How we imported Administrative Boundaries for Mexico from INEGI

The INEGI boundaries import project is focused on importing the data of the national, state, municipal and sub-municipal level divisions present in the MGN published by the INEGI in a community monitored process.

One of the current problems in OSM regarding Mexico’s data is the incompleteness of the administrative boundaries for municipalities. Municipalities are the second-level administrative division in Mexico, the first being the state. There are 2456 municipalities, including the ones in Mexico City which are also a second-level division just with a different name – delegations.

The main goal of this process is to enhance the current OSM administrative division coverage of Mexico with open data made available by the government at the end of 2014.

Import Process

The following steps describe the entire workflow we followed to import the boundary data.

  • Step 0 – Reprojection of INEGI dataset

Before any other step, the data released by INEGI has to be reprojected to WGS84 (EPSG:4326), from ITRF92, using QGIS, and saved as .shp file. An important thing to mention is that no simplification of the boundary geometries is considered whatsoever for this or any of the subsequent steps since the geometries are official government data.

  • Step 1 – Conversion to OSM data

Download the state boundary of interest relation from OSM and save it as an .osm file. In QGIS, using Vector > Research Tools > Select by Location, select the INEGI municipalities boundaries that are within the area of interest, in this case Quintana Roo state, and export the selection as .shp file.

Municipalities in Quintana Roo state.
Municipalities in Quintana Roo, as polygons.

The exported features will be polygons. In order to process them, they must be converted to lines in QGIS using the Polygons to Lines option, available in Vector > Geometry tools. Visually, the output will look the same as when the municipalities were polygons.

Municipalities in Quintana Roo, as lines.
Municipalities in Quintana Roo, as lines.

Using ogr2osm the .shp file containing the boundaries as lines is converted into an .osm file.

Before moving forward, the resulting .osm file has to be modified a bit. Using Notepad++, open the file and search and replace <nd ref=’ with <nd ref=’- and <node id=’  with <node id=’-, so the file will be with negative id.

The negative id is important because JOSM will know that this is new data, not yet added to the map.

Next, the .osm file can be converted to an .osm.pbf file using osmosis.

  • Step 2 – Processing

We load the .osm.pbf file from the previous step into an internal tool, called Mexico Split. The tool is designed to eliminate duplicate/overlapping ways by detaching them from their parent polygons and replacing them with a single common way of the two involved polygons.

mxsplit3
Detects overlapping ways and replaces them with a single common way.

Besides this main purpose, the tool also splits any resulting ways longer than 2000 segments in shorter ways, groups the ways in relationships according to the borders they define and adds some predefined tags to these ways and relations.

Tags added to relations:

type=boundary

INEGI:MUNID=<value_from_the_original_polygon>

name=<value_from_the_original_polygon>

Tags added to both ways and relations:

boundary=administrative

admin_level=6

source=INEGI, MGN 2014 v6.2

For example, data for Bacalar municipality contains the following information:

Bacalar municipality in Quintana Roo state.
Bacalar municipality in Quintana Roo state. (click for larger image)
  • Step 3 – Backup and metrics of existing OSM data

We took a backup of the current OSM data previous to the import of the regions that are going to be impacted, using Overpass API. Also, tag related metrics have been recorded – source, population, admin_center, admin_label, wikipedia etc. in order to have an overview of the newly added information.

  • Step 4 – Delete existing data from OSM and upload fresh data

In some cases the states already have some information regarding municipality boundaries (admin_level=6). These will be deleted, but before deletion we take a look at all the features and relations, to have a very good image of what we should put back in map data after the import.

Next, we upload the municipalities on a state by state basis.

  • Step 5 – Clean/verify the newly added data

This is a very important step because we verify the data that we’ve uploaded to make sure that there are no errors and manually re-link the admin_level=6 relations to the admin_level=4 boundaries, where required. Any other manual corrections are done at this step.

output_mSaeg1
An example of the newly added municipalities boundaries in Tabasco.

To ease the process of importing the municipality boundaries, we use the Mexico Import Map paint style for JOSM. It highlights the last node of every way, making it simple to see the length of every way.

Map styles - JOSM default vs. Mexico Import
Map styles – JOSM default vs. Mexico Import. (click for larger image)

The square node also has a certain degree of transparency, so we can see if there is a node under the node. To be able to work in a systematic way, it allows to quickly see duplicated nodes and see the difference between the admin_level=4 and admin_level=6.

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Author: mihai

Get in touch with me at twitter.com/ubermih

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