[mkgmap-dev] Issues with --housenumbers

Thu Jan 8 11:09:10 GMT 2015

Hi all,

during the last weeks I tried to improve the --housenumber option.

First of all: In most cases the existing code works quite well,
but in many special cases it fails.

I did not yet find a good solution, so I start to describe the problems
with the existing code.

1) No support for random house numbers. 
In some areas there is no obvious order in house numbers.
Nevertheless the current code in mkgmap always produces
house number data that assumes that the numbers are either
in ascending or descending order. We would need new data
structures to support this, or at least ignore random housenumbers.
The effect of the current code is that MapSource shows multiple 
possible places when you enter a road and a housenumber,
and maybe none of the places is correct.

2) No plausibility check is done.
The current code assigns a house (number element) to the closest road segment.
It orders the houses by sorting these closest points.
a) This doesn't work very well when multiple houses lie at the end of a road.
As an effect, a house with number 12 maybe assigned to the left side of a road
containing only odd numbers (or vice versa), or 
b) It also often fails when multiple houses are connected to the road with  an unnamed
service road. In many areas you have a group with odd numbers 1-9 followed by another group
with numbers 11-17. Depending on the position of the houses, the calculated order might be 
5,7,9,1,17,15,13,11 which results in an interval 5..11 instead of 1..17.
The result also depends on whether the service road is in the map or not .
c) In some areas, different road objects are created with the same road name, e.g. when 
a p-shaped road is split or the road forms some kind of grid like this a #  sign.
In such an area it is likely that some houses are assigned to the wrong (part of a )road.
d) In some cases we might be able to detect wrong OSM data as such and print
a corresponding message.

Both points 1) and 2) are correlated. Without a plausibilty check we cannot detect
the random house number case, so I think it is an interesting problem
of pattern recognition. The human brain is very good with that, but it is difficult to find
a quick and good algo for it.

Gerd

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