Marking distance on a map before launching your kayak, makes navigation and decision making while in the kayak easier. Why? Because you can quickly see distance travelled, distance to destination and distance of any alternative routes you might want to undertake.
Of course, when you know the distance you roughly know the time it will take you to travel that distance. I for instance know that I roughly paddle about 5 km/h. I therefore mark distances in 5 km intervals on the map.
Every map should have a scale, which tells you the relationship between a distance on the map and the corresponding distance in the real world.
I have a 1:50,000 map, where one unit of measurement (the blue grid squares) on the map equals 1 km. Put another way, the map represents the real world at 1/50,000 of its actual size. So, 1 cm on a 1:50,000 map is 50,000 cm (50,000 cm equals 500 meters).
To mark the distance I plan to travel I need something to write with and something to measure 5 km on the map with. In my case I used a tent guy line.
First, measure 5 km on the guyline and tie a knot.
Then simply lay the guyline on the map and plot your distance.
So after plotting the distance from my campsite to my destination I find out that the entire distance is about 25 km, which will take me about 5 hours. At any time during my paddle, I can check the map to see how long I have to go to my destination or any other point along the plotted route.
I can even plot alternative routes. Let`s say I wanted to check out the other side of the fjord. I can plot a route to an additional spot. When I get to the crossing of the fjord, I can evaluate if I have the strength or time to pursue this alternative spot.