topic Feature to Point does NOT center point in Geoprocessing Questions

Feature to Point does NOT center point

Anonymous User — Tue, 28 Aug 2012 20:57:57 GMT

I have a process whereby I create points from 2 different polygon feature classes and then compare whether the polygons have moved or changed shape based on XY coordinates of the point feature classes. To do this I use the Feature to Point tool (Data Management/Features) and set it to place the point inside the polygon.
In ArcGIS 9.3.1 the points would be placed in the center of the polygon. In ArcGIS 10 (sp4) the points seems to be placed almost anywhere in the polygon. Therefore if I add XY coordinates and compare the two new point feature classes the comparison finds points with differentiating XY coordinates but the polygons haven't changed.

Is there a way to get the feature to point tool to place the point in the center of a polygon? Or is there another tool? Any ideas are welcome.
thanks in advance

.... and for the heck of it I just tried NOT checking "inside" and voila, the points are in the middle of the polygon. Rather bassackwards and not-intuitive....

Re: Feature to Point does NOT center point

DanLee — Wed, 29 Aug 2012 00:03:59 GMT

Feature To Point tool is the right one to use. By design, If the Inside option on the dialog box is unchecked, for an input polygon feature the output point will be located at the center of gravity (centroid) of the polygon. Depending on the shape of the polygon, this centroid may or may not be inside the polygon. If you want to make sure to get a point inside the polygon, you should check the Indise box, then the point will be inside, but not necessarily at the "center".

Again, depending on polygon shape, for the Inside option, it may not be always true that the point is both inside and at the center.

We made some improvement on the location of the point for the Inside option in ArcGIS 10. You may see the differences between 9.3.1 result and 10 result; hopefully the 10 result is better overall. So for the comparison, you should use the same software version to process original polygons and the moved/changed polygons using the same Inside or Centroid option. If no shape change happens, the resulting points should be at the same inside or centroid locations relative to the polygons. If the polygon shape changed, then the point locations will change; it may not be a reliable comparison.

Help this helps you better understand the tool.

Re: Feature to Point does NOT center point

GraemeBrowning — Wed, 29 Aug 2012 00:14:53 GMT

The process you are describing sounds like the Feature Compare tool might be useful to consider.

Re: Feature to Point does NOT center point

MarkBoucher — Wed, 29 Aug 2012 13:40:47 GMT

You could also use "Select By Location" and choose the "Target layer(s) features are identical to the Source layer feature" option. Then if you "Switch the Selection" you would see which features have changed.

Of course, this wouldn't give you any way to track what has changed or measure how much change there was unless you took it a few steps further.

Re: Feature to Point does NOT center point

BruceHarold — Wed, 29 Aug 2012 14:14:05 GMT

Re: Feature to Point does NOT center point

Anonymous User — Wed, 29 Aug 2012 14:35:19 GMT

thanks all for the replies. I was frustrated b/c in 9.3.1 if I checked Inside the point was placed in the center of the polygon but it was not doing that in 10 (sp4).
I understand the rational behind inside vs not and I'll look into some of your suggestions such as the feature compare tool and the Change Detector (if I can upgrade to 10.1)

Re: Feature to Point does NOT center point

DanLee — Wed, 29 Aug 2012 14:56:44 GMT

If you could share some sample data and describe which Inside points are not right, we can have a look of the particular cases and explain the difference between software versions.

Re: Feature to Point does NOT center point

Anonymous User — Wed, 29 Aug 2012 15:40:28 GMT

The placing in the center was a bit random. Most of the points were placed in the center but around 5% were not. Maybe those few 5% had some weird geometry that I didn't notice (i.e. I needed to run Repair Geometry). I've attached 2 large shapefiles of the polygon data. I tried a sample set but using inside and not produced the same result, which was different from when I first ran the tool on the entire feature class.
I've got some data but it's too large to send via the forum. If you've got an FTP site I can post it to (or similar) let me know.

Re: Feature to Point does NOT center point

DanLee — Wed, 29 Aug 2012 16:44:59 GMT

The point location inside a polygon is calculated from the shape of the polygon, independently from other polygons. File size or number of polygons in the file should not matter.

Could you do the following in ArcGIS 10 SP4:
If you have spotted some point locations that are not right inside polygons, you can select a few (say 5) of those polygons and use Copy Features tool to copy them to a new feature class. Then run Feature To Point with Inside option and share the output points and the copied polygons on the forum. Or you can post screen shots to show where the points are inside polygons.

If the 9.3.1 result appears better, please use the same copied polygons to run Feature To Point with Inside option in 9.3.1 and share the output points or post screen shots.

Again it is expected that the Inside points may be different between software versions due to the improvement we made; and it is expected that the Inside point may not be at the center of a polygon because it is not calculated as the centroid.

Re: Feature to Point does NOT center point

Anonymous User — Wed, 29 Aug 2012 16:56:40 GMT

I tried exporting a portion of my parcels and then creating the points but I could not get the issue to repeat. However, the screenshot does show my problem.
There are a few points in the top left that show the blue dots on top of the purple hexagons and centred in the parcel. And then there are a few with the blue dots on the purple hexagons at one side of the parcel and then there are a couple in the middle where the purple hexagons are at one side of the parcel and the blue dots at the other side.
To create these points I used Feature to Point and checked Inside in ArcGIS 10 sp4

Re: Feature to Point does NOT center point

DanLee — Wed, 29 Aug 2012 17:35:08 GMT

Which sets of points, the blue dots or the purple hexagons, did you create using 10 SP4? And the other set of points were produced by 9.3.1?

As the image shows what's expected, the Inside points are not necessarily at centers. May I ask what made you to choose the Inside option instead of the centroid option (uncheck Inside)? For comparing shape changes, if the parcels have unique IDs or attributes and the changed parcels carry the same original IDs, then centroid points would be more reliable, even if they fall outside of polygons; you just need to make sure you compare the centroids with the matching attributes. Could that work?

Re: Feature to Point does NOT center point

Anonymous User — Wed, 29 Aug 2012 18:02:40 GMT

Both the blue dots AND the purple hexagons were made in ArcGIS 10 with the inside checked. When I did the same process before upgrading to ArcGIS 10 the dots would be placed in the center. That's why I chose Inside, b/c in 9.3.1 it placed the points in the center.
I would like to use the Feature Compare tool but one set of data comes from an outside source and does not maintain a unique ID so I can't use that tool. We create our own by combining a portion of 2 attributes and the XY coordinates, hence why I make the points and why it's ideal for them to be in the center.

Re: Feature to Point does NOT center point

DanLee — Wed, 29 Aug 2012 20:37:55 GMT

It is still not clear to me how both the blue dots AND the purple hexagons were made in ArcGIS 10 with the inside checked. What made them different? Are they from the same set of polygons (I can only see one set of polygons in the map)?

The Inside option doesn't produce centroid by design; it finds a "good-enough" location inside a polygon. This location may happen to be at the center or near the center, or just appear at the center, but not always at the center. This is true in 9.3.1 as well. Therefore, using the Inside option is not really reliable unless the matching features from both datasets have a common attribute.

If the shape of parcels are very similar in the two feature classes, perhaps you can try the Centroid option (uncheck Inside). Of course you would need to check for cases where a parcel has a concave "C" shape and the centroid location falls outside the polygon or in the wrong polygon.

Since you mentioned that your data are from different sources (internal and outside), is your goal to align them or to update one using the other? If so, you might have a use case for our on-going conflation project. Could you share a sample area of the two datasets, not too large to attach on the forum? Or if you like, you can send to me by email: dlee@esri.com. I'd appreciate it.

Re: Feature to Point does NOT center point

DanLee — Thu, 30 Aug 2012 16:44:13 GMT

I just want the readers to know that Hilary and I have taken this offline since yesterday. I investigated the issue on Inside point location using her parcel data; apparently our enhancement on the Inside option made after 9.3.1 has avoided some odd point locations, but affected some good locations. Her case is reproducible with a subset; we will try to look into further enhancement on this. Thanks, Hilary, for bringing this up and sharing the data with me. You may submit an enhancement request with Esri Support as well.

Re: Feature to Point does NOT center point

Anonymous User — Tue, 04 Sep 2012 17:42:21 GMT

This problem has been registered as a bug
NIM084360 - [Feature to Point Geoprocessing tool gives inconsistent results when running tool in v9.3.1 vs. v10]

thanks all for your assistance 😄