[mkgmap-dev] Explanation of the is_in function

Thu May 21 12:15:40 BST 2020

Hi Ticker,

the error is in the formular
distSqrd = lonDifSqrd * latDifSqrd / (lonDifSqrd + latDifSqrd);

If one of the values is small (< 1) and the other is big, the result is far from being correct.
What should this formular express?

Gerd

________________________________________
Von: mkgmap-dev <mkgmap-dev-bounces at lists.mkgmap.org.uk> im Auftrag von Ticker Berkin <rwb-mkgmap at jagit.co.uk>
Gesendet: Donnerstag, 21. Mai 2020 13:07
An: Development list for mkgmap
Betreff: Re: [mkgmap-dev] Explanation of the is_in function

Hi Gerd

Looking at your example, the end point of the line is inside the area,
but only by about 1 metre. This is the within the tolerance of a point
being considered ON a line with the current algo/EPS. All the other
points on the line ON or OUT of the area.

Changing the tolerance and/or the type of distance calculation might
change the overall answer to be IN & ON & OUT, but consider if the end
point of the line was exactly on the edge; the answer would be wrong.

The problem really is in the line following shape edge algo that should
spot that the line has diverged from the edge into the area.

I haven't looked in detail yet at Joris's failures, but did you notice
if they were of this pattern, where the line spans the area in one
bound?

In your change to isPointInShape, you've lost the general case (line
not horizontal or vertical):
  distSqrd = lonDifSqrd * latDifSqrd / (lonDifSqrd + latDifSqrd);

Regarding the comment about a small error area on the outside of the
polygon vertex where a point will report ON but shouldn't - this
area is very small but does exist and so the the comment is valid.

Ticker

On Wed, 2020-05-20 at 08:53 +0000, Gerd Petermann wrote:
> Hi Ticker
>
> the problem is in mkgmap. I can reproduce it with the attached simple
> example.
>
> I think the bug is in the distance calculation in isPointInShape(),
> see attached patch. It returned ON for a point which is clearly
> inside.
> I've also changed the way how test points are calculated so that the
> rounding tolerances are less likely to produce a problem.
> I don't understand the meaning of the comment
>                                         // there is a small area
> between the square EPS_HP*2 and the circle within, where, if polygon
> vertex and
>                                         // segments are the other
> side, it might still be calculated as ON.
> so maybe my patch makes things worse in other situations?
> My understanding is that we can use a² + b² = c² , so maybe the
> comment can be removed as well?
>
> Gerd
>
> ________________________________________
> Von: mkgmap-dev <mkgmap-dev-bounces at lists.mkgmap.org.uk> im Auftrag
> von Joris Bo <jorisbo at hotmail.com>
> Gesendet: Dienstag, 19. Mai 2020 21:12
> An: Development list for mkgmap
> Betreff: Re: [mkgmap-dev] Explanation of the is_in function
>
> Hi Ticker,
>
> Thx for the update, off course no problem.
>
> Gr Joris
>
>
>
> -----Oorspronkelijk bericht-----
> Van: mkgmap-dev <mkgmap-dev-bounces at lists.mkgmap.org.uk> Namens
> Ticker Berkin
> Verzonden: dinsdag 19 mei 2020 18:57
> Aan: Development list for mkgmap <mkgmap-dev at lists.mkgmap.org.uk>
> Onderwerp: Re: [mkgmap-dev] Explanation of the is_in function
>
> Hi Joris
>
> I can't do anything in the next few days to investigate this but I'll
> have a look when I can.
>
> Ticker
>
>
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