Ski orienteering is a unique sport that combines orienteering with cross country ski racing. Races are held entirely on trails, although some competitors may choose to take shortcuts through the woods. Elite ski orienteers combine excellent map reading skills with great skiing, although the sport can accommodate any abilities of orienteering and skiing. Racers are presented a map at the start, and must find the designated controls in the correct order, as fast as possible. How you choose to get to each control is in your hands!
Wikipedia has an informative page on the equipment necessary for ski-o. The skiing equipment is the same as that used for cross country ski racing. Many competitors modify their poles to have very large baskets to deal with soft conditions on the narrow trails, since the narrow trails can't be groomed with a piston bully, so are softer than the wide trails.
Orienteering equipment consists of the map holder, which attaches to the chest for hands-free map reading, and either the finger stick for electronic punching or a punch card, for pin punching. The punchcard is often pinned to the competitor's suit, and the finger stick (SI-card or EMIT brick) are pinned or tied to the competitor's glove, in order to leave the hands and arms free to ski. A compass, while less crucial than in summer orienteering, is usually either attached to the map holder or to the competitor's arm. U.S. Team member Alex Jospe has posted an article on what ski-o looks like.
The World Championships in Ski Orienteering (SWOC) currently consists of four events: The sprint, middle distance, long distance, and relay. Each country can enter four skiers of each sex in each race, except for the relay, which consists of teams of three. The country that won the previous World Championships is allowed five skiers in each race.
In the U.S., race formats are starting to follow that of the SWOC, although many local meets just do a long race. E-punching is starting to play a larger role in U.S. events.
Like ski races, some ski-o races are individual starts and some are mass starts. During mass starts, there are several different courses among the competitors, and usually at least one map exchange, so that everybody ends up doing the same course, but not necessarily at the same time, to discourage following. Some methods to keep competitors racing their own races are forked controls and butterfly loops (where one control is used multiple times, creating loops). Pictured is the mass start at the World Championships in 2007. Below is a map from the Long course at the European Championships in S-Chanf Switzerland, 2008 showing some forked controls.
Long course map 1 from the European Championships in 2008 in S-Chanf, Switzerland.
Some of the longer races have two or more maps, to add complexity to the course. In a map exchange, the skier drops the old map and picks up the new map. Here, two racers come through the map exchange in the Ski Orienteering World Championships in Moscow, Russia, 2007.
USA Team tag (Adrian Owens to Greg Walker) in the relay at the World Championships 2009.
The major difference between domestic races and international ones is the density of the trail networks - in Europe, there are tight sections of narrow trails, referred to as mazes. In the U.S., the trail networks tend to be less dense, and there are more prohibitions to grooming narrow trails through the woods.
Map of the F21+ course at the Empire State Games in 2008 in Lake Placid NY.
Middle distance map from Orsa Gronklitt, Sweden, January 2009. As you can see, the Swedish map has a much denser trail network than the Lake Placid one. The skiers stay completely on the trails, so the denser the trail network, the more decisions the skier must make on the fly.
Although the courses are set entirely on trails, there are some factors that play into the route choices of the competitors. Narrow trail versus wide trail, amount of climb or descent on the shortest route versus going around a hill, and safe routes versus the shortest routes are a couple examples. The narrow trails require a different technique than the wide trails, but skate skis can be used with great success for the entire course - that said, some people debate whether to use classic skis on hilly courses with lots of narrow trails or if they plan to do much bushwhacking. Although the ski orienteering maps are much simpler than the average summer orienteering map, skiers move much faster than runners, leading to many decisions that must be made on the fly.