About accuracy in the parcel fabric
This topic applies to ArcGIS for Desktop Standard and ArcGIS for Desktop Advanced only.
Each parcel dimension and thus each parcel in the parcel fabric has an associated accuracy. This is because parcel dimensions are derived from raw survey measurements, which always have associated accuracies. Measurement accuracy is a way for surveyors to measure the quality, or correctness, of their measurements.
By default, accuracy categories in the parcel fabric are derived from the survey date. This is because, in general, surveying equipment was less precise in the past than it is today.
You can assign an accuracy category at the parcel line level, the parcel polygon level, and at the plan level.
Accuracy and the fabric least-squares adjustment
Accuracy categories in the parcel fabric are only used in a fabric least-squares adjustment. Parcels with a high accuracy category will thus have a higher weight in the adjustment and will adjust less than those parcels with lower accuracies and thus lower weights. Low-accuracy parcels will adjust around the more accurate parcels.
Parcel fabric accuracy categories
The parcel fabric supports seven accuracy categories, with accuracy category 1 being the highest and accuracy category 7 being the lowest. Any parcel line assigned with accuracy category 7 is excluded from the fabric least-squares adjustment. This means the parcel line will have no influence on the outcome of the least-squares adjustment, but it will adjust along with the rest of the parcel fabric. By default, the parcel fabric assigns the following standard deviation and date ranges to accuracy categories:
|
Accuracy level |
Std. deviation bearing (secs) |
Std. deviation distance (m/ft) |
PPM (m) (parts per million) |
Description |
|---|---|---|---|---|
|
1 |
5 |
0.001/0.00328 |
5 |
Highest |
|
2 |
30 |
0.01/0.0328 |
25 |
After 1980 |
|
3 |
60 |
0.02/0.0656 |
50 |
1908–1980 |
|
4 |
120 |
0.05/0.164 |
125 |
1881–1907 |
|
5 |
300 |
0.2/0.656 |
125 |
Before 1881 |
|
6 |
3,600 |
1/3.28 |
1,000 |
1800 |
|
7 |
6,000 |
10/32.8 |
5,000 |
Lowest—excluded from adjustment |
In the table above, the standard deviations for distance are shown in both meters and feet. You can change the standard deviation and PPM values by editing the Accuracy table in the parcel fabric.
To view and edit the parcel fabric Accuracy table, use the Make Parcel Fabric Table View geoprocessing tool in ArcMap to create a table view of the fabric Accuracy table you want to edit.
Standard deviations and PPM
Deviation is a measure of the spread of values when measuring the same target. So if a surveyor were to measure the same target point many times with the same instrument, he/she would want the spread or range of values to be as close to each other as possible (in other words, the standard deviation as small as possible).
The standard deviation for the most accurate measurements in the parcel fabric is 5 seconds for bearings and 0.001 meters for distances.
Parts per million in surveying is a measure of change or uncertainty in measurements. The value given is out of a million. In the parcel fabric, the PPM value is an indication of accuracy. For accuracy level 1 in the table above, 5 PPM means an accuracy of 5 millimeters over a kilometer (1 kilometer = 1,000,000 millimeters).
Assigning accuracy categories
Accuracy categories can be assigned at the parcel line level, the parcel polygon level, and at the plan level.
When assigning an accuracy category at the plan level, all new parcels created in the plan will inherit that accuracy category. Even though the accuracy category on an individual parcel is NULL, internally it is assigned the accuracy category of its plan. Plan accuracy is set under the Attributes on the Plan Properties dialog box. You can override the plan accuracy by specifying an accuracy category for an individual parcel. Parcel accuracy is set under the Properties tab on the Parcel Details dialog box.
All related parcel lines will inherit their accuracy category from the parcel. Even though the accuracy category for an individual parcel line is NULL, internally it is assigned the accuracy category of its parcel. You can override the parcel accuracy category by specifying an accuracy category for an individual parcel line in the traverse grid under the Lines tab of the Parcel Details dialog box.
The following default accuracies are used:
- If no accuracy category has been specified at the plan level, then the default accuracy category of 4 is used for parcels and their related lines.
The default accuracy can be overridden by specifying a different accuracy category on any parcel or line.
- If no plans have been defined when migrating parcel data, data is migrated to the default <map> plan, which has a default accuracy category of 4.
Tip:You can assign plans to parcel data after the data has been migrated by editing the PlanID field on the parcel table. Set the PlanID of the parcels to the ObjectID of the plan you want to assign.
- If dimensions are generated from line geometries (inversed) during data migration, then the accuracy category of the inversed lines are set to the value specified for Accuracy Category for Inversed Line on the Load A Topology To A Parcel Fabric geoprocessing tool.
- Precise connection lines are assigned by default an accuracy level that is one level higher than the parcel accuracy. For example, if the parcel accuracy level is 3, then precise connection line accuracies are 2.
- Digitized lines are automatically assigned a default accuracy category of 6.
- Radial lines with lengths greater than 500 meters are assigned an accuracy category of 7 (excluded from affecting the outcome of a fabric least-squares adjustment).