#OSM Tracing Guide for Dhaka, Bangladesh
Dhaka is a rapidly-growing megacity with extreme vulnerability to climate change. The city is home to about 15 million people, and lies just above sea level - putting it at very high risk for intense flooding and storms. Infrastructure is poor and poverty is high, limiting the city's capacity to adapt.
Despite being the world's densest city, Dhaka lacks accurate and comprehensive maps of the road network and buildings. We're interested in getting OSM up-to-date in order to help with resilience and urban planning efforts, and to act as a base map if a major weather event occurs.
Dhaka can be tricky to map because of its density, so we put together this tracing guide to go over how to map the most important features in the area: buildings, water bodies, and roads.
##Getting started
We'll be using the default Bing aerial imagery layer for tracing. The background imagery can be difficult to see, but increasing the brightness will help. You can do this in the "Background settings" tab (Shortcut: b).
##Buildings
Buildings can be tough because Dhaka is so dense and has lots of tall buildings.
When we trace a building, we are trying to trace its footprint. But when mapping tall buildings, the angle of the imagery often makes it difficult to see where the building meets the ground. Instead, you may find it easier to trace the outline of the roof and then move that into the correct place. You can do this by clicking "Area" (Shortcut:3) and tracing the roof outline. When you've completed the shape, hover over the outline and select "Move" (Shortcut: m), then drag it to the correct place. You may also wish to square the corners of the building (Shortcut: s)
###Tagging buildings
After tracing each building, we want to "tag". Select the building and, on the left side of the screen, click "Building". A second menu will appear. If we know what type of building this is, we could be more specific. But aerial imagery makes it difficult to be sure if something is an office tower or a residential apartment building, so it is better to be generic and just select "Building" again instead of specifying and being wrong.
##Water bodies
###Ponds and small catchments
Dhaka is dotted with "homestead water bodies", which are manmade water catchments near buildings or homes. Water colour will vary greatly depending on the purpose of the catchment.
Here's a shot showing what these water bodies look like before tracing:
And after tracing:
The procedure for tracing a homestead water body is the same for tracing a pond. Select "Area" (Shortcut: 3) and trace the water body. When you're finished, tag it as "Water", and then as "Pond".
###Rivers
Rivers get traced two ways: first as ways (that's OSM-talk for "lines"), then as areas.
First, click "Line" (Shortcut: 2) and trace the middle of the waterway. The direction you trace in represents the direction of flow. If you aren't sure about which way the water flows, it's safe to assume south (towards the ocean). If it's wrong, someone will correct this later. Once you're finished, tag the river as "Water", then as "River".
Next, trace the area of the waterway (Shortcut: 3). When you're finished, tag it as "Water" and then manually enter "Water: River".
###Streams
Streams are traced as ways (lines) and then tagged as "Water", then as "Stream".
##Roads
First we trace them, then we classify them. The whole process looks like this:
###Tracing roads
To trace a road, use the Line tool in iD Editor (Shortcut: 2) and trace along the middle of the road. See below for an example of too few nodes (left), too many nodes (center), and the right amount of nodes (right).
Make sure the roads connect to each other where they cross. You should see a node appear at each intersection (see below).
###Classifying roads
In Dhaka, you'll mostly see residential roads, trunk roads, (need some help here.....)
Residential roads are the roads you see between buildings, which don't have markings and aren't much bigger than two lanes. They should be tagged as "Road", then "Residential":
Tertiary roads are the majority of multi-lane main roads in the city:
There are also some primary highways in the area, but these are generally already mapped.
That's it! For more resources, see:
Thanks to the HOT OSM team, whose material we borrowed for this guide.