[OSM-talk] quality of drawn way

[Andy Robinson]

Steve Chilton wrote:
>Sent: 10 May 2007 10:00 AM
>To: Nick Whitelegg; talk at openstreetmap.org
>Subject: Re: [OSM-talk] quality of drawn way
>
>Does anyone have any hard facts about relative accuracies of NPE and
>Landsat, or is it variable and unquantifiable?
>I traced quite a few small areas of water in Lake District from NPE the
>other day and was wondering whether to use the shape as per NPE but
>shift each one to fit the Landsat for position. Windermere and Coniston
>Water were already there and seem to fit pretty much to Landsat (may of
>course have been traced from Landsat).
>Any thoughts?
>

I did some analysis around the UK a few months back on this. Overall I found
the maximum landsat error to be around 100m, varying depending upon location
but with the maximum error for a given location always appearing to be
roughly southwest to northeast (as far as I recall). The NPE accuracy varies
between sheet scans. The flatter the sheet generally the better the scan and
the less the error across the sheet. Again the max error generally around
100m. Also note that there is a lot of inaccuracy of the gridline spacing on
the 1:50,000 NPE. I was surprised just how bad they were when analysised up
close. The 1:25,000 provisional/1st series maps by contrast are
significantly better, so I'm expecting them to be generally to +/- 10m or
better (location and grid) when finally scanned and made available.

On top of all this though we have the triangulation errors in the original
surveying. We know the OS has been moving whole villages around over the
last few years as positional accuracy has improved. Generally though I think
these errors tend to be sub 10m, although I've not looked into it in depth.

Cheers

Andy


>Cheers
>STEVE
>
>Steve Chilton, Learning Support Fellow
>Learning and Technical Support Unit Manager
>School of Health and Social Sciences
>Middlesex University
>phone/fax: 020 8411 5355
>email: steve8 at mdx.ac.uk
>
>Chair of the Society of Cartographers:
>http://www.soc.org.uk/
>Mind the (Map) Gap:
>http://news.bbc.co.uk/1/hi/magazine/5413010.stm
>
>
>-----Original Message-----
>From: talk-bounces at openstreetmap.org
>[mailto:talk-bounces at openstreetmap.org] On Behalf Of Nick Whitelegg
>Sent: Thursday, May 10, 2007 9:22 AM
>To: talk at openstreetmap.org
>Subject: Re: [OSM-talk] quality of drawn way
>
>
>On Thursday 10 May 2007 00:59, Alilo wrote:
>> Hi,
>>
>> I have just made some roads using the landsat as background. Because
>> of the offset not corrected I would like to tag the road as not good
>> quality so if someone else drives through the same road in far future
>> (over 500 km road in the sahara) he could correct it.
>>
>> is there something like this in OSM tagging?
>> - Derived from GPS : Quality=1
>> - Derived from landsat : Quality=2
>> - Derived from very old map Quality=3
>> - VMAP Quality=4
>> .....
>
>I'd actually say that very old maps (NPE) are better quality than
>Landsat. Given a choice of the two for a feature I knew still existed
>(e.g. lake,
>country lane) I'd use the NPE.
>
>Nick
>
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