A Police cordon isn’t to prevent the media from filming or members of the public looking at a crime scene, although the absence of the press and public is a bonus. The purpose of a cordon is to protect and preserve the forensic integrity of the scene by preventing the loss or contamination of evidence. Evidence can be lost in numerous ways and no two crime scenes will ever be the same. Generally a crime scene within a building or under cover can be protected from loss or contamination easier than an outside crime scene. Knife fights in streets have often seen knives disposed of down drains and the investigating officers must consider the possibility of rain and the likelihood that the weapon might be flushed away or blood and DNA washed off it.
Rain, wind and snow can all destroy evidence and examiners must always be conscious that weather conditions can degrade evidence and as such weather conditions and the weather forecast will impact on decision making.
This is one of the reasons police erect large crime scene tents. Nobody wants to be filmed or watched as they examine a crime scene and thankfully cordons and tents provide a degree of privacy for the examiner, but the primary purpose is the protection of a crime scene.
A controlled cordon and by this I mean a cordon where the entry and exist of ever person entering is logged and a route in and route out is identified will lessen the chance of loss or contamination.
Loss is very easy to explain. Footprints in snow will either be covered over by additional falling snow or destroyed by other footprints. The value of footprint (Impression Evidence) is the subject of a separate blog entry, but most people can understand the importance of preserving a shoe or boot impression.
Contamination of a scene happens when somebody other than the suspect, victim or other participant in the crime introduces contaminates after the event. Simply smoking a cigarette and discarding the butt on the floor of a crime scene would be introducing a contaminate to that scene. No officers or examiner is going to throw away a discarded cigarette butt at a crime scene, but what if they inadvertently carry it to the crime scene on the soles of their shoes or boots. Many uniform officers wear boots with ridge pattern which are perfect for picking up stones, cigarette butts and other tiny objects.
Plastic overshoes prevent contaminates being carried in on the soles of examiners and white forensic suits prevents the transfer of fibres from the examiners clothes to the victims. With the development of DNA sampling and comparison technology examiner even have to take care not to breathe on items recovered.
On a side note.
The forensic suits are made of reinforced and treated paper with cross stitching for strength, and although only paper thin they make a wearer incredibly hot and as such many examiners stripe down to their underwear before donning a suit.