[Talk-ca] Canvec vs. GPS

[Emilie Laffray]

On 7 March 2011 01:44, Samuel Dyck <samueldyck at gmail.com> wrote:

> Hey everyone
>
> I am presently preparing a careful import of Canvec data into Mantario
> area. I have stumbled across a trail that appears to be a GPS track. The
> problem is that while this trail did not overlap with the old old low detail
> lake data, it conflicts in some areas with the Canvec data. Which data
> should I adjust? The overlap between the two ranges for 17cm to 30m.  An
> inspection using Landsat (sadly the best imagery for the region) favours
> Canvec. I realize that this is a tricky subject. I'm assuming Godwin's law
> does not apply to this list.
>

Hello,

One thing to keep in mind is that GPS tracks are not the ultimate answer to
everything. If you want to have reliable data from GPS, you would want to
have multiple traces from the same area. A mainstream GPS receiver will have
an average precision ranging from 5m to 10m. They can be quite useful though
to locate more precisely some features.
Also bear in mind that aerial imagery is also not perfect and might not be
properly aligned.
In the end, I would trust in your case the CANVEC data since it is supported
by the imagery, but nothing prevents you at some point to go over with your
GPS device to go and take some measurement to check if CANVEC and/or landsat
are really accurate.
It is a great strength of OSM to be able to import some data and then go and
check if it really makes sense on the ground, as it could have changed since
the landsat picture was taken. One perfect example is one street in where I
used to live which has a roundabout. Only a commercial provider had it right
and all aerial imagery that I have seen was not showing it, including the
French cadastre.
So don't worry too much, import the data if you can and then try to go and
check later.

Emilie Laffray
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