Something like Convex Hull but creates polygon around all outer points

[map1]

Hello,

What I want to be able to do is to create a polygon around the
outermost points of a point dataset. This points are the results from
a gps survey. I can use Convex Hull but that has the elastic band
effect and only draws a polygon around the outermost points. What I
want to be able to do is create a polygon around all the outer points
to I can calculate the area that has been surveyed.

I'm sure a tool must have been developed in the past to do this.

Thanks for your help!

[Bo Victor Thomsen]

Google "alpha shapes". I don't know if someone has programmed a MapBasic
application, which generates alpha shapes.

Regards
Bo Victor Thomsen
Aestas-GIS
Denmark

[map1]

Thanks Bo. Yes that's exactly what I want but it doesnt look like
anyone has created a tool to do it.

I'm sure this would be a very handy tool to have with mapinfo!

[college.atlas]

Try this thread where this question was asked and answered to the OPs satisfaction.

https://groups.google.com/forum/?fromgroups#!searchin/mapinfo-l/Concave/mapinfo-l/0tVkXobJBw8/HsXXho1xGJEJ%5B1-25%5D 

[Tony Pilkington]

Using Delaunay triangulation, a set of points like this

 

 

can be triangulated to this

 

 

The triangles coloured red have a maximum angle above a user set maximum angle. (Default 120 degrees)

 

If you eliminate the red triangles, you get a concave group of triangles that may be said to cover your survey area.

 

The MBX should is best run within a projective coordinate system and a maximum of about 250 points. (It has run at over 1000, but you need a lot of coffee)

Ensure you have no coincident points.

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[Nyall Dawson]

Here's an alternative Alpha Shapes tool I wrote sometime ago. It includes routines for both Delaunay Triangles and Alpha Shapes. 

Similar restrictions as Tony's program apply - only use with projected coordinates, and don't try with too many points! (Also, it's MI10+ only.)

Hope it helps,

Nyall

On Wednesday, 8 August 2012 18:33:56 UTC+10, map1 wrote:

[KTG Infotech]

can i ask you whats wrong in this, i am not clear on your query.

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Data Integration, Analysis, Capturing and Data Conversion
for Geochemistry,IP and Drill logs
Skypes-ktgcad

[Tony Pilkington]

The voronoi diagram covers areas outside the control points.

 

The basic principles of classical ground and air survey are that you work within your control and you look for intersections of rays (hence angles) of between 60 and 90 degrees; aka a strong cut. Assuming that the GPS survey has been done to set up control for denser work, then the long thin triangles are areas where small errors can lead to unacceptable errors, probably way outside any good error budget. This is best illustrated by reference to any manual on plane table surveying.

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[Jamie]

Nyall,

any chance you could upload a MapInfo V10 compatible version is there?

[Jamie]

the one you have uploaded is only for MapInfo 10.5+

[Nyall Dawson]

No worries -- give me a couple of days to clean up the code and I'll post the source.

Kind regards,

Nyall

[Jamie]

that would be great

[map1]

Thanks for the input guys.

Nyall have you got any instruction for the alpha shapes tool? I just
gave it a go but got an error.

I managed to find a tool called Concave Hull Estimator for ESRI ArcGIS
that does the trick but id rather not have to keep swapping between
platforms.

http://resources.arcgis.com/gallery/file/Geoprocessing-Model-and-Script-Tool-Gallery/details?entryID=AA23C5C6-1422-2418-8811-1365253A756A

[map1]

Hi Nyall I keep getting the following error message

"(delaunay.mb.79) File convex_hull.dat not found. Unable to create new
table."

Im using mapinfo 11.5

[Nyall Dawson]

This should be fixed now -- thanks for reporting.

I've uploaded new binaries for both 10.5+ and 8+ together with the source code here:

http://nyalldawson.net/2012/08/alpha-shapes-for-mapinfo/ 

Hope it's of use to you!

Nyall

[Jamie]

looks as though it still doesn't work with mapinfo v10

[Nyall Dawson]

Is that using the version for MapInfo 8+? There's two versions, one is only for MapInfo 10.5 and above, the other should work in 8.5+.

Nyall

[Jamie]

Ive tried both neither seem to work in version 10

[Nyall Dawson]

On Friday, 17 August 2012 08:16:01 UTC+10, Jamie wrote:

Ive tried both neither seem to work in version 10

I must have messed up the compilation. Can you try this version please?

Nyall

[Jamie]

Works perfectly now thank you!