Tracing the source of nautical depth data

[rptb1]

I was directed to post here by OSMAnd support, because my conversation with them ended with a suggestion.

Summary: I tried to trace the source of the nautical depth data but was unsuccessful. My suggestion is that the OSMAnd app documents (via permalinks) the source of its data in a way that would allow it to be reproduced. This suggestion could apply to all map data.

Here are the edited highlights of the conversation with support.

I wrote:

I have been trying to find out what the nautical depths provided by OSMAnd mean. I have read https://osmand.net/features/nautical-charts and I use both the "Nautical depth contours Europe contours" and "Nautical depth contours Europe points" maps. I have spent some time searching through the OpenSeaMap wiki, but with no luck.

On a nautical chart, such as for example https://www.visitmyharbour.com/harbours/solent/gosport-marinas/chart/F19A60E53A4B1/portsmouth-gosport the depths are relative to a "chart datum" which is usually the lowest astronomical tide for the local port. That means that a "zero" depth on the chart is somewhere that is at sea level at the lowest possible local tide. It also means that the depths aren't consistent between charts if compared to, e.g., the WGS84 Geoid and the national datum.

I compared the depths shown by OSMAnd around Portsmouth Harbour with those on the Admiralty chart, and found that they were between 1.8m and 11m deeper.

I'm not expecting OSMAnd to show me perfect quality data suitable for navigation, of course! But whoever compiled the depths shown by OSMAnd must have some idea of their reference datum, and their reference geoid, and I can't seem to find a connection between your UI or your documentation and that information.

Support wrote:

Right now OsmAnd compiled data from various open sources which might be far from perfect.
https://github.com/osmandapp/OsmAnd/blob/master/LICENSE#L132

So, we're going to recompile this year and try to find new sources of information.

I wrote:

May I suggest that, as well as finding better information sources, you also find an accessible way to document the sources?

It could be something simple. For example, on the map downloads list you currently only have names and descriptions for the maps. Perhaps you could also have a persistent link that leads to further information on the web, for people who need to know more details.

Ideally, the link would allow anyone to reproduce the data -- e.g. a git hash to the data or a procedure that reproduces it, so that there's a permanent record.

[vshcherb]

Yes some source provided in https://github.com/osmandapp/OsmAnd/blob/master/LICENSE#L132.
All our sources are compiled and manually crafted i.e. there was some manual work done in QGis to compile sources, I'm not sure if we distribute these results publicly or not.

Main person behind this data generation @xmd5a2 is currently away probably he could enlight more information how it was done.

[quantenschaum]

I have been asking myself the same question. I looked into the depth data for the Netherlands. It was impossible to find information about the depth datum (LAT/NAP), where the data actually comes from and how up to date it is. A comparison with current and official data revealed that the NL depth data is complete rubbish, it is probably very old and should not be used at. It's a proof of concept of depth data in OsmAnd IMHO.

[quantenschaum]

• How could I prepare my own depth data file?
• How is depth data entered into OSM?

[quantenschaum]

Cool! Is there some documentation out there that describes how the tags in the OSM XML should look like?

[quantenschaum]

How do I have to tag depth contours and spot soundings that they appear on the map? I cannot find any docs on this topic.

[quantenschaum]

If I tag them with contour=depth and depth=12 nothing is shown on the map.

[vshcherb]

Did you check with inspector? https://docs.osmand.net/docs/technical/map-creation/create-offline-maps-yourself - check rendering_types.xml parameter it documents which tags are indexed for rendering. https://docs.osmand.net/docs/technical/map-creation/how-to-inspect-an-obf

[quantenschaum]

Thanks! I found this, your link is helpful as well.

I did download some OSM data with JOSM, added some points and lines and ran the OMC. It worked. I can import the OBF into OsmAnd and the buoys I added are shown on the map, but not the contour lines and points.

The docs do not tell much, it's more trying to find the right sections and trial and error. 😞

From rendering_types.xmlI would guess that tagging with contour=depth, depth=10 or point=depth depth=10 would do the trick, but it does not work.

There are depth data available for download from within OsmAnd. How were these OFBs created?

[quantenschaum]

Just to mention it:

Tags needed for contour lines

• contour=depth
• depth=<value in m>
• contourtype=[5m,10m,20m,50m,100m]

Tags needed for spot soundings

• point=depth
• depth=<value in m>

[quantenschaum]

see also #18116