[OSM-talk] New Popular Edition in JOSM
Nick Whitelegg
Nick.Whitelegg at solent.ac.uk
Tue Nov 14 10:22:28 GMT 2006
Sent by: talk-bounces at openstreetmap.org
To: "'Andrew Rowbottom'" <andrew.rowbottom at gmail.com>,
<talk at openstreetmap.org>
cc:
Subject: Re: [OSM-talk] New Popular Edition in JOSM
Andrew Rowbottom wrote:
>Sent: 14 November 2006 9:18 AM
>To: talk at openstreetmap.org
>Subject: [OSM-talk] New Popular Edition in JOSM
>
>Ahoy,
>
>>If this is true this would probably have been an even more common
>>occurrence in early NPE maps. Not being a map historian I can't be
>>sure of how big an effect this was, though I can say with reasonable
>>certainty that in the previous series (Popular Edition 1920-30's) even
>>after ortho-rectifying on a 2mile grid and reprojecting some of the
>>mapped trig pillars can be adrift by up to 200m.
>This makes an important point about using old maps generally. You cannot
>rely on a drawn map with any certainty because there are a number of
>contained errors. There are survey baseline errors (what became the trig
>pillars and is now done with survey grade GPS), surveyor's errors, and
>draughting errors (projection errors are less of a problem on a grid
based
>map). All of these are likely to be present in the NPE and other old
>mapping.
>On the whole, aerial imagery (and I include Landsat in this) should
provide
>a more reliable source for physical location, but as we have seen not
even
>these are without positional error. What they are good for though is
>defining the shape of physical objects, meandering rivers being a very
good
>example, which are much better defined on aerial imagery even at low
>resolution, than is typically found on traditional maps.
>I suspect we are going to find in the long run that OSM will be a more
>accurate positional map than many maps that have gone before, but that's
not
>particularly important if the traditional errors are less than a couple
of
>hundred metres or so. You can still navigate and find your way.
I think that's probably true. In particular, I still think that the NPE
(both for adding data through JOSM and visualising data through
'Freemap-NPE') is a good resource for rural areas with a low density of
roads, providing you're careful with your alignment. As I said yesterday I
intend to use NPE to add a lot of the rural roads round here, and focus
actual GPS surveying activity on the footpaths. It's less useful in towns
though.
>What's more
>important is that the route is the right shape. This is especially true
of
>walking maps where I've found over the years that the OS maps have been
>woefully inaccurate at defining the true shape of a path, especially when
>trying to find your way off some remote fell top in the fog!
I've noticed that too, it could be Easter eggs but I think it's more
likely to be due to the path deviating from the official route as laid
down by the Council, to overcome obstacles etc. One of the earliest paths
I surveyed is shown as a dead-straight line on OS maps, but in practice it
actually meanders somewhat. So for that path, OSM is certainly more
accurate.
Nick
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