[postgis-users] How to generalize or simplify a Polygon
Matt Lynch
matt at terraEngine.com
Thu Aug 28 09:20:24 PDT 2003
Hi Oscar,
Thanks for the code!
I am not sure I can use it, however. I haven't been totally clear on
how I plan to use the generalization routine.
My goal is to have the shapefile submitted via a webservice, generalized
by the webservice, and then imported into postgis.
This means I need one of the following:
1) a postgis operation
2) a commandline utility (DOS executable)
3) VB,C#,C++, or C code
That is unless there is a runtime engine for Avenue. I am not very
familiar with Avenue, and I did do a search on the ESRI site for an
Avenue runtime, but did not find one. If you could point me to one,
your solution would be perfect.
Regards,
Matt
-----Original Message-----
From: postgis-users-bounces at postgis.refractions.net
[mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of
oscar.gomez at telvent.abengoa.com
Sent: Thursday, August 28, 2003 9:49 AM
To: PostGIS Users Discussion
Subject: Re: [postgis-users] How to generalize or simplify a Polygon
This is an ArcView-Avenue implementation of the Douglas - Peucker
algorithm to generalize a polyline or polygon feature's vertices.
Good luck,
Oscar Gomez
IT Consultant
Telvent
theView = av.GetActiveDoc
thePrj = theView.GetProjection
if (thePrj.IsNull) then
hasPrj = false
else
hasPrj = true
thePrj = theView.GetProjection
end
' Get active themes
theThemes = theView.GetActiveThemes
if (theThemes.Count < 1) then
MsgBox.Error("Please make a polyline or polygon theme active","***
Error
***")
exit
end
' Process each active theme
for each t in theThemes
' Check that the theme is of the correct type
if (t.Is( FTHEME).Not) then
MsgBox.Error (t.GetName++"is not a polyline or polygon theme","***
Error ***")
continue
end
theClassName = t.GetFTab.GetShapeClass.GetClassName
if ((theClassName = "Polygon").Not and (theClassName =
"Polyline").Not)
then
MsgBox.Error (t.GetName++"is not a polyline or polygon theme","***
Error ***")
continue
end
' Get a tolerance in (distance units) to be used (must be > 0)
theTol = 0
theMUnits = theView.GetDisplay.GetUnits
theDUnits = theView.GetDisplay.GetDistanceUnits
theUStr = Units.GetFullUnitString(theDUnits).LCase
while ((theTol <= 0) or (theTol.AsString.IsNumber).Not)
theTol = MsgBox.Input("Positive tolerance value (in "+theUStr+
") for theme"++t.GetName,"Tolerance",theTol.AsString)
if (theTol = NIL) then exit end
if (theTol.IsNumber) then
theTol = theTol.AsNumber
end
end
theTol = Units.Convert(theTol,theDUnits,theMUnits)
' Get the name for a new theme, from View.Export script
def = av.GetProject.MakeFileName("theme", "shp")
def = FileDialog.Put(def, "*.shp", "New file for simplified "+
t.getName)
if (def = NIL) then return nil end
' Create a new shapefile theme, from View.Export script
tbl = t.GetFtab
shpfld = (tbl.FindField("Shape"))
if (shpfld.IsVisible.Not) then
shpfld.SetVisible(shpfld.IsVisible.Not)
WasNotVisible = TRUE
else
WasNotVisible =FALSE
end
' This will only export the selected shapes, if any have been selected
av.ShowMsg("Creating new shape file from"++ t.GetName)
anFTab = tbl.Export(def, Shape, tbl.GetSelection.Count > 0)
anFTab.SetEditable(true)
if (WasNotVisible) then
shpfld.SetVisible(FALSE)
end
' Find the shape field
theSField = anFTab.FindField("Shape")
' Set up status bar
av.ShowMsg("Generalizing new shapes from theme"++ t.GetName)
theSize = anFTab.GetNumRecords
' Initialize counters for reporting
count = 0
oldCount = 0
newCount = 0
' Process each shape
for each r in anFTab
av.SetStatus(100*count/theSize)
' Retrieve shape
if (theClassName = "Polygon") then
theOShape = Polygon.MakeNull
else
theOShape = Polyline.MakeNull
end
anFTab.QueryShape(r,theView.GetProjection,theOShape)
' Make a list for collecting the new set of point lists
theNShape = List.Make
' Process each list in the ListofListofPoints
for each p in theOShape.AsList
' Make a list for collecting the vertices to keep
theNList = List.Make
oldCount = oldCount + p.Count
' Set up stack for holding a set of lists of form {index,point}
theStack = Stack.Make
' Add first point to the list being assembled, and make it the
anchor
theAnchor = p.Get(0)
if (hasPrj) then
theNList.Add(theAnchor.ReturnUnProjected(thePrj))
else
theNList.Add(theAnchor)
end
aIndex = 0
' Add last point to stack
fIndex = p.Count - 1
theStack.Push({fIndex,p.Get(fIndex)})
' Process the points with the Douglas - Peucker approach
while (theStack.IsEmpty.Not)
' Process from Anchor (beginning) to Floater (top of stack)
fIndex = theStack.Top.Get(0)
' Create graphic shape for comparing distances
theVect = Line.Make(theAnchor,theStack.Top.Get(1))
' Initialize values for comparison
theMax = theTol
mxIndex = 0
' Find distance
for each i in (aIndex + 1) .. (fIndex - 1) ' process middle
points
dist = theVect.Distance(p.Get(i))
' Check against previous maximum
if (dist >= theMax) then ' this point is out of the tolerance
theMax = dist
mxIndex = i
end
end
' If a vertex is found outside the current tolerance corridor,
' push it onto the stack
if (mxIndex > 0) then 'a new floater has been found
theStack.Push({mxIndex,p.Get(mxIndex)})
else ' add floater to the list for the new shape and move
forward
if (hasPrj) then
theNList.Add(theStack.Top.Get(1).ReturnUnProjected(thePrj))
else
theNList.Add(theStack.Top.Get(1))
end
' Make the floater the new anchor
theAnchor = theStack.Pop.Get(1)
aIndex = fIndex
end
end
' Check for collapsed polygons
if ((theNList.Count < 4) and (theClassName = "Polygon")) then
' Revert to original polygon
if (hasPrj) then
theNList = List.Make
for each pt in p
theNList.Add(pt.ReturnUnProjected(thePrj))
end
else
theNList = p
end
end
' Finish up individual lists
newCount = newCount + theNList.Count
theNShape.Add(theNList)
end
' Finish up the shape
if (theClassName = "Polygon") then
theNShape = Polygon.Make(theNShape)
else
theNShape = Polyline.Make(theNShape)
end
anFTab.SetValue(theSField, r, theNShape)
count = count + 1
end
' Clear status message
av.ClearMsg
av.ClearStatus
' Report on counts of old and new theme
MsgBox.Info("The old theme with"++ oldCount.AsString ++
"vertices has been generalized to"++
newCount.AsString++"vertices","Results")
' Create a theme and add it to the View
anFTab.SetEditable(false)
fthm = FTheme.Make(anFTab)
theView.AddTheme(fthm)
end
' Bring the View to the front
theView.GetWin.Activate
"Matt Lynch" <matt at terraEngine.com>
"Matt Lynch" <matt at terraEngine.com>
Enviado por: postgis-users-bounces at postgis.refractions.net
28/08/2003 17:45
Por favor, responda a PostGIS Users Discussion
Para: <postgis-users at postgis.refractions.net>
cc:
Asunto: [postgis-users] How to generalize or simplify a Polygon
Hi,
I have an application where I will be accepting a shape file (polygon)
to define a region of permitted access for a user which will be imported
to postgis. That shape file has the potential to have many points if
the region defined has a border like a river or lake. Are there any
functions or tools available that would take a polygon shape and reduce
or generalize the number of points in the polygon? The resulting
polygon must cover at least as much area as the original.
Thanks,
Matt
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