Chi Square test failed...

546
6
05-18-2021 01:52 PM
chadhickman
Occasional Contributor

Hello I am trying to adjust a portion of my parcel fabric and keep receiving the attached warning message, could anyone shed some light on what this means? How and/or what should I correct the distance or direction accuracy values to?

Thank you.

0 Kudos
6 Replies
ChristineLeslie
Esri Contributor

Hi -

Here is an explanation of the CHI test from the documentation:

"If the Chi-square test fails and is too low, the estimated a priori accuracies may be too low (meaning that the numeric accuracy values may be too high). To improve the test, increase the estimated accuracies (decrease the numeric values). For example, decrease a 30 seconds accuracy estimate for directions to 20 seconds and rerun the adjustment.

If the Chi-square test fails and is too high, the estimated a priori accuracies may be too high (meaning that the numeric accuracy values may be too low). To improve the test, first use the adjustment feature classes to identify and correct any measurements that are flagged as outliers. Then rerun the adjustment. If there are no measurements flagged as outliers, decrease the estimated accuracies (increase the numeric values). For example, increase a 0.59-foot estimate for distances to 0.8 feet."

So for you, you need to go ahead and increase your a-priori accuracies (decrease the numeric values). 

"A priori accuracies are specified in the Direction Accuracy and Distance Accuracy fields of the parcel line or connection line feature class. If the fields are missing accuracies, the defaults of 30 seconds for directions and 0.15 meters (0.49 feet) for distances are used."

See this help topic for more information

https://pro.arcgis.com/en/pro-app/latest/help/data/parcel-editing/runleastsquaresadjustment.htm

Let us know if this helps answer your question or if you have any more questions.

Regards

Christine

chadhickman
Occasional Contributor

Christine,

Thank you for the reply. I have been incrementally decreasing the numeric value for both fields in all pertinent parcel line features but I still keep getting the warning message. Should I just keep decreasing the values until the warning message disappears?

I feel like I am just picking numeric value out of thin air, is there a “systematic” way I should be going about this?

Chad

0 Kudos
chadhickman
Occasional Contributor

Another question I had is, am I supposed to select the constrained control points when performing a adjustment?

 

Thanks 

0 Kudos
ChristineLeslie
Esri Contributor

Hi Chad

Are the dimensions on your parcel lines generated from the geometry (inversed)?

If dimensions are generated from the line geometry there is no error. So any a priori accuracies you put in will always return a Too Low Chi test - because there is zero error in the measurements. In this case the CHI test is not really meaningful. 

Chi test will start to become meaningful once "real" dimensions are added from the record.  How does your data look - are the dimensions from the record?

For the constrained adjustment:

A selection of both parcels and lines can be used as input to the least-squares analysis. When parcels are selected, dimensions on the parcel lines and coordinates of the endpoints of the parcel lines will be used as input to the adjustment. If lines or connection lines are individually selected, the dimensions on the lines and the coordinates of its endpoints will be used as input to the adjustment.

It is not necessary to select the control points.

 

Christine

 

0 Kudos
AmirBar-Maor
Esri Regular Contributor

@chadhickman  Would it make sense to meet with you where you can:

- Explain your use-case

- Show us your data

- Better understand your challenge

- Examine the results  

Thanks,

Amir

0 Kudos
chadhickman
Occasional Contributor

Amir (& Christine)

Thank you for your replies! A meeting would be great! 

Brief background, our PF is in a statewide database, it houses the lands the Illinois Department of Natural Resources (IDNR) owns, manages, and leases. Each site (State Park XYZ) is made up of parcels conveyed via Deeds, Easement Deed's, Lease Agreements, etc. The PLSS was also digitized for any parcel IDNR owns, manages, and leases. PLSS goes from the section to quarter-quarter (1/16th sections) level. Previous SDE-GDB was based on The ArcGIS Parcel Data Model, von Meyer. Properties and PLSS were digitized from legal descriptions, original survey notes and plats, aerial photography, etc. Recently migrated our previous GDB to the Pro PF and have Enterprise stood up and can edit via branch versioning.


I work closely with the IDNR Surveyor and would like to begin the process of importing his survey points as control points and have them replace corresponding PF points too. This will likely be done incrementally, on a site-by-site basis. His control points are either PLSS related (section corners, quarter corners, etc.) or parcel related (parcel control corners). Furthermore, we want existing features related to the point(s) to move too.


I have been testing with Transform (modify pane), LSA, and most recently the Import PF Points GP tool. I can get all three to execute but I am not 100% sure which method I should be using? Thus far it seems that the Import PF Points GP tool works best but it is a manual process. LSA I am unclear on some of the accuracy values I should be entering and what kind of adjustment should I be seeing?


Thank you,

 

0 Kudos