OpenStreetMap
Deriving smaller multipolygons from larger ones
Posted by Robert Copithorne on 1 July 2017 in English.Recent discussion on breaking up multipolygons in to smaller units. Note that the original polygon contains many members. The objective was to show the original land use of an area adjacent to, and draining into, the Alberni Inlet as a natural wood (forest), then break the area in to smaller units based on the nature of the tenure. The tenure in most cases sets the major purpose of the land use.
The initial comment could be based on my saving a session with a number of errors which I intend to correct through the work to be done on setting smaller units, as described below.
Comment from (OSM contributor) about 6 hours ago
Do you know what you are doing?
Comment from Robert Copithorne about 3 hours ago
Hello. Yes, I believe I am doing what is required, but I am aware that I created a big potential problem for myself when I created a very large complex multipolygon related to land use in Alberni land areas. Things became more difficult when I started to break the large multipolygon in to smaller pieces; specifically portions related to Strathcona Park, and Western Forest Products operations at Great Central Lake. Currently I face a large task of separating the elements of the three multipolygons, but I am working on that, and I believe will be able to straighten it out.. Any suggestions you may have at this point that would help to reduce the work involved would be appreciated.
Comments from other users relevant to the issue of splitting multipolygons in to smaller units would be appreciated.
Additional Info: Having worked in Forestry in areas around Port Alberni, including updating Forest Inventory maps, I am intensely interested in developing a map of land uses, features and access roads in the Alberni Inlet drainage area, and adjacent areas, for the benefit of all users.
Discussion
Comment from Warin61 on 1 July 2017 at 06:19
Hi,
There are 2 things here,
Land COVER. This is what natural=wood is, a land cover .. it does not signify what the lands is used for but by what is covering the land.
Land USE. Landuse=forest is a use of the land, usually to produce lumber/wood. The land is usually covered by trees, however when harvesting takes place the trees are thinned if not removed and so the land cover may well change as part of the harvesting.
It is unfortunate that the key ‘natural’ exists at all .. it could be better divided up into the keys landcover (trees, grass etc) and landform (peak, valley etc). But OSM is slow to change. I do use the landcover and landform tags in conjunction with the natural keys to give, I hope, future usefulness to hte map.
Comment from Warin61 on 1 July 2017 at 06:28
Ok, Now on the editing thing .. multipolygons etc.
Firstly using JOSM you can save your work locally to your hard drive and work on it later without uploading it to OSM. This means you don’t have to save the rough work with those errors.
JOSM also has a layer feature - you can merge a small area to another layer .. so you could work on the north east corner of your area - merge the correct bits off to a new layer and then up load that layer with the correct bits to OSM. Save the working layer back to your local hard drive and you can come back to do the rest later.
Does that help?
Comment from Robert Copithorne on 1 July 2017 at 19:13
Thanks, Warin61. I will try out your suggestions of layering, which I haven’t tried, and saving sessions, which I have done sometimes.
I was saving the particular session discussed above as I made my revisions, but chose to upload it at one point, although some errors needed to be corrected, to get an update on the map, and avoid conflicts, as others may be revising the map. I fully intend to address the errors shortly.
As to the designations of Land Cover and Land Use:
I would like to retain the tag natural=wood. I see it as the preliminary stage of land use. In most of the land area I am dealing with, there are large portions of original untouched forest as well as lots of rocks, and water (all of which can be considered part of a natural wood). In the case of managed forests, the management plan is to leave many of these areas untouched, perhaps for many years, so the natural=wood is appropriate. In this way, I think I am in agreement with your definition.
Land use is signified by a change in the forest cover, as you have described, either by forest operations (harvesting / road building) or by land clearing not covered by a forest management plan. To signify this, I change what was natural=wood to landuse=farmland. These changes are readily seen on aerial photos, so I don’t have to get in to worrying about what is a managed forest, and how to tag it. I use the tag farmland as opposed to forest, because visually it stands out on rendered maps replicating what is actually what is seen on the ground. I reject the proposal I’ve heard from others, to call regenerating forests “scrub” as they are far from it, being regenerated according to a very rigid formula. I would like to stick with the tag farmland for regenerating land, because under forest management licenses, as well as on private managed forest lands, the regeneration process is a form of farming, other than the longer time frame.
What will be needed when the currently designated (tree) farmland areas mature in to second growth stands remains to be seen. Visually they are very hard to distinguish from natural stands, without a closeup view, but it may be possible to use other mapping techniques to achieve this. My inclination would be to tag these area as “forest” under “Landuse” but this doesn’t exist in OSM yet, although there is a “Forest” under “Natural”. This could be changed by consensus, I imagine.
Sorry to go on at length about these land use issues but I am very interested in removing as much ambiguity as possible.
Hope that we are in agreement on these descriptions. If you have questions, or wish to discuss it further, please contact me.