Key GIS Terms: Geometry, Layers, and Topology

Introduction

When you work with GIS, you often run into terms you have never heard before. This article introduces some technical terms worth knowing when you use GIS.

Geometry

In vector data, the shapes of the data, such as points, lines, and polygons, are called “geometry.”

For details on vector data, see the article below.

Layers and layer structure

GIS represents a map by stacking data in layers, so each set of data is called a “layer.”

This way of stacking several layers to represent a map is called “layer structure.”

In QGIS, the Layers panel lets you manage the data (layers) added to a project intuitively.

To learn more about working with layers in QGIS, see the article below.

Features

Each individual object in a layer is called a “feature.” You can think of a feature as standing for an individual thing (or phenomenon) in the real world.

For example, in point data showing railway stations, each feature is the location of one station. In polygon data showing building shapes, each feature is the shape of one building.

Nodes and segments

The points that make up a line, such as the junctions where several lines cross and the start and end points of a line, are called “nodes.”

On the other hand, the line pieces that connect nodes are called “segments” (also known as edges or links).

Topology

In GIS, the spatial relationships between pieces of data, such as how they touch or overlap, are called “topology.”

Here are four spatial relationships between data: connectivity, adjacency, containment, and intersection. For some data, if these relationships do not hold, the data is treated as invalid.

Connectivity
Describes whether a piece of line data is connected to other line data.

For example, in line data showing roads, roads that join at an intersection are “connected.” On the other hand, an expressway that crosses over an ordinary road may look as if the lines overlap, but they are not connected.

Whether line data is connected matters in analyses such as route calculation and river systems.

Adjacency
Describes whether features are adjacent to each other. For example, in municipality polygon data, if there is a gap between City A and the neighboring City B, or if the two overlap, the adjacency is “invalid,” since in the real world there are no gaps or overlaps between cities.

Containment
Refers to whether one feature contains (or is contained by) another feature.

For example, if there is a lake inside a park, the park polygon “contains” the lake polygon.

Intersection
Describes two or more features crossing each other.

For example, a point for a railroad crossing is located where road line data and railway line data cross, and line data that passes through a polygon for an area intersects that polygon.

Conclusion

It helps to remember these terms. “Geometry” is the shape of vector data, such as points, lines, and polygons. Each individual shape is called a “feature,” and the spatial relationships between features are called “topology.”

GIS represents data by stacking it like “layers,” so each set of data is called a layer. Understanding these concepts will help you use GIS more effectively.