So we want to measure certain distance on the ship. We can either stretch out the measurement tape and measure it but it would be a tedious process and with error too.
This is because it would be difficult to measure across the ship's edges and some areas might be inaccessible.
Alternatively we can easily measure these distances from the ship's plans.
Let us say we want to measure distance from keel to the bridge deck. Lay out the General arrangement plan or use the one displayed in the accomodation.
Take a scale and measure the distance.
Measure this reading accurately as any small error in this reading will get amplified when you convert this reading to the actual distance.
Avoid the parallex error during measurement, i.e keep your eye in line with the reading to be measured on the scale.
Let us say we get this reading as 19.5 centimeters.
Now look at the scale of the general arrangement plan. The scale is usually mentioned at the bottom right cornor of the plan.
In this case, the scale is 1/200. This means that whatever reading we get from the plan, we need to multiply it by 200.
So the actual distance of the ship corresponding to the the distnace of 19.5 cms on the drawing will be equal to 19.5 x 200 = 3900 cms.
This means that the actual distance from the keel to the bridge deck for this ship is 39 meters.