About accuracy
This topic applies to ArcGIS for Desktop Standard and ArcGIS for Desktop Advanced only.
Parcel dimensions are derived from raw survey measurements observed in the field. Raw survey measurements always have an associated accuracy, which is a reflection of the quality of the measurements. The closer the measurement is to its true value (the more correct it is), the more accurate the measurement. Because parcel dimensions are derived from raw survey measurements, parcel dimensions have an associated accuracy as well.
Each parcel and each parcel line in the parcel fabric can have an associated accuracy category. By default, accuracy categories in the parcel fabric are derived from the survey date on the parcel's plan. This is because, in general, surveying equipment was less precise in the past than it is today. You can assign an accuracy category to a parcel, a parcel line and to a parcel plan.
Accuracy and the parcel fabric adjustment
Accuracy categories in the parcel fabric are only used in a parcel fabric adjustment. Parcels with a high accuracy category will 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 6 being the lowest. Any parcel or parcel line assigned with accuracy category 7 is excluded from the fabric least-squares adjustment. This means the parcel lines will have no influence on the outcome of the least-squares adjustment, but they will adjust along with the rest of the parcel fabric. By default, the parcel fabric assigns the following standard deviations 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.
To view and edit the Accuracy table, use the Make Parcel Fabric Table View geoprocessing tool located under System Toolboxes > Parcel Fabric Tools > Layers and Table Views.
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 to a parcel, a parcel line and to a parcel plan. When assigning an accuracy category to a parcel plan, all new parcels created in the plan will inherit that accuracy category. Even though the accuracy category on the 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 a different 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 a different 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 for a plan, a default accuracy category of 4 is used for the plans parcels and their related lines.
- 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, 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 category that is one level higher than the parcel accuracy. For example, if the parcel accuracy category is 3, precise connection line accuracy categories are set to 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).