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Buffering using VBA

RamaGangavall

I am using the following code to draw a 1 mile buffer around a polygon. When I measure the distance from the edge of the polygon to the buffer, its not accurate. North and South are fine, but East and West are off.

Dim pMxDoc As IMxDocument
Dim pActiveView As IActiveView
Dim pGraphicsContainer As IGraphicsContainer
Dim pEnumFeature As IEnumFeature
Dim pFeature As IFeature
Dim pTopoOp As ITopologicalOperator
Dim pElement As IElement
Dim strBufferDistance As String

Set pMxDoc = Application.Document
Set pActiveView = pMxDoc.FocusMap
Set pGraphicsContainer = pMxDoc.FocusMap

'Verify there is a feature selection
If pMxDoc.FocusMap.SelectionCount = 0 Then Exit Sub

'Get a buffer distance from the user
strBufferDistance = InputBox("Enter Distance:", "Buffer")
If strBufferDistance = "" Or Not IsNumeric(strBufferDistance) Then Exit Sub

'Buffer all the selected features by the BufferDistance
'and create a new polygon element from each result
Set pEnumFeature = pMxDoc.FocusMap.FeatureSelection
pEnumFeature.Reset
Set pFeature = pEnumFeature.Next
Do While Not pFeature Is Nothing
    Set pTopoOp = pFeature.Shape
    Set pElement = New PolygonElement
    pElement.Geometry = pTopoOp.Buffer(CInt(strBufferDistance))
    pGraphicsContainer.AddElement pElement, 0
    Set pFeature = pEnumFeature.Next
Loop

'Redraw the graphics

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RamaGangavall

Once I switched to the right Projection (UTM) it worked. Thanks for your replies.

NeilClemmons_GISinc

You should reread Melita's last post. Your screenshots clearly indicate you are looking at the geodesic distance.

RamaGangavall

Attaching a couple of screenshots of the buffers. East is measured at 0.75 mile. and North at 1mile. This is using NAD83 projection. But when I change the projection to Mercator, East and West measure fine, but North and South are off.

East Measure.jpg

North Measure.jpg

MelitaKennedy

The geodesic distance is the distance on the ellipsoid/spheroid. It will be in linear units like miles or meters.

The ConvertUnits is making an assumption to convert miles to degrees, probably using a mile at the equator to calculate the equivalent length in degrees. When you check that with the measure tool, the fact that the meridians (longitude lines) converge toward the pole means that the east-west distance is shorter than the north-south distance. My guess, based on the difference in the lengths, is that the data is at approximately 41 North latitude.

Melita

RamaGangavall

If you're using the measure tool, check what type of distance it's returning. A problem with east-west distances implies to me that the geodesic distance is being returned.

Melita

It returns me in miles.

RamaGangavall

I notice that you aren't giving your graphic element a spatial reference. Without a spatial reference defined, it's quite possible that ArcMap will not properly project the element. Have you determined that this is not a spatial reference issue? I've been using ITopoOp.Buffer for over a decade and have never had any problems with it.

Modified my code to the following, but still it doesn't work. When I measure the distance using the measuring tool, E is 0.75 miles and W is 0.78 miles, but N and S returns 1 mile.

Dim pMxDoc As IMxDocument
Dim pxMap As IMap
Dim pActiveView As IActiveView
Dim pGraphicsContainer As IGraphicsContainer
Dim pEnumFeature As IEnumFeature
Dim pFeature As iFeature
Dim pTopoOp As ITopologicalOperator
Dim pElement As IElement
Dim strBufferDistance As String

Set pMxDoc = ThisDocument
Set pxMap = pMxDoc.FocusMap

Dim pSpatRefFact As ISpatialReferenceFactory
Set pSpatRefFact = New SpatialReferenceEnvironment
Dim pspatref As ISpatialReference
Set pspatref = pSpatRefFact.CreateESRISpatialReferenceFromPRJFile("NAD83.prj")

Set pxMap.SpatialReference = pspatref

Set pActiveView = pxMap
Set pGraphicsContainer = pMxDoc.FocusMap

'Verify there is a feature selection
If pMxDoc.FocusMap.SelectionCount = 0 Then Exit Sub

Set pEnumFeature = pMxDoc.FocusMap.FeatureSelection
pEnumFeature.Reset
Set pFeature = pEnumFeature.Next
Do While Not pFeature Is Nothing
    Set pTopoOp = pFeature.Shape
    Set pElement = New PolygonElement
    pElement.Geometry = pTopoOp.Buffer(ConvertUnits(1#, esriMiles, esriDecimalDegrees))
    pGraphicsContainer.AddElement pElement, 0
    Set pFeature = pEnumFeature.Next
Loop

'Redraw the graphics
pActiveView.PartialRefresh esriViewGraphics, Nothing, Nothing

MelitaKennedy

If you're using the measure tool, check what type of distance it's returning. A problem with east-west distances implies to me that the geodesic distance is being returned.

Melita

NeilClemmons_GISinc

I notice that you aren't giving your graphic element a spatial reference. Without a spatial reference defined, it's quite possible that ArcMap will not properly project the element. Have you determined that this is not a spatial reference issue? I've been using ITopoOp.Buffer for over a decade and have never had any problems with it.

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