|地物 : 河川|
For small rivers it is not necessary to draw an area for the riverbank, but it might still be interesting, especially if it is irregular.
The area of the river should be marked by a area drawn along the riverbanks. In practice, as wide rivers tend to be long, the river will be drawn as a series of adjacent areas (ways 1 & 3 in image above).
- tag objects with both variants.
- convert from waterway=riverbank to natural=water tagging scheme without strong reasons
If there are only a few islands, terminate the water area at the island and start a new one, excluding the island.
For more complex cases, a multipolygon relation should be used and the island and the main river bank included in the relation. The main riverbank way (way 1 in image above) will have the outer and the way for the island (way 2 in image above) will have the role inner.
Another way to tag those large rivers using multipolygon relations would be to create only one relation for the whole river. This avoids the need to create arbitrary cuts along the river. However, longer rivers mapped with that approach cause problems for some data users. As multipolygons for the entire river offer no tangible advantages, but place an unnecessary burden on data consumers, they should not be used. Use multiple small multipolygons instead. (See discussion)
Many rivers vary strongly in water levels over time making it difficult to decide where exactly the boundary between water and land should be drawn. There is no definitive agreement on what water level should be taken as reference but there are a few guidelines that can help:
- In case of rivers with tidal variation of the water level the high tide level should be mapped like in case of natural=coastline.
- In case of rivers with strongly varying water levels but without a marked flooding period it is usually better to map the high rather than the low water level.
- In the very common case of rivers subject to seasonal flooding with much higher water levels for a relatively short period for example during monsoon season or snow melt and low and relatively constant water levels for the rest of the year it is best not to map the water extent during flooding. A useful rule of thumb would be to map the median of the maximum water levels of all months of the year. For those areas water covered during floods there are some ideas for tagging like Proposed features/floodplain and JA:Key:flood_prone.
- Rivers with a fairly constant water level for most of the year often have a clearly visible (though sometimes frequently changing) river bed indicating the normal extent of the river even during the dry season when the actual water levels are much lower.
- In case of intermittent rivers it makes sense to map the water extent at times the river is not dry and add intermittent=yes. Care should be taken in regions were water is diverted from the river for irrigation and other purposes since the river often does no more fill the river bed even during the wet season in such cases.
In general these rules can be followed more closely if we have better data (ideally someone living locally observing the river's position). When sketching from imagery, it is acceptable to just do our best based on what we can see (e.g., assume visible sandbanks get washed over as pictured below). Note that you should always be careful to avoid overriding valuable on-the-ground mapping with information sketched from imagery.
It is fairly common practice not to map non-permanent and quickly changing islands (sandbanks) in rivers. Vegetation cover on river islands can sometimes be used as an indicator for their permanency although it can also be misleading - there are many examples of vegetation free river islands that are stable for many years. When mapping from aerial or satellite imagery comparing images from different years can be very helpful.
This describes how to use the "create parallel way" tool in Potlatch 2 to create a riverbank from a river. This method works best when mapping a section of river that has a consistent width and each side of the riverbank is nearly identical in shape.
- Create a waterway=river down the middle of the river
- Select a section of the river that has a consistent width
- Select the parallel tool or type "p"
- Adjust the new parallel way's distance from the original way so the new way is half the width of the river (zooming in helps)
- Create a parallel way on the opposite side of the original river
- Label both new ways identically as waterway=riverbank
- Connect the ends of each new riverbank to create a closed loop
- Use the "merge ways" tool to merge these into a single way representing the riverbank area.