It has been a long-held practice in OSM (2008 or earlier) to map coastlines at the Mean High Water Spring as stated here:

People have questioned the wisdom of this on the Discussion Page.

There has been little discussion on the above page since 2009.

Can somebody direct me to more up-to-date guidance?

If there is no up-to-date guidance, how does one petition the OSM community to adopt standards for tidelands and other marine features such buoys, bathymetry, submerged hazard/rocks, etc?

asked 22 May '16, 15:22

flockfinder's gravatar image

accept rate: 0%

Fundamentally the problem is that, with some limited exceptions, we have no good global source for any candidate data for coastlines (at least 4 of each of High & Low Water marks). For some countries data are available for the nationally recognised convention (e.g., OSGB Open Data for Great Britain). National conventions will often depend on specific legal issues (for instance the foreshore in Britain belongs to the Queen, so it's historically been important to delineate property boundaries on the coast).

I think its reasonably fair to say that the most widely used '''cartographic convention''' is to use one of the High Water Mark standards as the basis for defining '''land'''. Therefore in principle we are free in OSM to map any/all High & Low Water marks. Ideally they should just be another set of taggable elements. Use of Low Water definitions produce far too many absurdities (not least of which is that they render places unrecognisable).

However, coastlines are also anomalous in many ways from other OSM data: particular rules (directionality of ways), particular processing etc.

In the ideal world we'd have better sources for different coastal water marks, and much better ways to deal with some aspects of coastlines (and remember Jochen Topf has already moved things a long way ahead of where they were a few years ago).

As it is we have to be pragmatic. In most cases we only have aerial imagery at unpredictable points in the ebb & flow of the tide. Where national data showing the locally conventional land/sea boundary exists we should follow this; elsewhere we just have to do as well as we can.

In summary a hard-and-fast rule has no point at present because we have no way of sourcing such data.

P.S. I would recommend this article on technical aspects of coastline mapping in Britain by the Ordnance Survey. I think it demonstrates that any solution is non-trivial.

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answered 22 May '16, 23:17

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SK53 ♦
accept rate: 20%

With accretion and erosion and subsidence and uplift and sea level rise at differing rates geographically, the Natural=Coastline will always be a moving target. Mouths of rivers regularly move hundreds of meters seasonally. The waves and the winds push the sands and gravels around. The best that we can do is present an approximation.

In such a dynamic environment, I suggest that it is imperative that local knowledge trump any global or national mapping authority. I would hope that the OSM community would adopt some logical rendering scheme that would highlight vehicular and pedestrian access to sand and mudflats at low tides and show which navigational hazards (rocks, shoals, sunken wrecks etc) are exposed at low tides or more importantly close tot he surface at low tides. Furthermore, it is important from a mariner's standpoint, that bathymetric contours or spot mapping of depth be measured from a low water datum. For these reasons, I think it is important to decide on a tagging scheme and a logically and valuable rendering scheme sooner than later. I am uncertain how much progress OpenSeaMaps has made of late. But I didn't see much progress for a couple of years.

Currently the ocean realm is a Terra Incognita in OpenStreetMaps and I hope that changes in the coming months. I have been mapping for 3 years and waiting somewhat patiently for progress in how potlatch renders the ocean realm.

I am of the mind that a unified map might be more valuable. After all, the same GPS (thanks Lambertus!) moves with me from hiking off trail in the mountains, to navigating unknown cities, to plying my waters near to my home or far removed from it. I would say, rather than splinter the various projects. I wonder if OpenStreetMaps can just absorb the rendering of the marine environment.

So I'll ask again: If there is no up-to-date guidance, how does one petition the OSM community to adopt rendering standards for tidelands and other marine features such buoys, bathymetry, submerged hazard/rocks, etc?

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answered 23 May '16, 01:29

flockfinder's gravatar image

accept rate: 0%


There is no way to petition the OSM community for rendering. Each rendering based on OSM data (the standard style, OpenSeaMap, Humanitarian, the garmin Style your refer to, OsmAnd, etc.) is developed by different groups. Each group has its own way to gather requests from "the public". But in the end, the developers decide what and how it will be rendered.

So you can try to convince one of those groups, or start developing your own style.

(23 May '16, 06:56) escada

I hesitate to answer further as this looks to be turning into a discussion rather than the Q&A format which this site is for. I would dispute that OSM has ignored the ocean realm: look at OpenSeaMap. There is plenty of mapping of the littoral zone: I used OSM for navigation on a low-tide only route two weeks ago. I also have no idea what you mean by Potlatch rendering the ocean: it's an editor. As for most things OSM the mantra is "patches welcome".

(23 May '16, 08:03) SK53 ♦
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question asked: 22 May '16, 15:22

question was seen: 2,990 times

last updated: 23 May '16, 08:03

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