Potlatch 1/readme

Things you really need to know about Potlatch and OSM

Potlatch is an ONLINE editor, any change you make is instantaneously saved in the general OSM database, so be careful.

If you are unsure about what you want to do (and we assume any beginner to be unsure), you are strongly encouraged to start Potlatch by clicking the play/practice button. None of the edits you make in play mode are saved. Once you get the grip, you may switch to normal edit mode.

Please respect the work of others and the integrity of the database, it is so easy to screw up things! At least learn to use the 'Z' key to make an instant "undo".

But once you're comfortable with doing simple edits, do get started on adding your data - we really do welcome new users and need as many mappers as we can get!

Read the Beginners Guide, because due to the fact that Potlatch is written in Flash, is is not as intuitive to use as other software!

Things you really need to know about the OSM database

All map data is made up of “nodes”, “ways”, "relations", and combinations of these. Nodes and ways represent physical objects while relations describe the relationship between nodes, ways and other relations.

• A single node may e.g. represent a stand-alone object like a park bench. Nodes are the only objects in OSM that have a geographical position assigned. Therefore nodes indicate where something is.

• Ways always consist of two or more nodes. A way may be used to represent a street or waterway. A way basically is a line joining all nodes it consists of. The line starts at the first node and ends at the last one. It may contain the same node several times and even start and end at the same node resulting in a closed way. Such a closed way may be used to represent areas like e.g. a parking lot or a forest, but it can also just be a circular road.

• Relations usually don't represent physical objects. They are e.g. used to describe the streets a tram line consists of or a set of cycleways forming some bike trail. The concept of relations is quite new but greatly simplifies mapping more complex situations. It is not difficult and is well worth the effort of learning as early as possible.

All objects need properties called “tags” to describe their purpose. A tag is a combination of a "key" and a "value". An example of a tag is highway=residential where "highway" is the key and "residential" is the value. These tags contain the description of an object and thus a way tagged highway=residential represents a residential street.

Tagging Examples

• Node tags: waterway=dock or highway=traffic_signals
• Way tags: highway=cycleway, highway=primary , name=Piccadilly Circus

There are long lists of recommended tags (see: Map Features) that are supported by the map. You can also use your own unsupported tags which may be very tempting as it doesn't require you to look up the recommended tagging. While this is possible, it is not recommended. There are several reasons why you should really try to stick to established ways of tagging such as those described on the Map Features. These are:

• Tags are used by the so-called "renderers" to actually draw the maps (e.g. the one you see on the OSM start page) from the data you enter. The renderer needs to know what the meaning of the tags you use. If a renderer doesn't have a rule to interpret a tag, the object tagged in this way will probably not show up in the final maps - which is probably what you want!
• Potlatch, like most other OSM editors, offers so-called tagging "presets". Using these may help you use tags correctly e.g. without the risk of typos. But if there isn't a preset, do look up the recommended tagging before - and only as a very last resort - inventing one of your own. There are many more recommended tags than those in the Potlatch preset.
• As there is no further description for any tag you invent, another person may not understand what your tagging was supposed to mean. Using the established tags allows others to understand what you are trying to do and gives other people a better chance to make further edits or corrections.

Tags exist for most common kinds of objects and the sets of commonly accepted tags - and presets - are extended from time to time. So if you really think a new tag is needed, do consider discussing this with other OSM users, e.g. on the OSM mailing list, rather than just using something you've come up with.

Areas

A closed way (having the same start and end nodes) may describe an area. Lakes and forests and many more area-like objects are mapped in this way.

Whether such a way actually describes a real area depends on its tagging. As a simple rule of thumb, ways (highways, waterways and railways) are never considered areas, even if they are closed. Everything else is considered an area. This is in fact the rule used by the Potlatch software to distinguish between a way and an area.

A rule of this kind exists in every editor or renderer. Unfortunately, these rules sometimes differ between the different editors and renderers. So it is quite possible that Potlatch considers something an area while the renderer drawing the map on the main OSM page does not. This depends on the way each program interprets the tagging. This is another reason to stick with existing and well-established tags, as these are likely to be treated the same way by all the different editors and renderers.