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Generating a centroid for a polygon

YousafHassan

Hi
I am trying to help my colleague who wants to select a polygon and then click a button, which would generate a centroid for that polygon. Ideally, he wants a pop-up to appear which would display a copy&pasteable X and Y. He wants this automation as he wants X and Y to be generated for polygons using the same algorithm.

I was thinking of building a custom tool for him using a model. Could anyone please point me to the tools I need in order to achieve this? If you have already written a tool, would you like share it with me? I am quite comfortable in python and VBA but am not so familiar with Arc tools. I would actually prefer to do this in python as the support for VBA won't be there in future.

Any help would be extremely appreciated.
Thanks.

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YousafHassan

I see what you mean!

In that case, let's take Alexander's advice and use the LabelPoint of the pArea. Change this:
pPoint = pArea.Centroid
To this:
pPoint = pArea.LabelPoint

Great! It's working. All points are now appearing inside the polygons. Thanks, James and Alexander. You have been really helpful.

JamesCrandall

Sorry, forgot to show you where the point is appearing. See attached.

I see what you mean!

In that case, let's take Alexander's advice and use the LabelPoint of the pArea. Change this:

pPoint = pArea.Centroid

To this:

pPoint = pArea.LabelPoint

YousafHassan

Sorry, forgot to show you where the point is appearing. See attached.

strangepolygonWithPoint.JPG

AlexanderGray

The reason they are called centroids and not centre is that there are many definitions of centre. Centre of the polygon's envelope, centre of gravity, etc. With regular shapes such as your example, all the centroid definitions are pretty much the same and inside the polygon. There is a whole debate in geography about what constitutes the centre. I think geographers are still debating about what constitutes the centre of Scotland, many definitions place it in the sea.

Please refer to esri documentation on centroid for examples on how it can end up outside of the polygon
http://help.arcgis.com/en/sdk/10.0/arcobjects_net/componenthelp/index.html#/Centroid_Property/002m0000009p000000/

GeomCentroid.gif

YousafHassan

Well, your do loop contains a few lines which are different from the code I have in VBA:

Option Explicit

Sub CreateCentroid()
''provides access to members that control the MXD
Dim pDoc As IMxDocument
''Provides access to members that control the map.
Dim pMap As IMap

''Set the MXD to this mxd file
Set pDoc = Application.Document
''Set the focus to the current map
Set pMap = pDoc.FocusMap

''Provides access to members that hand out enumerated features,
''field collections and allows for the updating, deleting and inserting of features.
Dim pFCursor As IFeatureCursor
''Provides access to members that manage a set of selected table rows or features.
Dim pSelectionSet As ISelectionSet2
''Provides access to members that control feature selection.
Dim pFSelection As IFeatureSelection
''Set the first layer as the active layer
Set pFSelection = pMap.Layer(0)
''Set the selected features
Set pSelectionSet = pFSelection.SelectionSet
''set the pfeaturecursor
Set pFCursor = Nothing
''Get a cursor from the selected features
pSelectionSet.Search Nothing, False, pFCursor
''Provides access to members that identify a polygon and permit controlled access to its inner and outer rings.
Dim pPoly As IPolygon4
''Provides access to members that return properties common to rings and polygons.
Dim pArea As IArea
''Provides access to members that define two dimensional points.
Dim pPoint As IPoint
''Variable to hold the X coordinate
Dim lat As Long
''Variable to hold the Y coordinate
Dim lon As Long
''Provides access to members that return and set properties of a feature.
Dim pFeat As IFeature
Set pFeat = pFCursor.NextFeature
Do Until pFeat Is Nothing
    Set pPoly = pFeat.ShapeCopy
    Set pArea = pPoly
    Set pPoint = pArea.Centroid
    lat = pPoint.X
    lon = pPoint.Y
    Set pFeat = pFCursor.NextFeature
Loop

MsgBox "The Eastings:" & lat & " The Northings:" & lon

End Sub

Could this be the reason?

See the attached screenshot for an example of a polygon that I am trying to work with. With square or rectangle polygons, it is fine but with any other strange shape it is not appearing inside the polygon.

strangepolygon.JPG

JamesCrandall

I am not sure why you are seeing centroids outside of any selected polygons --- it really isn't the centroid of the polygon then, right? Not sure what you have going on there. I did some limited testing of the code below and it places graphic points at the centroid locations of each of the selected polygons, or at least what appears to be the centroid. None are placed outside of any selected polygon.

            Dim pGraphics As IGraphicsContainer
            pGraphics = pMap
            Dim pMarkerElem As IMarkerElement
            pMarkerElem = New MarkerElement
            Dim pElement As IElement
            pElement = pMarkerElem


            Dim pFCursor As IFeatureCursor
            Dim pSelectionSet As ISelectionSet
            Dim pFSelection As IFeatureSelection
            pFSelection = pMap.Layer(0)

            pSelectionSet = pFSelection.SelectionSet
            ''set the pfeaturecursor 
            pFCursor = Nothing
            ''Get a cursor from the selected features
            pSelectionSet.Search(Nothing, False, pFCursor)

            Dim pPoly As IPolygon
            Dim pArea As IArea
            Dim pPoint As IPoint

            Dim pFeat As IFeature
            pFeat = pFCursor.NextFeature
            Do Until pFeat Is Nothing
                pPoly = pFeat.ShapeCopy
                pArea = pPoly
                pPoint = pArea.Centroid

                pElement = New MarkerElement
                pElement.Geometry = pPoint
                pGraphics.AddElement(pElement, 0)

                pFeat = pFCursor.NextFeature
            Loop

            pDoc.ActiveView.PartialRefresh(esriViewDrawPhase.esriViewGeography, Nothing, Nothing)
            pDoc.ActiveView.Refresh()

AlexanderGray

the area labelpoint is guaranteed to be inside the polygon. It may not be in the center (whatever that means to you) though.

YousafHassan

Hi
Still stuck at this final bit. I was reading about the IArea.Centroid Property and in the remarks at the end of the page, it says:

"The Centroid does not always occur inside the Area of the geometry. The Centroid is not the same as the center of the geometry or the Envelope binding the geometry (but it may be if and only if that is also the center of the weighted area)."

Is there anyway I can keep the centroid inside the selected polygon?

Thanks

YousafHassan

Thanks for your help again. One final thing is that at the moment, some points are being created outside the selected polygons. Is there a way to keep them inside polygons?

AlexanderGray

You might also want to look at the IConversionNotation interface on the point. Depends on the coordinate system you start out in. James' link is a more generic way.

JamesCrandall

Also, am I getting eastings and northings or latitude and longitude here?

Thanks again for your generous help.

You are probably not going to like hearing this, but its not quite that simple. You are getting the x/y of the point's spatial reference (I have no way of knowing what that is, it is your point layer). To make sure you are getting the Lat/Lon, you will have to make sure the point has a geographic coordinate system assigned and in order to do this I think you will have to project it from something.

...In essence, you will need to know where you are coming from in order to get it to project to something.

Luckily the old forums are full of examples. You might start here:

http://forums.esri.com/Thread.asp?c=93&f=993&t=227713&mc=1#msgid691091

Found with these search terms: IPoint.X

YousafHassan

Thanks a lot guys. The code is running without errors and is giving me the X and Y:

Option Explicit

Sub CreateCentroid()

Dim pDoc As IMxDocument
Dim pMap As IMap

Set pDoc = Application.Document
Set pMap = pDoc.FocusMap

Dim pFCursor As IFeatureCursor
Dim pSelectionSet As ISelectionSet
Dim pFSelection As IFeatureSelection
Set pFSelection = pMap.Layer(0)

Set pSelectionSet = pFSelection.SelectionSet
''set the pfeaturecursor
Set pFCursor = Nothing
''Get a cursor from the selected features
pSelectionSet.Search Nothing, False, pFCursor
Dim pPoly As IPolygon
Dim pArea As IArea
Dim pPoint As IPoint
Dim lat As Long
Dim lon As Long

Dim pFeat As IFeature
Set pFeat = pFCursor.NextFeature
Do Until pFeat Is Nothing
    Set pPoly = pFeat.ShapeCopy
    Set pArea = pPoly
    Set pPoint = pArea.Centroid
    lat = pPoint.X
    lon = pPoint.Y
    Set pFeat = pFCursor.NextFeature
Loop

MsgBox "The Eastings:" & lat & " The Northings:" & lon

End Sub

Is there a way of getting the lat and lon in a copy and pastable text field instead?

Also, am I getting eastings and northings or latitude and longitude here?

Thanks again for your generous help.

AlexanderGray

In VBA, ThisDocument is a shortcut to Application.Document. Both are equivalent.

JamesCrandall

Also, after looking at some older posts I noticed for VBA that pDoc should be set to "ThisDocument" instead of how the OP has it:


Set pDoc = Application.Document

Should this be:


Set pDoc = ThisDocument

Sorry it's been a long time since I've used VBA and just trying to help out as best I can!

AlexanderGray

In VBA you only use brackets in a method call when you expect a return. In the case of the ISelectionSet.Search, the cursor variable is passed in the method and returns populated. It is one of my peeves with ArcObjects, sometimes the method returns the value you want, other times you have to pass in a variable that gets populated by the function. Both methods are legitimate, I just wish esri picked one and stuck with it.

Dim pFCursor as IFeatureCursor
pFCursor = nothing 
pSelectionSet.Search Nothing, False, pFCursor

YousafHassan

Also, you have specified the lat and lon variables as LONG data type. Why? This would produce a whole number, which I don't know of any Latitude/Longitude coordinate that is a whole number. Is that what you want?

Well, the reason for that is that I wanted Eastings and Northings instead of latitude and longitude. So I take it that this code produces latitude and longitude? I assumed that lat and lon were just variable names and didn't mean that the code was actually producing latitude and longitude.

The thing is that I am running this as a Macro in the VB editor under Customise menu in ArcMap. And you have access to full visual studio environment. Maybe that's why this is not going to work as a macro. I am trying my best to get hold of VS 2008 or above, in order to install the SDK for .net

I wonder if some one can help us run this as a macro in the mean time!

JamesCrandall

Also, you have specified the lat and lon variables as LONG data type. Why? This would produce a whole number, which I don't know of any Latitude/Longitude coordinate that is a whole number. Is that what you want?

JamesCrandall

The pSelectionSet.Search(Nothing, False, pFCursor) is appearing in red. I am guessing it should be:
pFCursor = Nothing
pFCursor = pSelectionSet.Search(Nothing, False, pFCursor)

No the way you have the pFCursor would not work --- the code I posted works just fine for me. I really don't know why it would appear in red or do I even know what that means. I am using Visual Studio 2008 with all of the proper SDK's installed, along with ArcGIS v10.

YousafHassan

The pSelectionSet.Search(Nothing, False, pFCursor) is appearing in red. I am guessing it should be:

pFCursor = Nothing
pFCursor = pSelectionSet.Search(Nothing, False, pFCursor)

JamesCrandall

But now I am getting a run-time error:

Run-time error:91 Object Variable or With Block Variable not set

When I click on debug, it takes me to the following line in code:
Set pFeat = pFCursor.NextFeature
Thanks again.

That's probably because you are not setting pFCursor anywhere (I don't think I provided that in my original post. sorry I am picking snippets from my own implementation which is different). See if this helps/completes the code.

Sub CreateCentroid()

Dim pDoc As IMxDocument
Dim pMap As IMap

Set pDoc = Application.Document
Set pMap = pDoc.FocusMap

Dim pFCursor As IFeatureCursor
Dim pSelectionSet As ISelectionSet
Dim pFSelection As IFeatureSelection
Set pFSelection = pMap.Layer(0)

pSelectionSet = pFSelection.SelectionSet
''set the pfeaturecursor 
pFCursor = Nothing
''Get a cursor from the selected features
pSelectionSet.Search(Nothing, False, pFCursor)
Dim pPoly As IPolygon
Dim pArea As IArea
Dim pPoint As IPoint
Dim lat As Long
Dim lon As Long

Dim pFeat As IFeature
Set pFeat = pFCursor.NextFeature
Do Until pFeat Is Nothing
    Set pPoly = pFeat.ShapeCopy
    Set pArea = pPoly
    Set pPoint = pArea.Centroid
    lat = pPoint.X
    lon = pPoint.Y
    Set pFeat = pFCursor.NextFeature
Loop

End Sub

YousafHassan

Thanks for all your help so far. I have now amended the code to the following:

Option Explicit

Sub CreateCentroid()

Dim pDoc As IMxDocument
Dim pMap As IMap

Set pDoc = Application.Document
Set pMap = pDoc.FocusMap

Dim pFCursor As IFeatureCursor
Dim pSelectionSet As ISelectionSet
Dim pFSelection As IFeatureSelection
Set pFSelection = pMap.Layer(0)

Dim pPoly As IPolygon
Dim pArea As IArea
Dim pPoint As IPoint
Dim lat As Long
Dim lon As Long

Dim pFeat As IFeature
Set pFeat = pFCursor.NextFeature
Do Until pFeat Is Nothing
    Set pPoly = pFeat.ShapeCopy
    Set pArea = pPoly
    Set pPoint = pArea.Centroid
    lat = pPoint.X
    lon = pPoint.Y
    Set pFeat = pFCursor.NextFeature
Loop

End Sub

But now I am getting a run-time error:

Run-time error:91 Object Variable or With Block Variable not set

When I click on debug, it takes me to the following line in code:

Set pFeat = pFCursor.NextFeature

Thanks again.

AlexanderGray

in VBA you need to do 'set' with a pointer. A pointer is anything not a literal (a literal usually you can write down on a paper).

set pFSelection = pMap.Layer(0)

The part about declaring the variables is useful too since it avoids all kind of errors, you only need it if you put the option explicit at the top of the class (before any methods.) You should always use option explicit.

JamesCrandall

First, are you declaring pDoc and pMap somewhere else?

Dim pDoc As IMxDocument
Dim pMap As IMap

If not, you will need to put these at the top of your CreateCentroid Sub or make them Private variables in the entire class.

It has been a long time since working with VBA, but I don't think this will work:


pDoc = m_pApp.Document

Try to replace it with:


Dim pDoc As IMxDocument
pDoc = Application.Document

YousafHassan

Hi

I have made a slight amendment to your code:

Sub CreateCentroid()

pDoc = m_pApp.Document
pMap = pDoc.FocusMap

Dim pFCursor As IFeatureCursor
Dim pSelectionSet As ISelectionSet
Dim pFSelection As IFeatureSelection
pFSelection = pMap.Layer(0)

Dim pPoly As IPolygon
Dim pArea As IArea
Dim pPoint As IPoint
Dim lat As Long
Dim lon As Long

Dim pFeat As IFeature
IFeature = pFCursor.NextFeature
Do Until pFeat Is Nothing
    pPoly = pFeat.ShapeCopy
    pArea = pPoly
    pPoint = pArea.Centroid
    lat = pPoint.X
    lon = pPoint.Y
    pFeat = pFCursor.NextFeature
Loop

End Sub

And I am trying to run this code as a VBA macro but it is failing at this line:

pFSelection = pMap.Layer(0)

Compile Error: Method or data member not found.

Any ideas as to why is failing here?
Thanks

YousafHassan

So, I am not certain if the VS IDE Framework is what you need. I do believe that the reason why no ArcGIS Project templates show up is because you simply do not have the ArcGIS .NET SDK installed.

Yes, you are right. I have to get it installed in order to try your code.

JamesCrandall

I think I need ArcGIS Visual Studio Integration Framework!

Is there any other way I can acheive this? i.e. build a custom tool based on a model?

Just fyi, from: http://edndoc.esri.com/arcobjects/9.2/NET/73516929-d390-456c-84bc-c8f8a58b645a.htm

"...The ArcGIS Visual Studio IDE Integration Framework is installed as part of the Software Developer Kit (SDK) for .NET for each ArcGIS product and is an optional feature during installation. This framework is available for Visual Studio 2005, Visual Basic 2005 Express, Visual C# 2005 Express, and Visual Web Developer 2005 Express editions."

So, I am not certain if the VS IDE Framework is what you need. I do believe that the reason why no ArcGIS Project templates show up is because you simply do not have the ArcGIS .NET SDK installed.

YousafHassan

I *think* this is because you do not have the ArcGIS .NET SDK ("Developer Kit") installed. Can someone else confirm this?

I think I need ArcGIS Visual Studio Integration Framework!

Is there any other way I can acheive this? i.e. build a custom tool based on a model?

JamesCrandall

James, I am trying to create an Add-In following this link:

http://help.arcgis.com/en/sdk/10.0/arcobjects_net/conceptualhelp/index.html#//0001000001ms000000

But I cannot find ArcGIS node under project type. What do I need to do in order to get that in Visual Studio?
I apologise for these questions as I am a bit of a beginner in Arc.

I will be adding this to ArcMap and hopefully, once it is up and running, my colleagues will be using it too.
Thanks.

I *think* this is because you do not have the ArcGIS .NET SDK ("Developer Kit") installed. Can someone else confirm this?

YousafHassan

James, I am trying to create an Add-In following this link:

http://help.arcgis.com/en/sdk/10.0/arcobjects_net/conceptualhelp/index.html#//0001000001ms000000

But I cannot find ArcGIS node under project type. What do I need to do in order to get that in Visual Studio?

I apologise for these questions as I am a bit of a beginner in Arc.

I will be adding this to ArcMap and hopefully, once it is up and running, my colleagues will be using it too.
Thanks.

YousafHassan

Thanks for this. But what do you mean by creating an Add In or COM implementation? Actually, don't answer that. I've found the link and am reading about them now.

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