[Talk-ca] Importing buildings in Canada

Daniel @jfd553 jfd553 at hotmail.com
Fri Jan 3 20:40:54 UTC 2020

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.
[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!
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