordinate system (the datum) of the control points is transferred to the parcel fabric.
Control point coordinate values are held fixed in a fabric least-squares adjustment.
Parcels in the fabric are defined by a series of dimensioned lines and corner points. Parcel corner points are the from- and to-points of the parcel lines. In many cases, records of survey or plans display parcel dimensions but do not include coordinate information for the parcel corners. The parcel fabric provides an environment that allows you to add parcels without knowing anything about the coordinates while also letting you add, at a later date, control points.
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.
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.
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.
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 fabricand 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.
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.
Any control points showing large residuals should be investigated before proceeding with a least-squares adjustment. 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.
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. If check fit residuals are large and inconsistent with each other, it makes no sense to run a least-squares adjustment. Parcel lines are going to have to adjust at least by that amount, and the result will be an unstable adjustment. The reasons for the large and inconsistent residuals should be identified first, and the problems should be rectified. In most cases, very large check fit residuals are caused by control points that are linked to the wrong fabric point.