[HOT] Data Quality comments

[John Whelan]

I notein weeklyosmthat ngumenawesamson has made some comments about data 
quality in HOT mapping.

I've just finished going through Ghana and doing a bit of clean up.  I 
tend to map in Africa these days and often end up cleaning up after HOT 
mappers and I've been around a long time.

I think we need to split the mapping into armchair from imagery and on 
the ground.

A couple of comments on the armchair side.

First buildings, from imagery realistically I think all you can do is 
draw a building and say building=yes.  I see untagged building outlines, 
I see them labelled house, with levels, I see them labelled as layer -1 
etc. The NGOs aren't really interested in knowing exactly where the 
building is to the nearest centimeter they're more interested in knowing 
it exists so they can estimate population.  Adding levels from imagery 
just contaminates the map.

Second highways, realistically in Africa we need to map highways between 
settlements or between settlements and a highway and these can be 
labelled unclassified.  Let the locals sort out any other classification 
including paths etc.  This allows routing.  One highway connection per 
settlement at least provides a route to it.  Let the locals add more, it 
will help them feel as if they own the map.

Personally I'd ignore paths and tracks going to fields.  They tend to 
clutter the map and take up limited mapping resources.  They only other 
highway type I'd map in HOT would be highway=residential.

It also means that in JOSM you can just draw in the highways without 
tags.  Then JOSM validation will select them as being untagged.  Add 
highway unclassified and you're done.

I think you need to understand a bit about your target audience before 
planning your project.  Sometime ago I worked with a group of six 
American university geography students.  Their lecturer gave them the 
task of mapping one tile each.  I was validating on the project and gave 
quite a lot of feedback about four times a day.  Only one student 
completely mapped a tile and that took two weeks.  The others dropped 
out after a week.  The lecturer's comment was she hadn't realised it was 
so complex and wouldn't have asked them to do it if she had realised it 
was so much work.  None of those six mappers mapped again in OSM.

The instructions on that project needed to be fleshed out.  You needed 
to go to other places to find out exactly how something should be tagged 
and the project asked for everything to be mapped.

My expectation is an American University student should have a good 
command of English.  Realistically HOT Mappers first language will not 
be English so instructions need to be simple.

Also HOT projects tend to appeal to high self monitors, ( think of a 
group of people who go out on a Friday night and together do crazy 
things because it's fun.)  They don't have time or interest to read 
boring instructions they just want to map so remove their choices as 
much as possible and make the instructions simple such as:

Raw satellite imagery is accurate to 60 meters.  To get better accuracy 
we align it to existing buildings etc.  However expect the different 
imagery to be aligned slightly differently and when you map a building 
don't place it across a highway. Put it to the side even if the imagery 
says put it in the middle of the highway.

For buildings I and other experienced mappers running mapathons have 
found if you give them JOSM and the buildings_tool plugin you get a lot 
more buildings mapped by your mappers and they are more accurately 
drawn. It takes two or three mouse clicks using the buildings_tool 
compared to five or more in iD. I have yet to see a validator invalidate 
a building mapped with this tool. iD gives too many choices of tags.

JOSM using Microsoft's OpenJDK is fine and is simple to use for new 
mappers. It does take some planning though.  Have your mappers install 
it before arriving.  Yes it takes bandwidth but on a laptop you can set 
up the laptop to connect to the internet at off peak hours.

Bandwidth for a mapathon can be a problem.  Technically the image tiles 
can be cached on a local server.  A Raspberry Pi running SAMBA and using 
an SSD works well. However one laptop can be used as a server to the 
other machines.  I'll leave it to HOT's technical team to sort out the 
details and create a set of clear instructions.  Just remember JOSM 
holds the data locally and it can be uploaded at a later date so 
technically you could download the tile beforehand into JOSM, work with 
off line imagery and uploaded at a later date.  The whole mapathon need 
not be connected online which could be useful in places with poor or 
expensive internet connections.

On the ground mapping is different and needs a different approach.

Cheerio John


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