Rotameter
Also called: Opisometer · Topic: Maps and Map Scale · NCERT: Class 11, Ch 1 "Introduction to Maps"
Meaning
A rotameter (also called an opisometer) is a small instrument with a wheel. It measures the length of a winding line on a map. You roll the wheel along the line, for example a river, a coastline or a mountain road. The wheel records how far it has rolled. You then convert this map distance into ground distance using the map scale. A ruler only works on straight lines, so the rotameter is useful for curves and bends.
Example
To find how long the Ganga is between Kanpur and Varanasi in Uttar Pradesh, you roll a rotameter along the river's bends on the map. You then convert the reading into kilometres using the map's scale.
Don't confuse with
- Polar planimeter: it measures the area inside a boundary, not the length of a line.
- Dividers or ruler: these are used for straight features such as roads, railways and canals.