Snapping and shared edges

Snapping pulls the point you’re placing onto something already on the map: a corner of another shape, a point along its edge, a pin, or a road or building on the base map. Use it when shapes have to meet exactly, for example building footprints that share a wall, or sectors that share a boundary.

Turn snapping on

Choose Snap in the dark bar at the top of the map while you draw, or turn on Draw › Options › Snap to features. Both do the same thing.

Snapping stays on or off in this browser until you change it.

Tip

Hold Shift while you click to place one point freely, without snapping.

What it snaps to

When the pointer comes close to something it can snap to, an amber ring marks the spot and the bar says what it is, for example Snapped to Building 4. It snaps to, in this order of preference:

1. Corners of lines and areas, pins and cordon centres
2. Edges of lines and areas: the nearest point along them
3. Roads, paths, buildings and water of the base map

It only snaps to layers that are showing. Hide a layer to stop snapping to it.

Make areas meet with no gaps

Two shapes only meet perfectly when they hold the very same points. Snapping makes sure of that in three ways.

• Shared corners are exact. A point snapped onto a corner takes that corner’s exact position.
• A point on an edge joins both shapes. When you click on the edge of another line or area, the point is added to that shape too, so both shapes have it.
• Shared edges are followed. Click two points in a row on the outline of the same line or area, and the new shape follows that outline between them, corner for corner. It doesn’t cut straight across.

So a footprint drawn against its neighbour shares the neighbour’s wall exactly. There’s no sliver of gap or overlap, however far you zoom in.

Example: a footprint next to another

1. Choose Edit map, then Draw area, and make sure Snap is on.
2. Click the first corner on the neighbour’s wall. The ring shows it has snapped.
3. Click the other end of the shared wall, also on the neighbour. The new area picks up every corner of the neighbour’s wall between your two clicks.
4. Click the rest of the corners of the new footprint as usual.
5. Click the last point again, or choose Finish shape, then Save.

An area is followed the shorter way round between your two clicks. A line is followed along its length, in the direction you clicked.

Turn off following edges

Following edges is on whenever snapping is on. To draw straight across a shape from one edge to the other, either:

• hold Shift for the second click, or
• turn off Draw › Options › Follow shared edges.

What it doesn’t do

• Shapes stay separate. Moving a shared corner later doesn’t move the neighbour’s. Snap the dragged point onto the neighbour’s corner again to keep them together.
• Following edges only happens while you draw. Dragging the points of an existing shape still snaps, but doesn’t follow edges or add points to the neighbour.
• Base map features aren’t followed. Points snap onto roads and buildings of the base map, but a new shape won’t trace along them. Use Draw along roads to follow roads.
• Some shapes aren’t changed. A point on the edge of a cordon, a line drawn along roads, or a shape in a layer you can’t edit lands exactly on that edge, but no point is added to that shape. Following its edge still works.