When a polygon forming the boundary of water like river or lake is not closed "water might pour out" and tiles area drawn in blue where there should be no water. To avoid this all areas of water should be closed before saving them when using Potlach or uploading them when using JOSM or another offline editor. Mapnik might start immediately with rendering and afterwards not re-render for a while. In general JOSM shows the outpouring and gives a warning. When mapping is continued the polygon can be opened again or a new one can be started. A closed polygon is either one single closed way or a closed multipolygon. A closed multipolygon consists of multiple polygons which are members of a multipolygon relation and form together a closed way, also called ring. The multipolygon relation can contain several closed single polygons and closed multipolygons:
http://wiki.openstreetmap.org/wiki/Relation:multipolygon. As usual there are different ways to map large riverbanks. I prefer one multipolygon relation for the whole river. This keeps all together, there are none or few artificial lines and sections. In addition I can download the whole relation for editing in one shot and make "global" changes easier. Recently I mapped about 1000 km riverbank of the Ping River in Thailand in this way: http://www.openstreetmap.org/browse/relation/1151384. The single way segments (polygons) are about 10 km long. The multipolygons forming the riverbanks close at the beginning, at the mouth of the river and at the two lakes through which the river flows. One of them is a large reservoir with a dam. In the river there are more than 60 islands mapped. The lakes are also multipolygon relations with islands. Mapnik renders the river, the islands and the lakes correctly. Osmarender has sometimes problems with multipolygons, also in this case: http://www.openstreetmap.org/?lat=17.96519&lon=98.64856&zoom=15&layers=M. When there is an island in a river or lake and a multipolygon isn't used than Osmarender might draw the island and Mapnik not as in this case: http://www.openstreetmap.org/?lat=41.07168&lon=29.38923&zoom=17&layers=O. No matter which way you choose the most work and the most valuable work is drawing the riverbank. By the way: "Tagging for the renderer" is often misunderstood. The meaning of the phrase is probably closer to the following: "Don't deliberately tag incorrectly for the renderer." http://wiki.openstreetmap.org/wiki/Tagging_for_the_renderer. Happy mapping Willi > On 06/09/2010 19:00, Charlotte Wolter wrote: > > Hello everyone, > > I've been mapping the Little Colorado River, which, despite its > name, is rather large, bank to bank. When it's not encased in a canyon--such > as when it gets close to joining the big Colorado--it's a typical Western > U.S. river: a quarter-mile wide and a foot deep. So, in the places where it > is wide, I've been mapping it as two riverbanks. However, it sometimes > renders as a large area of water, covering places that are not under water. > I know we are not supposed to map for rendering, but this result > makes me think that I am doing something wrong. > So, what is the correct way to handle waterway=riverbank? Do we > join the two banks from time to time (as I have done to try to correct the > problem)? > The area I have mapped is in northern Arizona, around the towns of > Cameron and Winslow. > Thanks in advance for your help.
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