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<p class=MsoNormal>As the weather improves we are all going to be out and about
tracing roads for OSM. So there are various discussions, and work under way to
help find, prioritise and then fill the most important gaps in the map. Larger towns
and bigger areas that need attention are fairly well known, but experience suggests
that it can be more difficult to find smaller and more local places where a short
session could make a difference quickly.<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>So this is an attempt to help, at a bit more of a
micro-level – <a
href="http://www.reedhome.org.uk/Documents/osmembed.html?kml=KML/osmcategory.kml">http://www.reedhome.org.uk/Documents/osmembed.html?kml=KML/osmcategory.kml</a><o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>(to view the same thing on Google maps use this <a
href="http://maps.google.co.uk/maps?f=q&source=s_q&hl=en&geocode=&q=http:%2F%2Fwww.reedhome.org.uk%2FDocuments%2FKML%2Fosmcategory.kml&sll=51.525483,-0.746802&sspn=0.01287,0.033023&ie=UTF8&z=6">http://maps.google.co.uk/maps?f=q&source=s_q&hl=en&geocode=&q=http:%2F%2Fwww.reedhome.org.uk%2FDocuments%2FKML%2Fosmcategory.kml&sll=51.525483,-0.746802&sspn=0.01287,0.033023&ie=UTF8&z=6</a>
)<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>Broadly – green areas look as though they are already
fairly well covered, red areas look as though they would benefit from some attention,
and the rest are grey.<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>WARNINGS<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>This is very much a first cut, pre-beta, etc. We know there
are places missing, some gaps in the data, and we are aware that it
doesn’t always get things right (we think it does often enough to
be useful). <o:p></o:p></p>
<p class=MsoNormal>It really cannot tell how well an area is mapped from the
data. It can only try to find areas that look a bit thin - and some areas look
thin when they are not.<o:p></o:p></p>
<p class=MsoNormal>At the moment we are concentrating on roads, not the full
list of features in areas that are most thoroughly mapped.<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>As a result this sometimes flags up an area as
“poor” when in reality it has been perfectly well mapped. For example,
there are two areas near here that a lot of work has gone into, where the
classification is not right. So if your favourite areas shows up wrongly,
please don’t take it personally. <o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>Still, it seems to point in the right direction quite a lot
of the time, and it’s offered up on that basis. In my own area (which is
already fairly well covered) it has flagged up some towns that I already knew were
a bit thin, and another half dozen options that I can reach fairly easily, but didn’t
know about.<o:p></o:p></p>
<p class=MsoNormal>If it looks as though it will prove useful then future plans
will address some of the current limitations by refining the borders, filling
gaps, correcting errors in the underlying measures, and tweaking the arithmetic.
<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>However, some mis-classification will always be inevitable.
Read on to see why.<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>THIS IS HOW IT WORKS:<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>We start with a list of about 1,600 UK settlements, and a
figure for the population that lives there. Baring a few errors and omissions,
the settlements are the same ones that Cyclestreets uses for local areas - <a
href="http://www.cyclestreets.net/area/">http://www.cyclestreets.net/area/</a> <o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>We then try to find a boundary for each of these settlements.
This is based either on the local authority admin area where there is one, the
naptan pay-scale area if there is one, or if all else fails, a guesstimate of
how big the settlement must be based on population density. <o:p></o:p></p>
<p class=MsoNormal>(FWIW we already realise that some of the pre-defined areas
are too big, some of guesstimates are off-centre, or the wrong size, and the
guesstimates don’t work well on the coastline. But it’s a start,
and most settlements don’t look too far out.)<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>Where we know the actual length of roads in a settlement
from Department for Transport data we use that to classify the area, but this
only works for bigger towns and cities, so for the rest we are trying to figure
out how well they are covered without knowing the true length of roads on the
ground.<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>At the moment we do this using various ratios. Within each
settlement boundary we measure the length of roads in the OSM database, and
from that we calculate three measures: the length of roads per sq km (the road
density within the settlement), the length of road per head of population ,and
the proportion of roads that are major (primary, secondary and motorway).<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>The underlying hypothesis is that a thoroughly mapped area
should have a relatively high road density, plenty of road per head of
population, and a relatively low proportion of major roads (because it’s
the unclassified and residential roads that tend to be missing, not the major
roads). <o:p></o:p></p>
<p class=MsoNormal>However, there are anomalies - some areas are thinly
populated, some are at the intersection of a lot of major roads, some have more
tightly packed houses (so a high road density), while others have big gardens
(and hence a low road density). So inevitably our hypothesis sometimes breaks
down.<o:p></o:p></p>
<p class=MsoNormal>To avoid tripping over some of the more extreme cases, we
therefore highlight as “good” only those areas that fall into the
top quartile on at least two of the ratios, and to be classified as
“poor” we pick only those areas that fall into the bottom quartile
on at least two ratios. Everywhere else is in the middle.<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>That covers complete settlements. On top of that we have plotted
all the bigger residential areas (landuse = residential) where there
don’t seem to be many roads. We can’t know the population of these areas
so we just highlight those with a low road density (road length / area). If the
road density in a residential area is above the threshold, or if it is a very
small residential area (where the measures can be unreliable), then we just
ignore it. We have only plotted the bigger, low density residential areas.<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>One final caveat – the base data on which this is all
based was extracted several weeks ago, so it’s a little bit out of date.
An updated version will follow when time permits.<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>IN CONCLUSION<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>We have several ideas on how this can be improved, based on earlier
suggestions. We welcome more comments and ideas. <o:p></o:p></p>
<p class=MsoNormal>Even more importantly, we welcome more roads on the map.<o:p></o:p></p>
<p class=MsoNormal>So most of all we hope this helps members of the community
find some handy places where they can quickly make a difference by plugging a
few gaps. Remember that it’s all a bit hit-and-miss though. It’s probably
a good idea to check some other sources as well before rushing out with the
GPS.<o:p></o:p></p>
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