Tag Archives: invention

The length of the Derwent River

Past readers of Frequently Asked Questions know that I have not been able to discover the ‘real’ length of the Derwent River.  In conversation recently with blog follower Yo, I was reminded of the challenges involved with determining river lengths.

If a river empties into a sea by more than one waterflow, then which is the ‘real’ river? Do we say there are two river lengths for the same river in such a circumstance?

If a river enters into a bay which opens onto a sea, where does the river end?

If the mouth of a river is a complex delta receiving silt flows and forever changing, can we pinpoint the location of the river mouth? At which time could a reliable measurement be made? No and never, are the answers to these questions.

At the source, rivers sometimes start with a dribble of water oozing from the ground. Is that enough to be able to define the water source as a river?  Some rivers pour from Lakes. Is the middle of the lake the start of the river or is that bank over which the water leaves the Lake, the official starting point? If a dam has been built at the junction of the lake and river, then the starting point may be the water at the top of the dam or at the bottom of the dam – leading to a different final river length.

What about rivers such as the Nile River which have at least two waterflows starting inland and which then meet to form one river?  Which waterflow counts towards the measurement of the river’s length? Or does the river start where the two or more waterflows merge?

Rivers do not travel in neat straight lines so should we measure the distance along the riverbank? If so, which bank?  The shape of the shoreline on either side of a river can be markedly different from each other and one side is usually longer than the other.

How about measuring the length of the river taking a centre line? Will that give the most accurate measurement?  This is the preferred method, however many of our measuring devices are straight – think ruler, think tape – so how can we make a reliable measurement?

On the first day in December 1873, the Commissioner of Patents for Inventions under England’s Patent Law Act 1852 recorded Edward Russell Morris’s invention of a pocket instrument which could measure distances on curved lines.  Since the 19th century, various variations have been developed.

If you check on EBay online, many historical versions of Opisometer Curvimeter Meilograph Map Measurers have been photographed, and are offered for sale. Opisometers have a tiny wheel at one end which, when rolled on a map along a road or river, connects with a graded scale in either a straight or circular format to read a distance. The unit markings refer either to kilometres or miles. You can watch a demonstration of one of these measuring tools on You Tube .

A bushwalking friend purchased an Opisometer many years ago and lent it to me so I could ascertain the length of the Derwent River more closely.  Today I used the Opisometer by rolling it on my maps, along an arbitrary ‘line’ in the centre of the river.


After using the Opisometer on all seventeen 1:25,000 maps that cover the Derwent River, it seems the length is 214 Kilometres (refer to Frequently Asked Questions for a table showing the breakdown per map).

Based on my measurements today, so far I have walked against 61 kilometres of that length.  I can see that any previous measurements reported in my blog, have been wildly inaccurate – generally too low.