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During validation on HOT’s Tasking Manager giving feedback to mappers is crucial to enable improvement and encourage them to continue mapping. Beginners make the most errors, catching them as early as possible to provide constructive feedback can limit the quantity of errors and steer them in the right direction. This can lead to an overall increase in data quality.

I’ve outlined some of the most frequent feedback that I give to mappers while validating projects that involve mapping buildings as well as explaining the importance of intervening. Please feel free to use these comments in your own validation efforts on HOT’s Tasking Manager.

Issue Feedback Comment Importance
Unsquared buildings Well mapped but please square your buildings after tagging them by pressing ‘q’ on your keyboard. Keep up the mapping - many thanks! When we are ‘free-hand’ drawing buildings it is impossible to avoid creating corners that are irregular (not right angled). In reality most buildings do have right angled corners and we need our mapping to reflect that. Accurate building footprints are important because building sizes are often used to estimate population sizes for an area to conduct humanitarian activities.
Drawing several buildings as one building. Thanks for mapping. Next time, you can improve by avoiding mapping several buildings as one building. Every building must be traced separately. Zoom in as close as possible to see the outline of the buildings, hit each corner of the building carefully. Look for shadows, open ground, changes in roof color and vegetation to differentiate buildings. The humanitarian teams use the size of the buildings to estimate the number of people living there. This helps them to work out the approximate total population in the area and how much equipment or medication to bring with them. Buildings that are too big do not fit in with these calculations and lead to inaccuracies in the population count.
Overlapping buildings and highways To ensure buildings/highways do not cross or connect, zoom right in and carefully place the building corners. If a road overlaps a building you should feel free to adjust the position of the road before mapping the building, thank you for mapping. Road data is used in sat navs by humanitarian workers. A sat nav will think there is a dead end if it enters a building. This could add hours to a journey.
Using a different imagery instead of the imagery as instructed Good work, thank you for mapping. The project instructions encourage switching different imagery so as to catch all the buildings in this location. In this project , you can also switch to Bing imagery as well. It has a lot of unmapped buildings. This avoids missing out many buildings and other features since in most cases project managers select the most updated imageries.
Duplicate mapping Thanks for mapping. You might need to zoom right in to clearly see if there are existing buildings that have already been mapped before you start adding buildings. Task squares on the Tasking Manager will automatically unlock after 2 hours, which opens them up for other mappers to work on. If you are continuing to map after two hours, then lock the square a second time, to avoid double drawing of features by different people. It is important to have all the buildings mapped to enable appropriate allocation of resources during a crisis.
Building sharing nodes Great work, thank you for your mapping! To improve for next time, please zoom in close enough when tracing to avoid buildings sharing nodes or else overlapping. If buildings need to be adjusted or improved later on, it is much harder to do that if they share nodes with nearby features.
Wrong Tag Eg. Building Tagged as Landuse = residential, Area= yes Thanks for your mapping! I noticed you tagged a few buildings as residential areas. Next time, remember all Buildings should be tagged as “Building”. When using satellite imagery to map it is best to assume that all the buildings we see are generic buildings rather than attempting to define their specific purpose - this is done later on by field mappers. It is important for the local mappers/ field mappers to know what they are approaching, wrongly tagged features can misguide them and also waste a lot of time.
Spaces, trees, rocks, cars and even elephants(!) mapped as buildings
Thank you for mapping! Please, zoom in and out in order to clearly see if an object is a building or not. Look for a color difference between the roof and ground, shadows and regular geometric shapes (e.g. rectangles or circles).You can also use different imagery to compare for clarity.
Local mappers / aid workers could waste a lot of time traveling to remote rocks or trees to find no buildings, so it is important to have correctly mapped objects to avoid misguidance.
Untagged features Thanks for your help! Please make sure to identify the objects that you are mapping. Several of them are untagged. Make sure to select the tag from the left hand side of your editor after you trace the object. It is important for the local mappers to know what they are approaching. An untagged rectangle could be anything (field? forest?). If local mappers were looking to add information about buildings they could waste a lot of time traveling to unidentified rectangular objects.
Unmapped task, marked as “DONE” Thank you for your mapping, this task was marked as done, however it had several buildings which still needed to be mapped. Next time please check your task very carefully before submitting it as complete, ensuring to map all the buildings. You can choose to unlock the task for someone else to continue mapping by selecting NO before submitting the task. It is important to have all the buildings mapped to enable appropriate allocation of resources during a crisis or to plan humanitarian activities effectively.
Other features mapped in project when not asked to Well mapped but remember that we are only mapping buildings in this project. This is detailed in the project instructions. Keep up the good work! Humanitarian organizations set the features to be mapped based on their priorities - any time spent mapping other features might slow down the completion of their projects.


Comment from Chisom on 25 April 2022 at 11:20


Comment from jwalseth on 25 April 2022 at 19:00

This is excellent. I hope you don’t mind if I use your suggested comments verbatim!

Comment from Becky Candy on 28 April 2022 at 11:25

Please feel free to use these comments in your own validation efforts on HOT’s Tasking Manager, thank you

Comment from janabau on 29 April 2022 at 14:06

Becky, great effort at cataloguing most common points to improve and sharing agood practice. To the Buildings sharing nodes, I usually also recommend (more so at mapathons than in comments) to the mappers to hold down alt when the corners are close, it prevents the nodes from snapping together. Will share a link to your entry from the Missing Maps, because this fits very well into our communications strategy.

Comment from Cyberjuan on 29 April 2022 at 15:08

Hi Becky! This is great! Can I translate your post to Spanish? In fact I have translated part of it as tweets and published under the @esopenstreetmap account but I would like to have it all translated, it would be useful for our validation community.

Comment from Becky Candy on 29 April 2022 at 15:13

Sure @Cyberjuan, are you slack??, you could also share it there on different channels and it could also be shared in WeeklyOSM , thanks

Comment from Becky Candy on 29 April 2022 at 15:14

@Janabua, awesome idea! thanks

Comment from charles chilufya on 1 May 2023 at 19:37

great writings @Becky

Comment from JFL-75 on 27 July 2023 at 17:41

Thanks lot Becky !

Comment from abdul_4ts on 21 October 2023 at 20:08

Awesome, worth sharing. Thanks @Becky

Comment from JohnW11 on 7 February 2024 at 11:17

very useful - thanks

Comment from vaston02 on 3 June 2024 at 09:24

This is very helpful. Thanks Becky.

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