Recommended Alignment of Toronto Orthoimagery?

Communities Canada
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Hi all,

I’ve just started using the Toronto Latest Orthoimagery instead of only Bing. I wanted to ask if I can trust the alignment of the layer without any adjustments? I’ve tried aligning it to GPS traces but they don’t feel accurate enough to actually make me confident in small adjustments.

If I shouldn’t just trust the alignment without adjustment, does anyone have tips on how to get the most accurate alignment in my neighbourhood?

Thanks!

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I had been looking for some guidance for this too (not in Toronto though).

  • Presumably one would start with points that are known to be correct.
  • Depending on the resolution of the ortho photos this can be easier or not.
  • Ortho photos can be incorrectly assembled
  • Old data in OSM isn’t necessarily correctly placed

In the end, I guess I was lucky to find high resolution and well adjusted ortho photos (or we are all consistently a few meters off in the region).

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Hi Jean,

So there are no easy answers here.

The obvious good news is that the different imageries and alignments in Toronto aren’t more than 1 or 2 metres off, which isn’t a problem in the large scale of things. Most of the time you won’t get better accuracy from a consumer-level device. Any individual GPS traces are just as likely to be off by that amount. An average or sum of several traces could be a useful indication, but I don’t know if we have that much traces for most of Toronto.

Recently I’ve been mapping off the City orthoimagery as well, and when I’m changing alignments, I’ve tried to align to that. My logic being: 1) it’s the latest/newest, so useful to map off that, and 2) the City probably has more motive to align their imagery properly than Bing or Esri does for one of a hundred cities they serve. (Though the flip side could be that Bing or Esri might have more of a means and technology to align very correctly – but whether they actually care to, who knows.)

When I see existing alignments that are fairly decent but clearly to another offset [1], I generally don’t change them, unless needed for other reasons (accuracy, out of date-ness). Adjustments of 1 or 2 metres are unlikely to be really useful to OSM anyway. That sort of resolves the issue of which alignment to use: use the alignment already in OSM, unless it’s obviously bad or very outdated.

(In some editors you can fairly easily adjust the offset and turn it on and off, to adjust to where you’re working, I know that’s fairly possible in JOSM, can’t speak much for others.)

  1. typical example:

    961×861 324 KB
    ↩︎

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