# Generalization

The idea of Cartographic Generalization is well described in Wikipedia [1]. One important aspect of generalization is the accuracy and number of details of collected data.

The generalization of curves in map is the answer to the question: how detailed existing way or outline should be mapped?

The answer is different in different situations. On the one hand OSM says: we map the reality, on the other hand using too many nodes is not helpful - thousands of nodes is too much to represent a single roundabout.

## General discussion

Let´s look at this sketch:

According to different size of Delta L the results would be more or less generalized.

Looking upon the elliptical shape:

one might see: with the same Delta L the distance between nodes is as longer as more "straight" the way to be digitized is.

Looking at the real shape:

The lenght of one segment would be ca. 130,9 cm.

explains the difference between "good" and "bad" generalization. "Bad" has the same distance between nodes and wasted map:

"Good" generalization considers differences in curvature of digitized shape:

How small the Delta L value is, is the decision of the local community.

## Practical examples

Streets in western countries have often rounded boudaries in crossing area. Common radius for rounding is around 5 meter by crossing angle of 90°.

The curve may be generalized by use of some points. Given that the radius of such curve have exactly 5 meter and maper divide this in:

12 segments so is the Delta L= 2cm.

Divided in

7 segments - Delta L= 3 cm

Divided in

6 segments - Delta L= ca. 4 cm

See:

The lenght of one segment would be aroud 131 cm. This seems to be ok. No higher detail level is needed.

## Usecase JOSM PlugIn Fast draw

Parameters of PlugIn Fast draw:

- Epsilon multiplier=1.1

- Starting Epsilon=1

- Max points count per 1 km=300

- Enter key mode=Siplify with initial epsilon.

provides a good resolution for most of naturar, irregular areas like e.g. forests, rivers and lakes.