[OSM-talk-ie] Achill

Dermot McNally dermotm at gmail.com
Tue Jun 3 21:47:00 BST 2008

2008/6/3 Paul Cunnane <paul at cunnane.net>:

> Heh. Really just a question of moving from total noob to beginner
> status: I found out how to convert a GPS trace directly into ways so I
> can hack them into shape. Previously I had been tracing over the GPS
> tracks, which can be pretty laborious, especially for long and/or
> winding routes.

This is an important trick for long-distance mapping. There are those
who frown on it, but a lot of my mapping is of long roads with little
variation and there's no way I'm about to trace 100km of that. Have
you installed the utils plugin and got simplify way working? You may
want to tune its simplification parameters, since I found it a little
too aggressive in default mode. But do get it installed if you haven't
already, since it's fairly well a prerequisite when automatically
converting GPX trails.

> I also figured that turning off the background image layer speeds things
> up substantially, particularly as I wasn't generally bothering to
> download the images.

Achill is looking good. I've just adjusted the location of the
coastline of the island, BTW. It's not for the faint-hearted, but I
may as well explain how I do it (and why) in case anybody feels they
need to do it themselves. Basically the problem is that the Landsat
imagery is not that accurately calibrated, so anything derived from it
(including coastlines and lakes that are either traced manually from
landsat or via lakewalker) will also be off, sometimes by 100m or so.
Often the error is noticeable when roads end up underwater, as in the
case of Achill.

Anyway, here's how I fix the problem:

1. Load the affected area into JOSM.

2. Download Landsat imagery. You want as much context as you can get,
but you'll also need it to be at a high enough zoom level to make out
the details of roads - so you may need a few attempts. Occasionally
the contrast is too poor and you won't be able to proceed.

3. Zoom in and identify roads, preferably sections with big sweeping
curves, corners or junctions. You may find this easier if you make the
data layer invisible. You'll ultimately be looking to line up the
features you identify with the roads for which you know you have an
accurate GPS survey.

4. Click on the green four-headed arrow tool in JOSM. This allows you
to drag around the background relative to the data. Calibrate the
background against the known good roads.

5. Click on the arrow tool again. Now change edit tool (say, line to
selector back to line again). Otherwise I've found that the
calibration mode won't switch off.

6. Now for the scary part (and don't forget that undo is your friend
if you much it up): What you want to do is select all visible pieces
of coastline, lakes, whatever will have been out of calibration due to
the landsat error. Make sure they are all off by the same amount, as
others may have fixed individual lakes or perhaps repaired a coastline
from their own ground survey. The theory here is that all of the
things you select will be out by the same amount, so by selecting it
all and then dragging one piece into line with the newly calibrated
background, you'll fix everything in one go.

7. Find a good candidate for realigning (small bays or peninsulae are
good), zoom in close and drag your coastline to where it ought to be.
Slow machines may need to you hold the button down for a while before
redraw, but hang in there.

8. Redo if you mess it up.

9. Clean up the edges - you will have created sharp corners or
artefacts of some sort where your fixed coasts meet other bits beyond
your scope. See if you need to tidy it up. This won't happen in cases
where you select whole lakes or islands (like Achill).


7. Having selected all
Iren sind menschlich

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