Secure, photograph, sketch: documenting the scene

A scene can only be examined once. Every footstep, every opened door and every moved cup changes it. So the first job is not collecting, it is protecting and recording.

1. Secure. Limit who comes in. A logbook lists everyone who enters, with the time. Often a single marked path (with stepping plates) is used, so people do not walk over traces on the floor.
2. Dress to avoid adding your own traces. Gloves, shoe covers, a hair cover and a mask keep the examiner's fibres, hairs and prints out of the evidence.
3. Record in three ways: notes, photographs and a sketch.

Photographs go from far to near.

• Overview: the whole room from each corner and door, so anyone can see where things were.
• Mid-range: each item in its place, so the position is clear.
• Close-up: the item itself. Photograph it once as found and then again with a ruler or scale next to it, with the camera held square (parallel) to the surface. Without a scale nobody can later measure a shoe print or a fibre from the picture.

Each item gets a numbered marker placed beside it (never on it), so the photo, the sketch and the notes all use the same number.

The sketch. A rough sketch is drawn from above, like a map. It shows the room's walls, doors and furniture and the numbered items. Distances to each item are measured from fixed points, such as door frames and wall corners, so that anyone can put every item back on the map. A sketch records things a photograph can distort: true distances and the relation of one item to another.

Search patterns. Hunting for traces without a plan means gaps and repeats. These patterns are used:

• Strip (line): searchers walk side by side in lanes across the area. Good for wide, open ground.
• Grid: a strip search done twice, the second time at right angles to the first, so every spot is seen from two directions.
• Spiral: one searcher walks in a spiral, from the outside in or from the centre out. Good for a small area.
• Zone: the room is divided into zones and each zone is searched in turn. Good for rooms and buildings.

Fragile traces, such as a footprint in mud or loose fibres, are recorded and collected first, before anything that could disturb them.