[OSM-talk-ie] Noob question

Dermot McNally dermotm at gmail.com
Thu Apr 24 00:57:25 BST 2008


On 24/04/2008, Paul Cunnane <paul at cunnane.net> wrote:

> Actually, speaking of coastlines, have a look here:
>
>  http://openstreetmap.org/?lat=53.7947&lon=-9.5597&zoom=14&layers=B0FT
>
>  I've verified the route of the coast road with my own GPS, and it's
>  accurate. Any ideas on an approach to cleaning up the coastline?

You've answered your own question below, along with one I had...

>  Also, I have traced some rail lines from overhead imagery, but it seems
>  the imagery is slightly off. I'll hold off tracing any more until I can
>  get a GPS fix, or else an accurate sense of how much and in what
>  direction the imagery is off. Unfortunately the Yahoo imagery isn't very
>  detailed in Mayo...

Basically, the imagery is all off by a certain amount. The landsat
stuff is off by about 30m, based on  what I've read from others. I
wondered where you'd got your railway info from, and now I know. My
view is that it's a better source than none at all and it can all be
cleaned up sooner or later. The landsat imagery is ultimately where
the coastline came from (there's your first answer), and it's also
where the lakewalker plugin for JOSM finds the images from which to
autotrace lakes. You'll see a bunch of lakes I drew in Mayo and
elsewhere. That's where they came from. You'll see that any I haven't
adjusted (see below) will also be prone to intersect roads.

Something that is worth knowing, though: You can, in JOSM and
Potlatch, calibrate the imagery against known good data (in this case,
your GPS trails). In the case of the landsat stuff it takes a bit of
practice and you still need to use The Force a bit, since it can be
hard to make out the roads that correspond to your traces. For the
Yahoo stuff it's pretty simple.

In Potlatch, you calibrate using (I think) spacebar-drag and stop when
things line up. JOSM has a toolbar button that looks a bit like a
4-headed arrow. Click it once to enable calibration mode, again to
switch it off and then be sure to change mode (drawing mode to
selection mode, say), as it seems to need this clue that calibration
is finished.

Once you're happy that you have well-calibrated backgrounds you can
drag other stuff around. I find JOSM best for this. Existing shapes
like lakes can usually be selected whole and simply dragged to match
the calibrated backdrop. Coastline likewise, piece by piece. Because
the shape is correct (rough, but acceptable), you need only correct
the offset, it's far easier than moving every node. (Trust me, I
imported the Irish coast and cleaned it up the hard way, since there
were fewer automated tools at the time. There's a lot of it).

I find that calibrated Landsat imagery is very acceptable for placing
lakes, rivers and coastline. I'll only rarely use it for roads, and
usually in cases where I'm trying to stimulate interest in further
work in the area. I did this on the last bit of N13 across the border
from Bridge End recently, since it's nice to have the road "complete"
even if it can benefit from cleanup once some traces become available.

Dermot




More information about the Talk-ie mailing list