Points, Lines, Polygons, and Polylines in a GIS are dumb graphics. We make them intelligent by attaching attributes to them. That way we can tell that a point is a rubbish bin or a sampling location, a line is a road, a polyline is a river, or a polygon is a sports field or a parcel of land.
We can use attributes at a very simple level for small projects or at a very sophisticated level in a large corporate environment. The ability to attach geography to a database makes GISs far more powerful than the sum of their parts. Geographical functionality in a GIS draws a geographic object on a map, and the attribute database describes what that object is.
So let’s discuss the link between map graphics and their attributes with reference to an on-screen digitizing environment.
- Points: Points are one mouse click. Possible attributes are that a point is a power pole, that it’s wooden, and its installation date.
- Lines: Lines are two mouse clicks. Possible attributes are that it’s a segment of road and it’s called Smith Street.
- Polyline: Polylines are three or more mouse clicks. Possible attributes are that its a river, this it’s in poor condition, and it’s called Johnson River.
- Polygon: A polygon is four or more mouse clicks, with the last mouse click being absolutely identical to the first mouse click. Possible attributes are that it’s a land parcel and that its owned by Mrs Smith or Ms Smith.
Importantly, when an attribute is attached to geographical object we can click on that object and find out information about it. We can also query entire databases for both geographical and attribute information. For example, a query of all wooden power poles more than 30 years old would return not only a list of a relevant poles, but also a map showing where they are.
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See also What is Spatial Data
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