[Talk-ca] Importing buildings in Canada

Fri Jan 3 22:09:10 UTC 2020

Thank you, Daniel, for advancing on the import! I agree with most of 
what you said below (I will reply to some details on the wiki page).

I also agree to separate the different sources as I have great 
reservations about using Microsoft's building footprints for an import 
into OSM (the outlines are simply not good enough in their topology).

Let's get this going,
Tim

On 2020-01-03 15:40, Daniel @jfd553 wrote:
Bonjour groupe, mes excuses pour ce très long courriel !-)

I have reviewed everything that has been written on the ODB import (aka
Canada Building Import) in Talk-ca and the wiki. I proposed changes to
some wiki pages (via talk tabs) to ease the discussions about this
import and the following. Now, in order to restart the import, here are
some thoughts and a proposal on how to proceed to complete the task.

*1. Issues with the ODB Data Import*

Many concerns were raised about the import. One major concern was to
obtain local communities’ buy-in in the Canadian context. Another
concern was to improve the quality of the data prior the import. The
following paragraphs intend to clear most of these concerns.

*1.1. Which data import project?*

According to the import guidelines (steps 3 & 4), a data import
explicitly refers to a single data source (ODB in our case). Discussions
about the availability and quality of Microsoft or ESRI data, while
interesting, are not relevant as they should be dealt with as other
import projects.

*1.2. What has been imported so far?*

According to what I found [1], the ODB import is completed for 21
municipalities. These imports seem to have kept OSM content’s history,
at least for the samples checked, but many problems were found. In some
case, the imports brought swimming pools in OSM because they were
included in the dataset (e.g. Moncton). In other cases, importing
buildings with accurate locations (XY) over content mapped from less
accurate imagery resulted in buildings that now overlap the street
network (e.g. Squamish). It means that all these 21 imports need to be
carefully re-examined and corrected as required.

For 12 other municipalities, the import is partial, either suspended as
requested, or because previous imports had already provided most of the
buildings (often from the same municipal provider). That said the import
will definitely improve OSM accuracy and completeness if done properly.

*2. How should ODB Data be imported?*

I will copy the following paragraphs in the “Canada Building Import”
wiki page [3] for a detailed discussion…

Since the data (ODB, OSM and imagery) differ from one municipality to
another, there can be no imports at the national or provincial level. We
have to work on a municipal basis and make sure to identify all the
problems and the corrective measures to apply when dealing with issues
like those I identified [1].

*2.1 Importing Locally*

According to the import guidelines (step 2), we must not import the data
without local buy-in. However, and contrarily to some European country,
there is usually no such thing as a local OSM community in each
municipality. However, we may find a few local mappers from time to
time. Working on a municipal basis should allow identifying these local
mappers before doing the import. I often use this tool [2] to identify
and contact local mappers. Once identified, I suggest that…

- We contact them to explain our intents by referring to appropriate
wiki pages.

- We wait a week or two to let them respond nothing, that they have
concerns, or wish to help.

- Without negative answers we could proceed to the import.

I first suggest that when a contributor wishes to import ODB for a given
municipality, he first identifies himself as responsible for the import
(we need to create an entry for each municipality somewhere in the
wiki). He can then contact local mappers, as explain above, and go ahead
with the import once everything settled. For those who already made the
import, I suggest that they review their work since many issues were
detected with some of these imports.

Since there are only a few local OSM communities in Canada, and because
Canada is large, I suggest not limiting the import of a given
municipality to the people of the concerned province or region.

*2.2 Pre-processing*

Once local mappers have agreed, some pre-processing can be done if
required.

A few months ago, I developed a tool that could be used to process the
data [4]. Concerns were raised because the application was developed
using proprietary software. So I documented the whole process and
algorithms in order to see courageous coders converting it in open
source software. In the meantime, and as long as I have access to an FME
licence, I could process the data, when necessary, prior to make it
available through the task manager.

Proposed pre-processing [4] includes:

- Reading of original ODB data,

- Removal of near collinear nodes (simplification),

- Orthogonalization of buildings (for corners having near right angles),

- Tagging of building footprints,

- Providing files in OSM format.

/Proposed tagging:/ In addition to the tags produced by the
orthogonalization process [4] and the source tag (source
<https://wiki.openstreetmap.org/wiki/Key:source>=Statistics Canada -
Open Building Database), the name of the Census Subdivision provided in
ODB data [5] is used to add the addr:city tag to each building.

The pre-processing requires parameters that are specific to the data to
process. These parameters were estimated on a municipal basis using
actual ODB data. The processing time increases exponentially according
to the number of buildings so, it may take a couple of days before the
data is available for a given municipality. Currently, the proposed
pre-processing does not convert terrace buildings into individual houses
nor it tags topological errors.

*2.3. Import Process*

After the local mappers, if any, agreed to the import, the
pre-processing completed when required, we can proceed to the import.

1- Do not bulk import the data! Always use the task manager
(http://tasks.osmcanada.ca/). Select and open a task square in JOSM. If
it’s too big (e.g. too much work or request is too big to load in JOSM),
go back to the task manager and split the task into smaller squares.

2- Load imagery layer (Bing or ESRI World Imagery) and align the imagery
with ODB data (i.e. create a new image offset) if necessary because,
unless proven otherwise, ODB should be more accurate (XY) than most
available images especially in hilly areas.

3- Align the existing OSM content to the image (i.e. after the new
offset is applied) if required.

4- Currently step 2 and following as described in the wiki [2]. I
suggest merging the Conflation section [6] here and reviewing everything
to take into account the current proposal.

*References*

[1] https://wiki.openstreetmap.org/wiki/The_Open_Database_of_Buildings__

[2]
https://wiki.openstreetmap.org/wiki/Canada_Building_Import#Import_process

[3] http://resultmaps.neis-one.org(“Overview of OpenStreetMap
Contributors aka who’s around me?”)

[4] https://github.com/jfd553/OrthogonalizingBuildingFootprint__

[5]
https://www150.statcan.gc.ca/n1/pub/92-195-x/2011001/geo/csd-sdr/csd-sdr-eng.htm

[6] https://wiki.openstreetmap.org/wiki/Canada_Building_Import#Conflation

Let’s move ahead!

Daniel

_______________________________________________
Talk-ca mailing list
Talk-ca at openstreetmap.org
https://lists.openstreetmap.org/listinfo/talk-ca