What is Spatial Data
The four simple map structures in a GIS are Points, Lines, Polylines and Polygons. If you were to on-screen digitize the figure above then each of the circles (nodes) represent a mouse-click.
There are four basic mapping structures in a GIS – Points, Lines, Polylines and Polygons…
Points: You should think of a Point is a being a mouse click. A point could be something like a rubbish bin, a tree, or a fence post.
Lines: A Line has a start point and an end point. This could be a section of road, river, or water pipe.
Polyline: A polyline is just as the name suggests – a line consisting of many smaller lines. It has a start point and an end point and at least one additional node. To on-screen digitize a polyline would require at least three mouse clicks. One of the great things about polylines is that if the start point and the end point are identical, you could use a GIS map building routine to turn it into a polygon.
Polygon: Polygons consist of a whole bunch of lines, or polylines joined together with an identical start point and end point. When I say identical, I mean identical down to the sub-millimetre. This simple structure becomes far more complicated when you start drawing polygons next to each other.
For example, two adjoining land parcels would have to have identical coordinates for adjoining boundaries. If not, then the lines will either overlap or underlap, meaning that neighbours would appear either own portions of each other’s land, or an area would appear to be owned by nobody. When they are processed by GIS data cleaning routines, underlapping and overlapping polygons create erroneous polygons called slivers. GIS maps containing slivers create all sorts of problems, including erroneous reporting.
These four spatial data types seem simple, but attributed and in a sophisticated geographical information system they can be extremely powerful.
See also What is Attribute data
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