Kemnitz, a cop for more than 30 years and the only certified police tracker in Wisconsin, says it's up to each tracker to train himself to recognize such color changes in the area they are tracking.
He recommends the following training trick to novice trackers: "Break a branch of a low-hanging bush. Attach a tag and record the date, time of day the branch was snapped, and the amount of shade or sunlight at the time. Return to the bush at various times of the day for a week, making notes as to the amount of plant juice expressed, the time it takes to dry in the shade and/or sun, and the color change in the damaged area. Repeat this process for a variety of plants." The observer will notice the plant juice of most plants in direct sunlight dries in a matter of 2 or 3 hours during the summer months. In the shade, the same break will appear fresh for 24 to 36 hours, but the ends of the broken branch will discolor.
Master tracker, Joel Hardin, has tracked lawbreakers for the U.S. Border Patrol for three decades. He now runs a school designed to teach police officers and search-and-rescue personnel the science of tracking and aging footprints.
"Gravity, wind, rain and the sun will age footprints," Hardin pointed out at a workshop in Michigan. "Knowing recent weather conditions is critical for interpreting tracks at a crime scene. It's incumbent that investigators know when and how much rain fell in the 24 to 36 hours before a crime is discovered."
Training and experience in a variety of environments are essential for becoming a competent mantracker. If a clear footprint is discovered in dirt or mud, investigators may use several methods to estimate the time the track was laid. Gravity, Hardin mentions, will age a track in a predictable manner. The edges become less sharp, rounding off with age as gravity pulls the soil into the track. By pushing his thumb into the soil next to the track and observing how long it remains crisp, an investigator can estimate the time interval since the track was laid. Another factor to consider is wind. A print protected from the wind remains much fresher than one subjected to a breeze. Trucks and other traffic on a highway create a wind that will obliterate tracks in a matter of hours. And certain soils hold prints better than others. For example, a footprint in sand will appear to age rapidly, within minutes, while a track in loam or light mud will hold its freshness for hours or even days.
Determining the age of footprints left in snow at a crime scene can be a helpful skill for officers working in the northern areas of the country. "Ice will form in the track because foot pressure squeezes out some moisture," Kemnitz says. "Knock on the floor of the track with your knuckles. If you hear a hollow sound, the track is probably more than 24 hours old. But, if the track is in a shady area, it may be more recent. The officer can check his time estimate by placing his own footprint nearby and seeing how long it takes to freeze." Kemnitz, who spent much of his life tracking in the frozen north, adds this tip. "Frost in the track means the track was made before nightfall, and animal prints found inside your suspect's footprint were probably made during the night. Many animals are nocturnal, and it's up to the tracker to know the animal patterns in his area."
The missed evidence
The art and science of tracking has a long history in America, but it has fallen out of favor over the last century. Other parts of the world are more cognizant of the usefulness of footprints in law enforcement. In Europe, usable footprints are found at approximately 40 percent of crime scenes, and Australian police officers discover and process even more. However, American crime labs report only 1 to 2 percent of submitted evidence involves footprints. Crime scene investigators in the United States are seldom trained to proactively search for footprint evidence. "What we don't look for, we don't find" is a cliché that seems to apply to tracks. Footprints are truly "the missed evidence" in many jurisdictions.