Elements
From OpenStreetMap
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All views of maps consist of the basic elements "nodes" and "ways". Properties (A.K.A. "Tags") are assigned to these elements by use of additional values which are a "key" with a "value". All editing interfaces use the elements and allow the input of keys and values.
The basic elements have equivalents in the internal database as Data Primitives.
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Node
Nodes are the basic element of the OSM scheme. Nodes consist of latitude and longitude (a single geospacial point)
(We don't generally record the third dimension, altitude, although this is a tagging proposal)
Nodes are needed to define a "way" (see below), but a node can also be a standalone un-connected point representing something like a telephone box, a pub, a "place" name label, or all kinds of other points of interest (POI). Standalone nodes should always have at least one Tag such as amenity=telephone.
Nodes forming part of a way often do not have tags themselves (they only exist to describe the way) , but this isn't a hard and fast rule. For example, a node can be tagged railway=station and also appear along a way tagged railway=rail .
Way
A way is an interconnection of at least two nodes that describe a linear feature such as a street, or similar. Nodes can be members of multiple ways.
One way is characterized with uniform properties. Same properties are for example, same priority (motorway, federal highway, ...), same surface quality, same speed, etc. Ways can be split later into shorter ways if different properties exist (for instance, if a street has a one-way section, that section would be a different way from the two-way section, even though they share the same name).
To be considered complete, a way must have at least one "key" with a valid "value". See: Map Features.
Area
Areas are not real elements. These are closed ways with according tags. There is actually no other difference between a circular closed way and an area.
Areas are in general closed surfaces. Therefore they are separately used. Areas are pseudo elements.
Areas are created by use of keys like "area", "water" or other.
Areas should have at least one "key" with a valid "value". See: Map Features.
To map "holes" in an area, e.g a clearing in a wood, you need to make the wood a 'C' shape, and bring together the end's of the 'C' to create the correct closed shape 'O'. First draw all the way around the wood, then continue the way with a connecting line to the clearing in the woods. Continue in the reverse direction around the clearing, and finally complete the loop, then link back to the start point with a final section of the way. You will have created 2 sections of the way between the same 2 nodes if you have done this correctly. If there are multiple holes just repeat the same pattern, always returning to the same point on a loop, and doubling the sections of the way to get back.
Historical note: Segments
We used to have another element type called a "Segment". Segments were abolished in Protocol version 0.5, and the concept is no longer used within the editing interfaces.
