[Imports] Tanzania roads manual import

[Christoph Hormann]

These comments are based on the data made available now.  This covers 
only a small part of Tanzania which is mostly already quite well mapped 
and is fairly uncritical w.r.t. imagery (not much dense forest 
vegetation, no persistent cloud cover).  I am going to assume this is 
also what is going to be imported, if the actual import is supposed to 
cover other areas as well things might vary.

In general since a large portion of the roads in the data set are 
already in OSM in comparable or better quality it would make sense in 
terms of efficiency, avoidance of errors and ease for the mappers to 
remove those roads in advance (i.e. everything that is within some 
small distance of an existing road).  In some areas manually finding 
the roads not yet mapped in OSM comes close to a needle in a haystack 
problem.

Differences between OSM roads and this data are in most cases of the 
same order of magnitude as the satellite images' typical positional 
accuracy so imagery is not going to help much in deciding which data is 
better in that regard.

>
> > - if you advise the mappers to always run line simplifications on
> > the ways first it would be better to do this on the whole data set
> > in advance, this would avoid unnecessary work for the mappers.
>
> Running the Simplify Way plugin for each task takes just seconds,
> [...]

Since the lines only touch at the ends it is easy to run a 
simplification before converting to OSM format.

> The workflow is still unfinished, and I will add instructions when no
> Bing nor Mapbox are present. In those cases, when no OSM ways are
> present, we can safely import the MSD&JSI segments as they are,
> bearing in mind that they have been collected with GPS devices and,
> as you will see, they are most of the time of very good quality.

That would mean you generally consider cross checking with an 
independent source not necessary.  The question is how are the various 
decisions you ask the mapper to make (tagging, deciding which data is 
more accurate in case the road is already in OSM) to be made then?

> Sometimes the imagery let's see clearly that surface=asphalt. But I
> agree that in case of doubt we should stick to that paved/unpaved
> tagging.

Reliably distinguishing asphalt from concrete is generally very hard.  
This might be feasible based on quantitative analysis of low noise 
infrared data in case of relatively clean surfaces but not from visual 
color jpegs from a web service with dust during dry season.

> > - Instead of 'source:date=2014..2015' use 'source:date=2014;2015'
>
> I disagree. "2014..2015" means that the data has been collected any
> time during 2014 and 2015. [...]

That is not how source:date is generally used in OSM.  There are only 
eight cases of use of a '..' connector in source:date at the moment, 
all covering longer time spans with many in between values.  
source:date=2014;2015 means data is from the years 2014 and 2015 which 
seems exactly what is intended.

-- 
Christoph Hormann
http://www.imagico.de/