Third pass validation
Third pass validation is data quality control done by experienced mappers in areas that have been remotely mapped via the Tasking Manager (TM). It involves a relatively quick check across an entire project area once a project is no longer being mapped and validated - ideally the project is first 100% mapped and validated.
Why?
Third pass validation can pick up and fix issues common to TM projects caused by task edges - such as disconnected highways; highway classification changes from one task to the next; and un-mapped or duplicated buildings at task edges. Third pass validation also allows another chance to improve overall data quality and provide feedback to mappers and validators (eg. via changeset discussions).
Scope?
In general, third pass validation focuses on work done on the TM project, not the map as a whole. If the TM project is a buildings project, the focus will be on fixing anything to do with buildings (including their interaction with other mapping). If other problems are identified, they can be fixed too, but the goal is not to fix pre-existing problems caused outside the TM project.
Who is in charge?
The project manager is responsible for third pass validation of every TM project they manage. Ideally the project is 100% mapped and validated before starting the third pass validation, but even if it has not been fully mapped or validated, a third pass validation should still be done. It is expected from the project manager to do third pass validation themselves, or that they coordinate with an experienced mapper (or multiple experienced mappers) who can do third pass validation on their behalf. Once third pass validation is done the project manager should archive the project and is suggested to note for the awareness of mappers in the project questions and comments section that third pass validation is completed. Volunteers for third pass validation are often much welcomed by TM project managers. If you are interested in doing third pass validation of a project, before you start, please reach out to the project manager.
Suggested workflow
This is a suggested workflow for third pass validation. Other workflows are possible depending on the preferences of the individual doing the third pass validation.
- On the TM, go to the project that you want to do third pass validation on. Check if all tasks are mapped and validated. If not, it is best to stop third pass validation and to continue first the regular mapping and validation of all the tasks of the project.
- Scroll to the bottom of the project, download the task grid and open it in JOSM.
- Select all tasks (CTRL+A) and download the OSM data within them in a new layer. This can be done with the download along functionality (File > Download along). If the project is too large, you can start with a part of the task grid.
- Open the same imagery layer as was used in the TM project.
- Run the JOSM validator, and fix the errors you can fix. You may want to activate this validation setting.
- If you have a lot of data in JOSM it might be handy to create a filter to hide certain data. This will ensure that JOSM does not crash or slow down too much, and it will as well give you a clearer view on the data that is relevant. Best is to hide features that are not part of the project instructions and thus less relevant to fix. You can learn more on how to use filters here.
- HINT: You can also enable an inverted filter by checking I. This means that in a buildings filter it will show only the buildings and hide all other features instead of the other way around.
- For building projects:
- Run the duplicate buildings functionality of the mapathoner plugin and fix those. Add the duplicated buildings to your todo list which will allow you to correct them one by one.
- Run the non orthogonal building functionality of the mapathoner plugin and fix those. Do not square (press Q) the buildings in bulk if you have not visually checked them. It is better to use the todo list which will allow you to correct them one by one. You can also use the search tool (Ctrl+F) to avoid selecting more complex buildings:
- Enter building: yes nodes:4 and this will select all buildings with corners (avoiding circular buildings)
- Enter building: yes nodes:-10 and this will select all buildings with more than 4 corners (avoiding circular buildings)
- Select all buildings and check if there are unusual tags used, such as building=home, or if people have given strange names to features, such as mapathon tags (e.g. #awesomecompany2024) or descriptive names (e.g. edificio, ronde hut - if you don't recognize the language it's helpful to run the name through a translator and see if it comes out as "building" or similar.) You can eg. select on: building=* / name=* . Unusual tags can also be detected via paint styles - see point 10 below.
- For roads projects:
- Check that all roads are connected to the road network by selecting a road and pressing SHIFT+CTRL+E.
- Follow local tagging conventions. Refer to OSM Wiki Tagging by country for more details. If not available, always follow the general road clasification. It is often the case that they are not consistent across the whole area. Select highway=primary / highway=secondary / etc. and see if they form a network.
- For residential area projects:
- Usually a residential area is only tagged as landuse=residential, please check if they have other tags that might not be correct. Such as place=* / name=* . Those would be rather added to a point in the middle of the residential area, see here.
- Visually check if residential areas encompass all the buildings they should. You can use the following as guidance:
- if there is less than 50m between buildings, they should be the same residential area.
- if there is between 50m to 150m between buildings, it will depend on the context and it is your decision.
- if there is more than 150m between buildings, they usually are not part of the same residential area. This issue can be caused by HOT TM task edges, a mapper has only mapped the residential landuse within their task, but the residential landuse was larger than their task.
- HINT: If you want to be really thorough you can select all landuse=residential and use the todo list which will allow you to check them one by one.
- Visually check if there is residential landuse missing and there are clusters of buildings not within residential areas. Sometimes this is clearer to see if you switch off the imagery background.
- Use paint styles to visually discover and fix errors that have not come up yet visually:
- The Missing Maps/Youthmappers paint style, see here how it works.
- Other paint styles that maybe useful are: "Apple Paint Style", and "LayerChecker"
- If you have used filters, remove them and run a final check with the JOSM validator (you might have made errors during your validation) and fix everything that needs it!
- Please remember to update your OSM data in JOSM (CTRL+U) before uploading to minimize the potential for data conflicts.
Of course do regularly upload the fixes and adjustments you did, best is to use a similar changeset comment as in the original TM project, just adapt it with what you actually edited.
Other useful resources
- Additional data quality assurance tools you could use too:
- Third pass validation was teached by Rebecca Chandiru during the 2023 HOT Data Quality internship, see here their resources:
- Resources for validation: