[mkgmap-dev] Documentation for the new Douglas-Peucker options in low-res-opt branch

Tue Jun 15 05:36:00 BST 2021

Hi Mike,

some answers:
- the unit is metre, it is scaled with this formular: maxErrorDistance = filterDistance * (1<< (24 - resolution)), so factor 4 for resolution 22, 8 for 21, 16 for 20 and so on.
- The default is 2.6 if the option is not given or given without a value. The --no prefix doesn't work, but the Douglas-Peucker filter can be switched off with a value of 0.
- I used different option names because I think --reduce-point-density is a bit misleading. I didn't think about changing the syntax of the existing option, but it would be an alternative.
- Why different option for polygons? See http://gis.19327.n8.nabble.com/Include-the-following-patches-into-trunk-Patch3-quot-reduce-point-density-polygon-quot-tc5327721.html

I really have no clue how one (Felix) determines his favorite values and it's not interesting for me as a software developer.

@Steve
Reg. online doc: I don't know if this is intended. I'd prefer to have separate docs for each branch, or that branch builds don't change the web page, whatever is easier.

Gerd

________________________________________
Von: mkgmap-dev <mkgmap-dev-bounces at lists.mkgmap.org.uk> im Auftrag von Mike Baggaley <mike at tvage.co.uk>
Gesendet: Montag, 14. Juni 2021 22:32
An: mkgmap-dev at lists.mkgmap.org.uk
Betreff: Re: [mkgmap-dev] Documentation for the new Douglas-Peucker options     in low-res-opt branch

Hi Gerd,

I think the original documentation for --reduce-point-density and
--reduce-point-density-polygon could do with some improvement. It also seems
bizarre to have a recommended value that is not the default. Is 2.6 the
default if --reduce-point-density is specified without a value, or is it
also the default if the option is not specified? Are the units metres? Is
the distance the same no matter what resolution is used, or does the
distance increase at lower resolution? If the former, wouldn't it be better
to increase by a factor of 1.414 at successive resolutions? Would this be
sufficient to not need to be able to specify individual values for
resolutions?

I'm not keen on having two very differently named options that basically
achieve the same aim and suggest that it would be better to simply extend
the existing --reduce-point-density options with
--reduce-point-density=value|resolution:value[,...] or even better
--reduce-point-density=value[,...] where the first value applies to the
first used resolution and so on, with the last value being scaled for any
further resolutions that have not had a value specified.

Is there a reason why polygons need different values than lines? Shouldn't
reduce-point-density-polygon default to the reduce-point-density value?

I note that although the documentation belongs to the low-res-branch, it is
showing up on the mkgmap command line web page.

Regards,
Mike

-----Original Message-----
From: Gerd Petermann [mailto:GPetermann_muenchen at hotmail.com]
Sent: 14 June 2021 07:43
To: mkgmap-dev at lists.mkgmap.org.uk
Subject: [mkgmap-dev] Documentation for the new Douglas-Peucker options in
low-res-opt branch

Hi all,

I've now added documentation for these new options, see:
https://www.mkgmap.org.uk/websvn/diff.php?repname=mkgmap&path=%2Fbranches%2F
low-res-opt%2Fresources%2Fhelp%2Fen%2Foptions&rev=4775&peg=4775

Is it clear enough?
I think the recommend value --reduce-point-density-polygon=8 is far too high
at low resolutions. Should this be changed?

Together with the new --improve-overview option this branch version can
produce much better results for the lower resolutions.

Gerd

_______________________________________________
mkgmap-dev mailing list
mkgmap-dev at lists.mkgmap.org.uk
https://www.mkgmap.org.uk/mailman/listinfo/mkgmap-dev