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30 min

Building Digitization Fundamentals

Building digitization is the core skill of humanitarian mapping. Accurate building data enables population estimates, disaster response planning, health service delivery, and infrastructure development. This comprehensive guide covers every aspect of professional building mapping.

Building mapping requires precision, consistency, and attention to detail. Every building you map contributes to critical humanitarian work and must meet strict quality standards.

Part 1: The Building Tool in JOSM

JOSM's building tool (from the building_tools plugin) streamlines the process of creating rectangular structures. Master this tool before attempting manual building creation.

Using the Building Tool

  • Activate: Press keyboard shortcut 'B' or click the building icon
  • Click first corner: Place initial point at building corner
  • Click opposite corner: Drag to define rectangle size and orientation
  • Building appears: JOSM auto-creates perfectly rectangular shape
  • Building is selected: Ready for tag application
  • Apply tags: Tool automatically adds 'building=yes' tag

Critical: Always zoom in to maximum detail before drawing buildings. What looks like one building at low zoom may actually be multiple separate structures.

Part 2: Identifying Buildings from Satellite Imagery

Not everything that looks like a roof is a building you should map. Learn to distinguish buildings from other structures and shadows.

Building Identification Criteria

  • Permanent Structure: Fixed foundation, not temporary or mobile
  • Enclosed Walls: Complete walls on all sides (not open-air markets)
  • Roof Visible: Clear roof outline visible in satellite imagery
  • Appropriate Size: Typically >20m² (check project requirements)
  • Distinct Shadow: Consistent shadow pattern indicating elevation
  • Rectangular/Regular Shape: Most buildings have geometric patterns
  • Context Clues: Located in residential, commercial, or institutional areas

Do NOT Map These

  • Shadows on ground (very common beginner mistake)
  • Vehicles or parked cars
  • Small sheds or storage units (<20m²)
  • Temporary structures or tents
  • Open pavilions without walls
  • Swimming pools or water tanks
  • Outdoor sports courts
  • Construction sites (incomplete buildings)

Pro Tip: If you're uncertain whether something is a building, check multiple neighboring tasks that have been validated. This shows you what experienced mappers consider appropriate.

Part 3: Drawing Rectangular Buildings

Most buildings are rectangular. The building tool makes this fast and accurate.

Step-by-Step Process

  • 1. Identify building outline clearly visible in imagery
  • 2. Press 'B' to activate building tool
  • 3. Click precisely at one corner of the building
  • 4. Move mouse to diagonally opposite corner
  • 5. Click to complete the rectangle
  • 6. JOSM creates building and selects it automatically
  • 7. Check the Properties panel - 'building=yes' should be present
  • 8. If correct, press ESC to deselect and move to next building

Part 4: Complex Building Shapes

Not all buildings are simple rectangles. L-shapes, courtyards, and irregular structures require different techniques.

L-Shaped Buildings

  • Draw as TWO separate buildings that share a wall
  • Create first rectangle for one section
  • Create second rectangle for perpendicular section
  • Ensure shared wall nodes overlap exactly
  • Select both buildings (hold Shift while clicking)
  • Press 'M' to merge into single building
  • Tool menu → Merge overlapping ways

Buildings with Courtyards

  • Outer wall: Draw outer perimeter of building
  • Inner courtyard: Draw separate shape for open courtyard area
  • Select outer building first
  • Hold Shift, click inner courtyard to select both
  • Tools → Create multipolygon (or press Ctrl+B)
  • Inner area becomes 'hole' in building
  • Tag outer way with 'building=yes'
  • Inner way automatically becomes 'inner' role

Advanced Shapes Warning: Multipolygons and complex buildings are error-prone. Until you've completed 50+ basic tasks, stick to simple rectangles. Complex buildings can wait.

Part 5: Squaring Buildings

Building corners should be perfect 90-degree angles. JOSM provides a tool to fix slightly misaligned buildings.

The Squaring Tool

  • Select the building you want to square
  • Press 'Q' keyboard shortcut (or Tools → Orthogonalize)
  • JOSM adjusts all corners to exact 90-degree angles
  • Building maintains approximate size and position
  • Use this on every building unless it's truly non-rectangular
  • Check result - ensure building still aligns with imagery

Efficiency Tip: After drawing a building with the B tool, immediately press Q to square it, then ESC to deselect. This becomes muscle memory: B-click-click-Q-ESC for each building.

Part 6: Building Alignment and Accuracy

Your buildings must precisely align with the satellite imagery. Poor alignment creates unusable data.

Alignment Best Practices

  • Zoom to maximum (200% or higher) before drawing
  • Place nodes at exact corners, not approximate positions
  • Align with roof outline, not shadows
  • If imagery is blurry, draw conservatively smaller rather than too large
  • Check alignment from multiple zoom levels
  • Compare your work to validated adjacent tasks
  • If imagery has offset issues, check project instructions for correction method

Part 7: Building Tags

Proper tagging makes your building data useful. At minimum, every building needs the 'building=yes' tag.

Essential Building Tags

  • building=yes - Required for ALL buildings (auto-added by tool)
  • building=residential - If clearly a house/home
  • building=commercial - If clearly a shop/business
  • building=industrial - Factories, warehouses
  • building=school - Educational facility
  • building=hospital - Healthcare facility
  • building=church - Place of worship (if identifiable)
  • source=Bing or source=Maxar - Document imagery source

Tagging Warning: Only add specific building types (residential, school, etc.) if you are absolutely certain from imagery or project instructions. When in doubt, use building=yes.

Key Takeaways

  • Use 'B' tool for rectangular buildings, 'Q' to square corners
  • Zoom to maximum detail before drawing any building
  • Do not map shadows, vehicles, or temporary structures
  • Every building needs 'building=yes' tag at minimum
  • Complex shapes (L-shapes, courtyards) require advanced techniques
  • Alignment accuracy is critical - place nodes precisely
  • When uncertain about building type, use generic 'building=yes'
  • Practice on beginner projects before attempting complex urban areas