topological anomalies brochure

8/3/2019 Topological Anomalies Brochure

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CONTENT PagE

Anomaly #101: Loopbacks Sel-intersections 3Anomaly #102: Unclosed Polygons/Rings 5

Anomaly #103: Internal Polygons with Incorrect Rotation 7

Anomaly #104: Duplicated Points 9

Anomaly #105: Kickbacks 11

Anomaly #106: Spikes 13

Anomaly #107: Small Areas (polygon smaller than a specifed size in square metres) 15

Anomaly #108: Slivers or Gaps 17

Anomaly #109: Overlapping Polygons 19

Anomaly #110: Duplicate Polygons (polygons with identical attributes) 21

Anomaly #111: Short Segments 23

Anomaly #112: Null Geometry - Table Records with Null Shape 25

Anomaly #114: Empty Parts (geometry has multiple parts and one is empty) 27

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Reouton

This error is resolvedby modications to thegeometry within a GISediting session. It is notpossible to automate

this process.

Anomaly #101

loopbck se-nterecton:

Geometries aected1. Lines

2. Polygons

DescriptionA loop back or sel-intersecting polygon is when:

1. The boundary o the polygon crosses itsel. This error is sometimesdescribed as a Butterfy or Figure o Eight polygon, or

2. The line recrosses a vertex in a dierent direction.

These two events are illustrated in the diagrams to the let. Asno tolerance is used in the loopback or sel-intersecting polygontest, these errors are not created by points that are close to eachother, but are not necessarily sel-intersecting, snapping together.I a clean with a set uzzy tolerance is run beore or ater this test,it could be expected that more sel-intersecting polygons would

be identied.

A segment incorrectly touchesor crosses another segment othe same polygon

To rectiy, the vertex is moved

to its correct position resultingin only one polygon

The vertex is NOT crossedin two directions

The vertex is crossedin two directions

1.

2.


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Reouton

This error can beresolved by ensuringthat the start and endnodes are exactlythe same on the rstand the last segmentwithin the polygon.

This error correctionprocess can beautomated.

Anomaly #102

Uncoed Poyon/Rn:

Geometries aected1. Polygons

DescriptionA polygon geometry should have the same start and end point.This means that the end or to node on the last segment mustbe incident on the start or rom node on the rst segment.In case the start and end points are not the same, the polygonwill be unclosed.

Firstsegment

Firstsegme

nt

Last

seg

ment

Last

seg

ment

The rst and last vertex in thering are separated, so thepolygon is unclosed.

FromNode.

ToNode.

The rst and last vertex in the ringare incident on each other. In thiscase the polygon is closed.


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Anomaly #103

intern Poyon wth

incorrect Rotton:


DescriptionWhen the polygon is topologically simple, but its rings are notoriented correctly, the problem results in incorrect ring rotation.

The external ring o a polygon should be oriented clockwisewhereas internal rings should be counterclockwise.

Reouton

This error is resolvedby reversingor fipping theorientation o theoending ring.

This can be anautomated process.

incorrect rn rorton:

Outer ring - counterclockwiseInner ring - clockwise

Correct rn rorton:Outer ring - clockwise

Inner ring - counter clockwise


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Anomaly #104

Dupcted Pont:

Geometries aected1. Points

2. Lines

3. Polygons

DescriptionAs no tolerance is used in this test, a duplicate point is seen tohave exactly the same c o-ordinates as another point. For lines andpolygons this means that one point in the line or polygon eatureis incident upon another point. I a point o one eature is exactlyincident on a point o another eature, the two eatures are seenoverlapping.

The use o a tolerance to return points which are within a distanceo each other will return a di erent result to the approach with notolerance used. The short segment test replicates this approachwith a tolerance specied prior to the test being run.

Reouton

The error is resolvedby automatedtechniques. I notolerance is used,the geometry o

the object will notbe altered by suchtechniques.

A point is exactly co-incident onanother point. This means it hasexactly the same co-ordinates.It is not possible to see this errorwhitin a GIS.


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Anomaly #105

Kckbck:


2. Polygons

DescriptionAn event is dened as a kickback when:

1. The internal and external angles between two segments areless than

k(e. g. 55 degrees);

2. Both the distance between the external spike and theexternal polygon boundary and the distance between theinternal spike and the internal polygon boundary are less thand

k(e. g. 1 metre).

There is commonality between the kickback test algorithm andthe spike test algorithm, which means that errors can be labeledboth as a kickback and a spike. The parameters set or these twotests are seen to be the best compromise between picking upgenuine issues and not reporting too many alse positives.

Reouton

The error can beresolved by editingthe oendingkickback within a GISenvironment.

Internal and external anglesare less than

k.

Both distances

are less than dk.


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Anomaly #106

spke:


2. Polygons

DescriptionA Spike is ormed when:

1. The internal angle o the two segments that make up thespike is less than

a(e. g. 5 degrees); or

2. The internal angle between the two segments is less than

b(e. g. 55 degrees) and the segment length is less than

db

(e. g. 2 metres).

Reouton

The error can beresolved by editingthe oendingkickback within a GISenvironment.

Internal angle is less than a.

The length o the spike isnot considered.

Internal angle is lessthan

b.

The length o thesegment is lessthan d

b.

1.

2.


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Anomaly #107

s are (poyon e thn

pecfed ze n qure etre):


DescriptionA small area anomaly is reported when the area o the polygon isbelow a specied threshold, A

min(e. g. 10 m2).

Reouton

This error is resolvedby deleting thepolygon eaturewith an area belowthe threshold. The

deletion process caneither take place ina GIS environmentor a databaseenvironment as abatch process basedon selection criteriaapplied against thedataset.

A polygon is below theminimum specied area A

min.


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Anomaly #108

sver or gp:


DescriptionA sliver or gap is ormed when:

1. The area o inner rings remaining ater neighbours are mergedis less than A

s(e. g. 2 m2) or

2. The area o the gap is less than the perimeter (in metres).

Reouton

This error is resolvedby manually editingthe polygon verticesor running a cleanprocess against the

dataset which wouldresult in the polygonboundaries snappingtogether.

Poyon B

Poyon a

Gap or sliver where Polygon A andPolygon B do not share a commonboundary due to diering locationo vertices.


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Anomaly #109

Overppn Poyon:


DescriptionAn overlapping polygon is computed by looking at the geometricintersection o any contiguous polygons. I the result i s not empty,there is an overlap regardless o size.

Reouton

This error is resolvedby editing thepolygon verticesmanually, or runninga clean process

against the dataset,which will resultin the polygonboundaries snappingtogether i they arewithin the speciedtolerance.

Poyon B

Poyon a

Polygon A is overlapping Polygon B.

PolygonBboundary.

PolygonAboundary.

Ch k O O O C O s Ch k O O O C O s


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Anomaly #110

Dupcte Poyon (poyon

wth dentc ttrbute):


DescriptionA duplicate polygon is seen as an exact copy o another polygonboth in attribution and geometry. The only thing telling the twoeatures apart is that each one would have a dierent eatureidentier withing the dataset.

Reouton

This error is resolvedby deleting a rowrom the datasetto remove theduplicate polygon.

iD = 1000004

iD = 1000005

=

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Anomaly #111

short seent


2. Polygons

DescriptionA short segment is reported when two subsequent vertices allwithin a specied tolerance to each other. The deault value iss

min(e. g. 0.05 metres).

Reouton

This error is resolvedby deleting oneo the verticeswithin a GIS editingenvironment.

The distance between two verticesis less than the specied tolerancevalue s

min(e. g. 0.05 m). In this

example, the distance is 0.0132metres, which is less than the s

min.

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Anomaly #112

Nu geoetry - Tbe record

wth Nu shpe:


2. Lines3. Polygons

DescriptionThe line, point, or polygon eature has no values in the geometrycolumn o the dataset. As the geometry column is null, no eatureis associated with the row.

Reouton

This error is resolvedby deleting the rowwithin the dataset.

iD gEOmETRY aREa ValUE

1000001 POLYGON 1023,56 512

1000002 NULL 0 43

1000003 POLYGON 523,23 231



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Anomaly #114

Epty Prt (eoetry h

utpe prt nd one epty):


2. Lines3. Polygons

DescriptionEmpty parts occur within a multipart eature, such as a multipartline or polygon. An Empty Part occurs when a geometry withinthe multipart eature is null.

Reouton

This error is resolvedby editing themultipart eature.

mUlTiPOlYgON ((123..., 432...), (), (345..., 654...))

Second polygon part is empty.

2.1.

3.


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Sinergise d. o. o.Teov uc 30

1000 ljubjn, soven

T: +386 [0] 1 477 66 76

F: +386 [0] 1 477 66 10

E: [email protected]

ljubjn, deceber 2010


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topological anomalies brochure

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