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Points to Polygon "Footprint"

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04-22-2010 09:18 AM
AndrewBar
New Contributor
I have a large point feature class.  I need to produce a polygon that is identical to the x-y "footprint" of the points.  I could manually find the boundary points and digitizing these as the polygon vertices...but I have 10s of thousands of points so this is impractical.  Any ideas?  Using ArcMap 9.3 with all available extensions.  Thanks!
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26 Replies
ChrisMathers
Regular Contributor II
Have you tried to do a multi ring buffer on the whole thing? The rings taht overlap would merge into one poly. Do a tiny buffer so that the footprint isnt much larger than the point cloud. This will really only work if its a dense point cloud like LiDAR data. Just make the buffer = spatial resolution e.g. 1'. You may have to look at the polygons properties and delete and little holes in it.
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JonathanBaarda
New Contributor
If the suggestion above doesn't work for you, you might want to take a look at this help page from esri: http://webhelp.esri.com/arcgisdesktop/9.3/index.cfm?TopicName=Writing_geometries

It is fairly straightforwards using the script examples to write the X and Y coords of the points into a polygon.
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JoeFlannery
Regular Contributor II
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ChrisMathers
Regular Contributor II
I dont see that tool in my toolbox jrflannery. I think its new in 10.
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AndrewBar
New Contributor
Thanks to all for the responses

clm42 - multiring buffer seems to choke on the massive number of points in my feature class.  I waited for over half an hour and it wasn't even close to being finished.

jbaarda - your suggestion would work if I knew which points in the feature class represented the boundary points already....but I don't know this.

jrflannery - looks like convex hulls are exactly what I need, but as clm42 points out, I don't have access to the Minimum Bounding Geometry tool in 9.3.  Anyone know how to implement this in 9.3?

Thanks again
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DanPatterson_Retired
MVP Emeritus
http://arcscripts.esri.com/details.asp?dbid=14535
has a variety of containers including convex hull
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AndrewBar
New Contributor
Sorry to be a nuisance.

Thanks Dan for that link.  But is turns out that I misunderstood the definition of Convex Hulls....this is not exactly what I need. 

Convex hulls produces the minimum convex polygon containing all the points, but if the border of the point cloud has any concavities, the convex hull will actually include areas not within the point cloud "footprint".  I need a polygon that is EXACTLY the same geometry as the xy footprint of the points. 

Any further suggestions will be appreciated, and thanks!
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DarshaHardy
New Contributor II
Try the Convex Hull function in ET Geowizards

It's a great piece of software that includes functionality sadly lacking in ArcView 😄

ah, oops those post above weren't there when I started writing this post

Have a look at the ET Create Tin function - you could assign all points the same height, then do a dissolve. You'll end up with a 3D shapefile but you can copy the resultant feature in a new 2D file.

Edit: just realised that's still not going to work! You would though, end up with polygons that if deleted, would give you the outline you need. Just need to identify those polygons that have no neighbours on one side - some kind of topology check?
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DanPatterson_Retired
MVP Emeritus
if the points are sequential, then look at ET Geowizards for tools to convert points to poly* features, if the features are not sequential and you want some of the concavity to be maintained then you are looking for a concave hull for which there is no existing tools within the Arc environment.  if the pattern is something else, perhaps a illustrative picture might generate some ideas
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