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How to determine the changes between two polygon shapefiles of similar data

WhitneyNewcomb

The city I work for gets its parcel data from the county once a month so we have several years of almost the same data. I want to be able to determine where changes occur between every new month. So what are the changes between September to October, October to November, etc. And by changes I mean any change in geometry (like parcel splits or merges) or changes in attributes (especially changes in ownership).

I've contacted ESRI about this and they weren't able to do a whole lot for me.

Here's what I've tried so far:

I am able to find changes in geometry by using the Select by Location: Identical features and inverting the selection.

This works fine but does not get me changes in attributes.

I would use the Feature Compare tool but this does not work as it requires the same number of rows in both tables - which I don't have.

I also found an old arcgisscripting tool and converted it to arcpy and it worked with point datasets (but at a certain point in time the County changed how they did their parcel data and went to polygons only which the script does not work with).

So I'm at a loss for what to do now.

Does anyone have any advice for me?

Please let me know if there's any info I can give to help you help me.

-Whitney Newcomb

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MikeVolz

Does the data contain any date/time fields that you can use to differentiate the data? Do you know if the county enforces attribute changes to be tracked by populating a date/time field (Editor Tracking would be useful in this scenario, but you would need to contact the county to see if they are implementing this procedure)?

DanPatterson_Retired

for your last option... doesn't work with polygons....produce a centroid file of your polygons, then you can use that at least for the attributes

NicoleJohnson_4-1779366541485.png

WhitneyNewcomb

Unfortunately, no it doesn't. They don't even fill out their metadata. It's rather upsetting. Hah!

WhitneyNewcomb

Dan,

That actually might work for me to some extent, but I'm worried that I'll have a hard time converting it back to a polygon file.

The script I found grabs the different fields of the two parent files and dumps them into one new file so that for every point in it there is (usually) actuallly two points stacked on top of each other. One from the earlier month and one from the later month.

DanPatterson_Retired

you wouldn't need to convert the centroid file back to a polygon file... it would just be used to identify which record attributes have changed... Your polygon file would remain untouched

gpnobbir

Could you try first converting the polygons to polylines and then using the polylines to compare? If you want you can use Polygon to Line under Data Management Toolbox, in Features toolset.

http://support.esri.com/en/technical-article/000007707

WhitneyNewcomb

You're totally right.

Thank you Dan. You have been most helpful!

DanLee

Hi Whitney,

We are developing tools to help solve conflation problems. There is a GP tool Detect Feature Changes which currently only works on line features, such as roads, parcel lines, etc. This tool detects spatial (geometry) and attribute differences between two sets of line features.

We are investigating polygon use cases. What you described seems a good case for us. Would you be able to share some sample data, two or three monthly data of a small area? If so, you can attach your data in email to dlee@esri.com.

In the meantime, I think it's a good idea to first exclude the spatially identical ones, as you did. For the remaining parcels, perhaps try Spatial Join Oct vs. Nov, and then Spatial Join Nov. vs Oct. That could give you 1:n relationships both ways to catch what's been split or combined.

Thanks,

Dan (Esri Conflation Team)

WhitneyNewcomb

Hi Dan,

Let me just check with my higher-ups and make sure I can share the data. It shouldn't be a problem and I'll email you ASAP.

Also, Could you elaborate on how the spatial joins will show me these differences? Thank you.

DanLee

Great! I look forward to your sample data. Thanks again for sharing the data and for your time.

Spatial Join tool allows you to join 1-to-1 or 1-to-many; you can choose one of many Match Options, such as INTERSECT, CONTAINS, HAVE_THEIR_CENTER_IN, etc. If a polygon in Oct data Is split into 3 polys in Nov. data, the tool will find the relationship. If you choose 1-to-1 join, you will get a Join Count field with the value 3. If 2 polys are merged, the two polys will join the same merged poly. There are other tools to analyze duplicates, e.g. Find Identical, Frequency.

WhitneyNewcomb

Hi Nobbir,

This doesn't work for because attributes do not come along with the polylines. I really need to see the differences in attributes.

LindaRadomile

Hi Dan,

Has there been any movement on this tool? We too are looking for a polygon comparison to highlight additions, removals, or modifications.

DanLee

Hi Linda,

I would still like to ask for your data, if possible, to see the specific case. I did help Whitney to solve the problem he faced once I got his data; he actually had common attributes between the two sources, so Add Join followed by Select Layer By Attribute did the trick. Would you be able to share sample data? You can email to dlee@esri.com

Thanks,

Dan

GuillermoMarinez1

Hi, i have the same problem, i am going to implement the folowing process in model builder. When i finished the model i will share them.

looking for the solution i found an tool in datainteroperability that perform the "change detector" operation, if one have this extension can find the diferences between two featureclass.

deleted-user-_ZI9nJL9TGxh

Hello Guillermo,

Might your model merit a script that searches the tables to compare for changes in the APN before it finds spatial changes? We are working on that angle. My colleague and I are using the Union tool on the two different files with a unqiue field for APNCURRENT and APNPRIOR . I'm working at putting it into a python script but new to Python.

LIsa Pierce

gpnobbir

If you need help with Python scripting please shoot me a mail at nahmed@esri.com (also some sample data will be helpful)

ChrisDonohue__GISP

I do this all the time for the City I work for to "true-up" the land base layers. Here's two processes that seem to work well in identifying changes:

Detecting geometry changes. For both the existing polygons and the new polygons, convert the polygons to lines. Then run Erase on the line files to subtract the New from the Existing. Finally, run Multipart to Singlepart on the result. The output of all this will be lines that show where the geometry differs. This can be helpful in knowing exactly what needs to be edited, which can be an advantage over the "Select By Location, Identical" process (which is another way to do this).
Detecting Attribute changes. If you have a common unique ID, an Attribute Join can be used to combine the two attribute tables. Some examples: APN, TaxID. Obviously will vary depending on your attribute fields. Once the two datasets are joined, Queries can be run for each field of interest to see if the attribute has changed. Generalized example: Table1.APN <> Table2.APN.

Of course, one can do much more complex processing, but that will depend on knowing more of the specifics of the data.

Chris Donohue, GISP

JosephCornellisson

Dan

Has any work been done on this polygon comparison tool. I have a similar situation trying to compare two parcel data sets that have changed over time

thanks

Joe

DanLee

Hi Joe,

Thank you for checking. The short answer is no, we have not developed this tool/option yet. Part of the reasons is that we are still researching on use cases, not only parcel polygons but also other features, in order to fully understand the spectrum of scenarios and to see what's in common for a tool design.

Would you be able to share your parcel data with me? Could you describe in what way the two data sets are different, e.g. are they mostly identical except the changed features; do they have common attributes/IDs, such as APN?

Thanks,

Dan

JosephCornellisson

Dan

Unfortunately I am not the owner of the data, just a user. It is considered

proprietary data. I will check with them but I am not hopeful.

Please call and I can give you the use case.

Joe

Joseph Cornellisson

Geologist/GIS Manager

Division of Minerals Evaluation

Appraisal and Valuation Services Office (AVSO)

U.S. Department of the Interior

Denver Federal Center, Bldg 46, Suite 102

PO Box 25247

One Denver Federal Center

Denver, CO 80225

Phone: 303-969-5941

Fax: 303-969-5949

E-mail: Joseph_cornellisson@ios.doi.gov <Joseph_Cornellisson@ios.doi.gov>

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TedKowal

This is a conflation process.  I use MSAccess as my database and perform Attribute change detection using the following methodology using the Database SQL (Not using ArcGIS).

There are three kinds of differences:

-New records in tblDataToday
-New records in tblDataYesterday or lost records in tblDataToday
-Changed records that are both present in tblDataToday and tblDataYesterday

1.  To browse new records in tblDataToday:

SELECT tblDataToday.*
FROM tblDataToday LEFT JOIN tblDataYesterday ON tblDataToday.ID = tblDataYesterday.ID
WHERE (((tblDataYesterday.ID) Is Null))

 

2  To browse new records in tblDataYesterday or lost records in tblDataToday:

SELECT tblDataYesterday.*
FROM tblDataToday RIGHT JOIN tblDataYesterday ON tblDataToday.ID = tblDataYesterday.ID
WHERE (((tblDataToday.ID) Is Null))

 

3  To browse changed records simultaneously in tblDataToday tblDataYesterday:

	3.1 Create a query qChangedData:
		SELECT tblDataToday.*,"tblDataToday" as  ChangedIn
		FROM tblDataToday
		UNION ALL SELECT tblDataYesterday.*, "tblDataYesterday" as ChangedIn
		FROM tblDataYesterday

	3.2 Create a query qChangedIDs:
		SELECT All_IDs.ID
		FROM (SELECT DISTINCT * FROM (SELECT tblDataToday.* FROM tblDataToday union all SELECT tblDataYesterday.* FROM tblDataYesterday  )  AS uAll)  AS All_IDs
		GROUP BY All_IDs.ID
		HAVING (((Count(All_IDs.ID))=2));

	3.3 Create a query qChangedData:
		SELECT qChangedData.*
		FROM qChangedIDs INNER JOIN qChangedData ON qChangedIDs.ID = qChangedData_u.ID
		ORDER BY qChangedData.ID, qChangedData.ChangedIn
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MikeVolz

Ted:

Does this process work for you in Pro?

TedKowal

No I do not use pro nor currently willing to migrate. Pro does not recognize an Access personal geodatabase, until it does Pro is a no go for me. I am struggling learning qgis as my fallback.

This is a perfect use case demonstrating the time saving ability of access (even though it performs the geometry operations slow) but the flexibility and ease of use FAR OUTWEIGH any geometry processing.

Enable ArcGIS Pro to access ESRI Personal Geodatabases

bharold

Hello, please see this blog - which has a link to an updated sample for change detection

https://community.esri.com/community/open-platform-standards-and-interoperability/blog/2019/11/15/see-what-changed-and-where-it-changed

Requires ArcGIS Data Interoperability extension.

LLyons

Hi Dan,

Just wondering if there has been any updates? I'm trying to identify the changes between the 2010 Census TIGER boundaries and the 2020 Census TIGER boundaries. Thanks!

bharold

This tool wraps the Data Interoperability function in a Python script tool:

https://pm.maps.arcgis.com/home/item.html?id=834e3ba8034e4e7f83d9fc4fcfb5713c

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