Types of Location Errors in Spatial Relationships (Topology) (Especially for GATE-Geospatial 2022)

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A major commercial GIS should be able to provide these general spatial relationships, and you can find them to identify error. There are six major types of errors:

This Diagram Shows Major Types of Errors

Pseudo Node

Pseudo nodes are identified by ones that have only two arcs attached to it. This often occurs two lines are digitized and are connected at one end, but the connection point does not occur at a junction with other lines. If the lines are intended the same entity with the same attributes, then the pseudo node is an error and should be removed.

The most obvious example is an island polygon where the bounding line begins and ends at the same node. Removing a pseudo node involves joining the two arcs or either side of it into one. This usually means ensuring that the attributes of each arc are the same so that there is no problem in merging them.

This Figure Shows Island Polygon and Pseudo Node

Dangling Node

Dangling nodes also referred as Dangles. Dangling Nodes are identified as nodes with only one line attached.

They include both,

  • Overshoots – extends too far past another line and,
  • Undershoots – not quite reaching another line.

In such case, the line was intended to connect with another line and hence overshoots and undershoots are errors. However, dangles are also identified with lines that are connected to another line at one end, such as dead-end streets in a road network. Such, dangles are NOT errors. Such nodes are known as acceptable dangling nodes.

This Figure Shows Dangling Node

Dangles can result from three possible mistakes:

  • Failure to close a polygon
  • Failure to connect the node to the object it was supposed to be connected (undershoot)
  • Going beyond the entity you were supposed to connect (overshoot)

Label Error

Label points are inside each polygon that will act as a locator for a label on which you will display text information about that polygon. You need only one label point per polygon. In short, Label points ( “centroid” ) are associated with polygons and are used to attach the attributes for the polygon. The labels must be placed in a polygon with the correct attribute attached. Two types of errors can occur relating to label points in polygons

  • Missing labels
  • Too many labels

Correction of Label Errors

The correction of label errors requires the removal, editing or addition of labels. It should be noted that some label errors may be due to existing dangle errors where two adjacent polygons appear as ONE polygon simply because of an undershoot. In such case, the undershoot should be corrected.

This Figure Shows Missing Label and Too Many Labels

Silver Polygon

This error occurs when the software uses a vector data model. In this case, it is required to digitize the adjacent lines between polygons more than once. Failure to place the digitizing puck at exactly the correct location for each point along that line will often result in a series of tiny graphic polygon called “Silver Polygon.” In short, Silver Polygons are polygons that result when two lines are digitized twice along the same boundary. This can occur because of digitizing the same line twice or overlaying two layers. Silver polygons, also referred to as Spurious polygons.

This Figure Shows Silver Polygons and Corrections

The easiest way to avoid silver polygons on input is to use a GIS that does not require digitizing the same line twice. The other solutions are:

  • Try to use existing lines for boundaries that are common to two or more features
  • Eliminate all polygons with an area than a specified tolerance value

Weird Polygon

Weird polygons are tiny “knots” usually caused by digitizing errors where lines are accidentally crossed. This occurs when two or more lines cross over, producing the semblance of a polygon. The most frequent cause of this error is a point digitized in the wrong place or in the wrong order. In the “cleaning” process, the intersection points are identified resulting in additional nodes and lines. Detecting weird polygons is difficult but is not impossible.

This Image Shows Weired Polygon

Missing Arc

Arcs are missing. They may be detected by studying the original map, otherwise they may be impossible to identify. Any missing arcs detected must be captured and integrated into the appropriate dataset.

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