Hi Tomas,
The generalization I wrote about was just a crude basic generalization
of vector (building) data of OSM using a default tool of ESRI's ArcGIS.
The specific tool used (Simplify Polygon), has more advanced settings
than standard Douglas Peucker, but by itself does nothing really special
other than weeding out vertices / nodes. I just attempted to use it with
different tolerances to see what the results would be, and concluded the
resulting defects in building topology, were not worth the reduction in
file size.
Of course, if your city's buildings are far more detailed than the
average building in OSM, e.g. an import of official government data as
measured up to centimeter level using land surveying techniques, with
rather large vertex counts on average, I can imagine that even simple
techniques to generalize, may reduce vertex counts more than I achieved.
And also depends a lot on how much you personally tolerate artefacts...
(mine is low for buildings).
However, as to interesting stuff to read: our national Kadaster of the
Netherlands, is actually the very first and only national mapping
organization world wide, that has successfully managed to implement a
fully automated generalization work flow for generating 1:50k maps from
1:10k maps, including landuses, waterways, highways, but also
generalizing build-up areas and buildings. They used a range of
cartographic generalization tools from ArcGIS (that I didn't use...).
The results achieved by the Dutch Kadaster, closely mimic what Imagico
states as a more holistic approach to generalization, and are largely
truth to how cartographers traditionally manually generalized maps. In
fact, one of the key aspects of the workflow developed by the Dutch
Kadaster, was to mimic as closely as possible the inherent "rules" their
cartographers used and developed over decades or more than a century, to
"generalize" maps to smaller scales.
However, if you read the level of effort needed to achieve this (years
of development by a small team of employees / researchers, and a huge
tool chain build up), and the sheer processing power needed to do such a
sophisticated generalization, it is utterly clear you cannot do this in
real time. It is only worth the effort in organizations like the
national mapping agencies, where the ultimate gain of automatization to
fully replace manual conversion of topographic maps from one scale to
another, or to keep different workflows for different scale map series
(1:10k,1:25k,1:50k,1:100k,1:250k etc.) alive, far outweighs the effort
to develop such a generalization tool chain and workflow in the long run.
They now call this workflow "AUTOgen"
The Dutch Kadaster was actually awarded a prize by ESRI for this work.
See also this ArcNews bulletin (pages 19-21):
https://www.esri.com/~/media/Files/Pdfs/news/arcnews/winter1314/winter-2013-2014.pdf
Some links to this work by the Dutch Kadaster:
-
https://repository.tudelft.nl/islandora/object/uuid:12f0c152-958a-4688-a56d-5e30f7540a68/datastream/OBJ
-
https://www.kadaster.com/documents/33433/36597/An+overview+of+the+Dutch+approach+to+automatic+generalisation/6af54a07-3188-41db-81d2-fdc2e1d4094b
-
https://www.kadaster.com/documents/33433/36597/Feasibility+study+on+an+automated+generalisation/dbf664a7-160f-456d-9559-32263ef6793f
-
https://www.researchgate.net/publication/299455974_Automated_generalisation_in_production_at_Kadaster_NL
Links to English pages of Dutch Kadaster;
https://www.kadaster.com/international-consultancy
https://www.kadaster.com/automatic-generalisation
Other interesting information regarding buildings (LODs) from research
involving one the person also involved in the Kadaster work:
https://www.int-arch-photogramm-remote-sens-spatial-inf-sci.net/XXXVIII-4-C26/7/2012/isprsarchives-XXXVIII-4-C26-7-2012.pdf
(Note: I wasn't involved in any of this by the way, just know of this work)
Marco
Op 16-4-2018 om 19:23 schreef Tomas Straupis:
2018-04-16 19:34 GMT+03:00 Marco Boeringa wrote:
No, buildings are not the most interesting. I once generalized all buildings
in Denmark. It only reduced the storage by maybe 5%, at the high cost of
heavily distorting a large number of them. Most buildings in OSM are in fact
already in their most generalized state: just 4 nodes. Unless you think
triangles is a suitable representation ;-)
Interesting, what algorithm did you use?
I'm playing around in Vilnius which has urban houses, big block
houses, industrial zones and old town with lots of connected buildings
of very irregular shapes.
In Vilnius there are 54267 buildings tagged with 366979 vertexes.
Clustering them with distance of 5m gets 45810 objects (of course
with the same number of vertexes).
Removing buildings with area < 100 and having neighbours in < 500
meters I'm left with 28974 buildings with 299224 vertexes.
Simplification (amalgamating buildings in the cluster and trying to
remove edges < 20m) reduces the number of vertexes to 117108.
So this is much more than 5%.
There are still a lot of problems (no triangles:), but I do not
expect number of vertexes to rise considerably.
Even "dumb" generalisation (st_buffer+- with join=mitter) reduces
vertex count by ~25%.
Reducing storage/tile size is not the only/main purpose of generalisation.
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