Tracing rectangular buildings - iD editor
This image shows you which buttons or wheel on a mouse has been used, and which keyboard buttons pressed.
Points to note;
- If the building should have square corners, make sure you trace it with square corners, or use the tool to square the corners.
- Make sure you tag!
- Size matters - population estimates are based on the cumulative building area in a given area,
- the right size building the right way round will help people locate their village,
- Zoom in enough to see the details - a zoom scale of 20 metres or less is usually about right.
- Looking for buildings - scan with your scale somewhere between 50 metres and 80 metres. Watch out for shadows and straight lines (round buildings are harder to find!)
- Buildings and roads, rivers, landuse=residential boundaries or other features should not overlap.
- If you make a mistake, use the Undo button to undo your last change(s)
- We’re all improving the map. If the previous mapper has left the buildings a funny shape or wrong in some way, correct it and consider sending them a polite message.
Buildings obscured or seen at an angle
It is common for the satellite image to be taken at an angle making the building seem distorted. In the above image the image to the right shows how the building should be traced. It is often easiest to trace the outline of the roof, then drag this to the point where the walls meet the ground - this was the technique used to trace the building above. Notice also that a building partially obscured by trees has been correctly traced as rectangular.
Buildings with a ridged roof
Most buildings either have corners at 90 degrees, or they are round buildings.
This building has 90 degree corners, but has a ridge on its roof. The camera angle initially makes the building look a strange shape.
This building has a tree partially obscuring it. It is a rectangular building.
In isolated parts of the world it is common to use sheets of corrugated iron as roofing - when new these will reflect the sunlight which creates a very bright flare effect on the satellite imagery. It is common for buildings to have a verandah or be L shaped and you should ensure that your tracing follows the shape of the building.
When tracing the building outline, you must try to trace the building at the right size, and at the point where its walls meet the ground - do your best! In many parts of the world, buildings are partially constructed and then left without roofs until they are sold, or otherwise wanted for use. Map a building without a roof as a normal building as it does not take long to add a roof, and the roof may have been added already, in the time since the satellite imagery was taken.
Do your best - even the experts will disagree sometimes, and sometimes you just have to go there to see exactly what is really there. If in doubt add it as a building.
As we are tagging buildings we have never visited and are not familiar with, the safest and recommended tagging scheme is building=yes which you can see on a correctly tagged building if you scroll to the bottom of the tagging pane and click on All tags.