[mkgmap-dev] nearby-POI: What to do with several POI arranged on a circle?

Thu May 7 19:58:53 BST 2020

I had a few more thoughts on the multiple POI case. 

There seem to be three scenarios that possibly need handling in different
ways:

1. a cluster of POIS that are incorrectly tagged (e.g. every building in a
college is tagged as amenity=college). In this case if there are more than 2
POIS I would ideally like a higher severity level of message to be used so
that I can easily identify the problem and fix it in OSM.
2. a cluster of POIS that are correctly tagged, but I don't need to see
every one (e.g. bus stops within a bus station). In this case I would like
to reduce the bus stops to a single one (or even better if there is a bus
station tagged, remove all the bus stops).
3. a cluster of tags where I am happy to see several of them, but don't
necessarily need them all (e.g. with park benches it is useful to see there
are several, but it's not necessary to see every individual seat).

Regarding the position algorithm, I suggest the merge-at-mid-point option
only operates when there are two POIS. With multiple POIS just choose the
nearest POI to the centre and delete the others for both delete-poi and
merge-at-mid-point. That would solve any problems of it ending up in the
middle of a lake when used on objects surrounding the lake. Or we could just
drop merge-at-mid-point if it is too tricky to come up with something that
doesn't have unwanted side effects.

Regards,
Mike

-----Original Message-----
From: Mike Baggaley [mailto:mike at tvage.co.uk] 
Sent: 07 May 2020 16:10
To: 'mkgmap-dev at lists.mkgmap.org.uk' <mkgmap-dev at lists.mkgmap.org.uk>
Subject: RE: [mkgmap-dev] nearby-POI: What to do with several POI arranged
on a circle?

Hi Gerd, finally managed to try your updated code and it looks like a great
improvement over my starting point, which whilst it did work in some places,
in others it seemed to have no effect. The barrage of extra deletions also
pointed me to a problem in my points file which was creating a POI to
display the name of  power generators without any icon. Originally this just
showed big generators, but as there are now lots of solar panels on people's
houses mapped it has been adding thousands of POIS with no icon and no name,
which wasn't a great deal of use!

Regarding the multiple POI case, it really depends on what type of POI it
is. The logs pointed me to a tourist destination where every holiday home
had been tagged as a hotel - for that one I would ideally want just a single
point left, and preferably the severity of the message increased, so that it
is easy to pick out and go and fix at source. For ones like a line of park
benches, I'd be perfectly happy with them only being considered in pairs
(just thinning the POIs so that you can still see there are plenty of
seats).

Regards,
Mike

-----Original Message-----
From: Gerd Petermann [mailto:gpetermann_muenchen at hotmail.com] 
Sent: 07 May 2020 15:04
To: mkgmap-dev at lists.mkgmap.org.uk
Subject: Re: [mkgmap-dev] nearby-POI: What to do with several POI arranged
on a circle?

Hi Randolph,

I try to find out what the result should be when more than two POI are
involved. They may be arranged in a straight line or in a circle or whatever
random order.
At the moment I tend to follow Tickers suggestion and remove the option to
keep it simple.

Gerd

________________________________________
Von: mkgmap-dev <mkgmap-dev-bounces at lists.mkgmap.org.uk> im Auftrag von
Randolph J. Herber <army.bronze.star at gmail.com>
Gesendet: Donnerstag, 7. Mai 2020 15:21
An: mkgmap-dev at lists.mkgmap.org.uk
Betreff: Re: [mkgmap-dev] nearby-POI: What to do with several POI arranged
on a circle?

Perhaps averaging the latitude and longitude coordinates of all the points
separately would work. That is not quite the same as meet in the middle. If
the points are evenly spaced around the edge of a circle, even though the
latitude and longitude scalings (except at the equator) are not the same.
What I believe you are trying to do is minimize the total distance from the
selected point to the collection of points.

Randolph J. Herber

On 5/7/2020 2:42 AM, Gerd Petermann wrote:

Hi all,

while doing further tests I've found this edge case (see attached file)
Draw a circle with a diameter of ~56m and tag all nodes amenity=bank.
The default style has
amenity=bank [0x2f06 resolution 24]

With --nearby-poi-rules=0x2f06:30:merge-at-mid-point one might expect a
single POI in the middle of the circle, but the result is different:
Since there is no POI near the center of the circle the current algo finds 4
groups, all close to the edge of the circle.

I think for merge-at-mid-point I need a different algo. I'm looking at
https://en.wikipedia.org/wiki/Cluster_analysis now.

Gerd

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