# About accuracy in the parcel fabric

This topic applies to ArcEditor and ArcInfo only.

Each parcel dimension and thus each parcel in the parcel fabric can have 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 in the parcel fabric is defined by survey date. This is because, in general, surveying equipment was less precise in the past than it is today.

You can assign accuracy at the parcel line level, the parcel polygon level, and the plan level.

## Accuracy and the fabric least-squares adjustment

Accuracy in the parcel fabric is significant 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 or levels, 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 standard deviation and date ranges to accuracy categories as follows:

Accuracy categories in the parcel fabric

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

Table of accuracy categories in the parcel fabric

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. Once the table view is created, click List to start an edit session and open the table.

## 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 the plan level.

When assigning an accuracy at the plan level, all new parcels created in the plan will inherit that accuracy level. Plan accuracy is set on the Plan Properties dialog box under the Attributes tab. You can override the plan accuracy by specifying a different accuracy for an individual polygon. Parcel accuracy is set under the Properties tab on the Parcel Details dialog box. When assigning accuracy at the parcel level, all related parcel lines will inherit their accuracy from their parcel. You can override the parcel accuracy by specifying a different accuracy for an individual parcel line in the traverse grid under the Lines tab of the Parcel Details dialog box.

If no accuracy has been specified, the following default accuracies will be used:

• Boundary lines, connection lines, dependent lines, radial lines, road lines, and origin connection lines are assigned a default accuracy level of 3.
• Precise connection lines are assigned a default accuracy level of 2. (By default, precise connection accuracy is always one level higher than the parcel accuracy.)
• Digitized lines and lines with dimensions computed from the line geometry (inverted lines) are assigned a default accuracy level of 6.
• Compiled parcel lines are assigned a default accuracy level of 5. Compiled parcels are those parcels that have been assembled from previous plans and do not represent a survey carried out on the plan date. The Compiled field on the parcel is set to True.

5/6/2011