![]() ![]() “how many m2 is 10 Smith St”) can be made directly to these databases without the need to involve QGIS. Topology is where a polygon knows everything about every polygon that adjoins it. Spatial databases use what are known as topological data structures. The more sophisticated spatial databases that QGIS connects to such as Grass, Postgres, and Spatialite, are far more intelligent and far less forgiving than simple GIS formats such as shape files. In non-topological GIS maps, polygon C will report as being larger than it should be. In topological GIS maps, the sliver between polygon B and polygon C will become a very small unattributed polygon.Because there is a gap there’s no way to tell that polygon A is next to polygon B. ![]() But both will cause errors when you’re making geographical queries… Neither gaps or overlaps are major problems if you’re only creating thematic maps. ![]() Two of the errors you’re likely to create when you’re digitizing in QGIS are shown in figure 1.Īlthough the map in the inset seems OK, when you zoom in you can see… If you don’t you can create digitizing errors. Setting the QGIS snapping options is important. These are often difficult to see unless you zoom in closely. Figure 1: Two common QGIS digitizing errors are slivers and gaps. ![]()
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