Mathare Building import

From Map Kibera

(Difference between revisions)
Jump to: navigation, search
(Tagging scheme)
Line 57: Line 57:
! scope="col" | Shapefile  
! scope="col" | Shapefile  
! scope="col" | OSM
! scope="col" | OSM
 +
|-
 +
| id
 +
| ignored, since not used in shapefiles
|-
|-
| designated=*  
| designated=*  

Revision as of 11:49, 16 May 2011

In this page, I (Sébastien) will describe how to import the data of the building extraction of Mathare that we've got from AAAS. See Primož's blog for the initial analysis.


Contents

Changing the SRS

The data was made available as shapefiles in WGS84 Web Mercator Auxiliary Sphere projection, which is known as Web Mercator, spherical Mercator, Google Mercator and is referenced by various ids: EPSG:900913, EPSG:3785 (obsolete), EPSG:3857, ESRI WKID 102100... In QGIS, it's known as Google Mercator, EPSG:900913, ID 3644.

Before working with the data, we need to change the Spatial Reference System (SRS) to be compatible with OSM, that is moving to non-projected WGS84.
In QGIS: menu Vector -> Data Management Tools -> Export to new projection. Then choose the output CRS to be WGS84, EPSG:4326, ID 3452.
The two files that we're going to import are GroundTruthed-wgs84.shp and BuildingsOnTop-wgs84.shp

Setup PostGIS database

PostGIS is relatively easy to install on Ubuntu. I might describe it further later. Anyway, the current environment is PostgreSQL 8.4, PostGIS 1.5.
I found this script from GeoDjango very convenient to create a postgis db template:

$ ./create_template_postgis-debian.sh
$ createdb -T template_postgis mathare

Import the shapefiles to the db

Each shapefile will be imported a separate table with shp2pgsql:

shp2pgsql -s 4326 -c -g the_geom -I -S -W UTF-8 -N skip Groundtruthed-wgs84.shp | psql mathare
shp2pgsql -s 4326 -c -g the_geom -I -S -W UTF-8 -N skip BuildingsOnTop-wgs84.shp | psql mathare

Import OSM data to the db

In this case, we're only interested in buildings from OSM.

Get fresh data from OSM, keep only ways tagged with 'building' and their associated ways:

osmosis \
--read-api left=36.8425655 right=36.881876 top=-1.2480512 bottom=-1.2696752999999998 outPipe.0=1 \
--way-key keyList=building inPipe.0=1 outPipe.0=2 \
--used-node inPipe.0=2 outPipe.0=3 \
--write-xml file=mathare_building.osm inPipe.0=3

Import it to the database:

osm2pgsql -c -d mathare -l -s mathare_building.osm

The following tables will be created:

  • planet_osm_line
  • planet_osm_nodes
  • planet_osm_point
  • planet_osm_polygon
  • planet_osm_rels
  • planet_osm_roads
  • planet_osm_ways

Find collisions between OSM and the import candidate

Very few buildings have already been mapped in OSM in Mathare (139 as of 2011.05.09). Yet, we should be careful whilst importing the dataset from aerial imagery extraction. While we can discard the geometry of the existing buildings, we shall preserve their attributes.

Basically, we shall check for geometries that overlaps and apply the attributes of the OSM element to the imported one.

Note: I can see two tricky corner cases, which I don't need to worry about right now since this import does not feature them. However, overlooking these issues in another import will likely result in a loss of data:

  • nodes of the buildings bear attributes
  • multipolygons

We can find the overlapping elements with this query:

mathare#= select g.gid, o.osm_id, from "groundtruthed-wgs84" as g, planet_osm_polygon as o where ST_Intersects(g.the_geom, o.way) ORDER BY g.gid;

Tagging scheme

Shapefile OSM
id ignored, since not used in shapefiles
designated=* see table below
type=* building:type=*
name=* name=*
Designated
Shapefile Designated=* OSM
Toilet building=yes
amenity=toilet
NGO/CBO building=office
office=ngo
Administration building=office
office=government
CBO building=office
office=cbo
Self-Help Group
building=yes
note=Self-Help Group
fixme=Find an appriopriate tag for "Self help group"
Medical Clinic building=yes
amenity=hospital
health_facility:type=medical_clinic
Business building=commercial
Petrol Station building=yes
amenity=fuel
motor=yes
House building=residential
Apartment Building building=apartments
Hardware Shop building=commercial
shop=hardware
Church building=church
amenity=place_of_worship
religion=christian
Bar building=yes
amenity=bar
Bathroom building=yes
amenity=shower
Private Toilet building=yes
amenity=toilet
access=private
fixme=mathare preset scheme
Garage building=garage
Public Toilet building=yes
amenity=toilet
access=public
Market Place

building=yes
amenity=market_place

Cyber Cafe building=yes
amenity=cyber_cafe

Generate the OSM data to be imported

Personal tools