OpenStreetMap

CanVec Data

Posted by Adam Martin on 9 July 2013 in English (English)

Looking over my area, I am proud to say that there a ridiculous number of fresh water sources. Canada is renowned as possessing one of the largest supplies of natural fresh water in the world. My province does not buck that reputation - there are a tremendous number of bodies of water, both big and small, that serve to provide all of the water needs of the population. That's the good part.

The bad part is that most of these water sources were added via the CanVec data available from the Department of Natural Resources. This is good topographical data, but it is not completely accurate. In one case, a fresh water source is itemized in that data with no corresponding information in the satellite data available through Bing. Another has a water source that is significantly different in shape from the aerial data. I am hesitant to delete the CanVec markings as they are based on professional surveys of te region. Still, the lack of any 1 to 1 consideration in it gives me pause. For egregious examples, my instinct is to plan a visit to the site - I can probably settle the matter fairly swiftly that way.

The other interesting variance in the CanVec data is that it does not indicate the type of any of the sources of water that it outlines. Which is it - a pond or a lake? A check around the Internet provides a ridiculous amount of referencing, discussion and argument as to what the difference between these two geographical items are. Some draw distinctions in terms of size, depth, movement or content while noting quite openly numerous examples that violate those criteria. Local knowledge in lieu of naming is likewise of limited use as a enormous water body in one place is a pond while another just a kilometer away at half the size is a lake.

To combat this, I have adopted the official naming scheme for water bodies as prescribed by the OpenStreetMap Wiki. Lakes are henceforth all bodies of water that are flowing (as in a stream or river) that are natural - that is, formed by local conditions independent of the influence of man. Ponds are artificial, man-made lakes that may now compose a portion of a regular waterway, but which is no less created either intentionally or otherwise.

Comment from Vincent de Phily on 10 July 2013 at 10:34

Water bodies are difficult to map because their shape changes quickly. Tidal water is the obvious example, but the convention of mapping the high water mark is fairly well established.

Lakes may seem static but they are not. In Ireland we have a lot of [turloughs](http://en.wikipedia.org/wiki/Turlough_(lake\)) that can swell to hundreds of meters length in a matter of days, stay around for a few weeks or months, and disapear again. Even for an identical "fill factor", the shape may be different from year to year. This is an extreme case, but all the lakes of the world probably fit between "static" and "turlough".

So how do you map such a beast ? No one outline will be correct.

  • Figure out wether your satellite image was taken during a wet or dry spell.
  • Accept the fact that your mapping is going to be subjective, and that different sources (canvec, satellite, ooc maps, etc) will draw a different shape.
  • Keep the coastline's "mean high water" convention in mind.
  • Use natural=wetland around the natural=water where applicable.
  • Compound all that and use your best jugement.
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Comment from Adam Martin on 11 July 2013 at 14:45

Thanks for the pointers, @Vincent de Phily.

I have a related question - what do you do in circumstances where the coastline and the administrative boundary have been merged?

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Comment from Vincent de Phily on 11 July 2013 at 16:17

I haven't really formed an opinion on coastline vs boundary. I expect it differs legally from country to country. To me this looks awfully silly and makes me wish for county borders away from the coast, but I can understand the argument that only the country itself, not its constituent counties has sovereignty out in the see. To think that it I'm rowing between those islands I'm in Ireland but not in Mayo nor Connaght feels weird, but I haven't argued against the established order so far.

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Comment from pnorman on 12 July 2013 at 06:09

Much of the CanVec water data in many regions is over 30 years old, I'd be bold in deleting it if you can't see any signs.

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Comment from Andrew_Finn on 26 February 2014 at 09:47

The main difference between ponds and lakes that you can notice in satellite view is depth. Lakes are deeper, and generally (but not always) bigger. Lakes because they are deeper have little vegetation and the shores are rocky while ponds have lots of vegetation and the shoreline will have a greater percentage of grass and vegetation on it. There is a large number of ponds that could be called lakes or vice versa. I wouldn't worry too much about it and usually will go with the actual name from the Newfoundland and Labrador Geographical Names Board by the Department of Environment & Conservation. Wetland info can be found here http://www.ducks.ca/learn-about-wetlands/what-wetland/

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Comment from Adam Martin on 26 February 2014 at 12:03

That is usually the sort of difference I note for ponds and lakes as well. For OSM, any natural body of water is a lake and any man-made body is a pond. That's for that resource - it'll certainly help!

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