Feature to Point does NOT center point

466
14
08-28-2012 01:57 PM
by Anonymous User
Not applicable
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....
0 Kudos
14 Replies
DanLee
by Esri Regular Contributor
Esri Regular Contributor
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.
0 Kudos
GraemeBrowning
Regular Contributor
The process you are describing sounds like the Feature Compare tool might be useful to consider.
0 Kudos
MarkBoucher
Regular Contributor II
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.
0 Kudos
BruceHarold
Esri Regular Contributor
0 Kudos
by Anonymous User
Not applicable
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)
0 Kudos
DanLee
by Esri Regular Contributor
Esri Regular Contributor
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.
0 Kudos
by Anonymous User
Not applicable
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.
0 Kudos
DanLee
by Esri Regular Contributor
Esri Regular Contributor
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.
0 Kudos
by Anonymous User
Not applicable
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
0 Kudos