Quality Improvement

According to ISO-19117, the geospatial data quality elements are categorized as:
 * Completeness
 * Logical Consistency
 * Positional Accuracy
 * Usability
 * Thematic Accuracy; and
 * Temporal Accuracy

In our research methodology at this stage, we have focused on the completeness and logical consistency, since a number of errors in these categories are self-detective. The accuracy aspects are our next step which requires involving reference maps and/or datasets. The completeness validation consists of checking for commissions and omissions according to ISO-19115. Also the logical consistency may be about conceptual facts, domain-related facts, format or topology of the geographical features.

Among these groups, we particularly focus on the conceptual and topological consistency. In OSM context, most of the topological inconsistencies are due to the geometrical mistakes and most of the conceptual inconsistencies are made by errors in setting the features’ attributes. Thus we have categorized the detectable errors into the following three groups:
 * Geometry bugs - topological inconsistencies
 * Attribute bugs: conceptual inconsistencies
 * Geometry/Attribution bugs: A combined topological and conceptual inconsistencies

Having the above categorization, so far we are able to detect the following self-detectable errors and provide the results through tile service, WMS-for-bugs and WFS-for-bugs web services. that are common mistakes in OpenStreetMap data entry. Some of the bugs can be auto-fixed, but not all of the fixes are implemented yet.

Spikes and Kickbacks
Unusual sharp edges does not show a real-world feature either in the line or polygon geometries. This is a common drawing error in both lines and polygons and happens when GPS logging is used to draw the geometries in OSM and either the GPS device logs a wrong point or the user goes along a wrong path.

Self-intersected line
Although a way which intersects itself is a theoretically valid geometry but it is not a simple geometry according to OGC. Such a non-simple way shall logically be separated to separate simple ways particularly in order to avoid unnecessary complexity made for the routing algorithms.

Different-layer joint
The value of the “layer” tag in OSM context determines the relative elevation of the features when they cross or overlap. When two ways are “noded” -i. e. they have a common vertex- it will be an inconsistency if their layer tags are different. It is noticeable that duplicate nodes are not valid in OSM so two noded ways necessarily mean a physical junction between them, which shall be in the same elevation.

Intersection without junction
This is an opposite case of the above, when the two ways are not noded – i. e. not physically joint- but they are geometrically cross on the same elevation. This is a case of inconsistency and it can mislead any routing algorithm.

Overlapping roads
While the same-layer crossing of the roads is completely valid, their overlap is not. Two ways are considered to be overlapping when they have same layer tag, and are noded in a number of consequent nodes. In that case, the line feature along the consecutive nodes is shared between the two ways, so it will not be clear which road the line belongs to, which is logically invalid.

Ways intersecting Buildings
For the same reasons mentioned for the “Intersection without junction” bug, a way cannot cross a building geometry unless they have different elevation, i. e. different “layer” tag shall be set for the two features.

Overlapping Buildings
It is common in OSM that users draw the building geometries without a proper accuracy. Particularly because most of the buildings are small features in the map scale compared to the ways. Thus it is common to have buildings that are geometrically overlapped in small sections, which is a case of topological inconsistency.

Unclosed Area
OpenStreetMap uses “area” tag to determine the geometrically-closed areas. It is inconsistent to set the “area” tag for non-closed geometries.

Invalid motorway connection
This is a conceptual inconsistency made either by wrong geometries or wrong attribution setting or a combination of the two. It can also be considered as a case of domain inconsistency. OSM has certain rules for connecting Motorways to the other types of the road, particularly according to certain countries’ regulations [...]. For example a motorway cannot connect to a residential way in the UK context.

Doubled place
Although this is not an error by definition, but has been checked in order to avoid possible cartographic and statistical errors. Polygons and points can have “name” tag in OSM context. Although double points or double polygons are not permitted in OSM, but it is possible that a point and a polygon refer to the same place in the real world. Because they may even have the same “name” tags which are normally rendered on the map, it can make double names on the same place. It can also make duplicate search results when the name is searched around the area, leading to some statistical errors.

Un-tagged bridges and tunnels
A common mistake is missing the “highway” tag for bridges and tunnels. A bridge or a tunnel is primarily a way and the ways shall be tagged with “highway” tag in OSM context (although this is not the best tag naming). The problem with those un-tagged bridges or tunnels is not only make it inaccessible for routing algorithms, but also makes them invisible on the map – using the current OSM rendering stylesheet.

Wrong-tagged bridges and tunnels
Sometimes the “layer” tag for bridges and tunnels are set incorrectly, particularly a negative value is assigned to bridges or a positive value to the tunnels. This will cause rendering a "strange intersection".