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.

7 thoughts on “The length of the Derwent River

      1. MikeH

        I wouldn’t have thought the ABS was an authoritative source of this information – it is not within the gambit of their expertise – they would have got the information from somewhere else. You should be consulting a mapping and surveying organisation – they are the experts.

        One of your problems in answering this question is your lack of understanding of the concept of “accuracy”. Accuracy is all about “fit for purpose” – there is no point striving for an accuracy beyond what is necessary. Consider a 2 km stretch of road – for the person who has to walk it to catch a bus, for example, just knowing it is “about 2 km” is sufficient accuracy to allow enough time to get there before the bus leaves. For the contractor who has to re-pave the road, knowing that it is 2.05 km (within a few metres) is adequate to enable him to order enough material. For the surveyor, and the draftsman who has to produce the detailed design plans, an accuracy of a few cms is required, and 2052.78 m is the accuracy of measurement needed.

        You can’t compare your measurement from the maps against any others quoted because you don’t know how they derived their figure.

        There is some aspects of your methodology which are suspect, the main one being – the maps you used are a plane representation of a curved surface and therefore the accuracy of distances measured varies across the map sheet. The projection used on the 1:25000 series gives good accuracy across the centre of the sheet, but the accuracy of distances measured at the edges is not so good. Also, if you used 25 sheets, there were at least 24 sheet boundaries crossed, each one a potential error in alignment. The detail on maps is mostly schematic, derived from aerial photos, and if you read the notes in the margin the accuracy of position is not guaranteed. Non-linear features are very difficult to trace precisely, especially in areas of thick vegetation. Did you calibrate your measuring device beforehand?

        For your purposes I would have thought + or – 5 kms would be sufficient (and then the comments above are hardly relevant).

        You have been diligent in how you went about the process and documented the parameters – well done. Just remember when ever you quote your result to include your methodology.


        1. Tasmanian traveller Post author

          Thanks Mike and I agree with your comments. I was very happy with my ‘rough’ measurement of 214kms using the Opisometer over the various maps. I always believed (but never said it in a post) that if an ‘accurate’ measurement was made/could be made then the River (between the points I designated in one of my posts) would be between 210 and 220kms. My greatest concern was that Google is projecting a 247km length and there has been nothing I could do to create such a measurement, even adding in Bays and Lakes which I don’t believe comprise the Derwent River. I believe the ABS length has been taken off the same maps I have been using – and on that basis I felt they were as authoritative as possible – taking into account the variations you suggested and many more. So it has been an interesting exercise, and one which has generated a lot of views, comments and emails. Thanks again for your very worthwhile comments.


  1. Coreena

    189kms from one source, 249kms from another source, 214kms from your blog – and it’s your blog that backs it up with how you came to that result… thank you for explaining, and questioning, the method that may or may not have been used. Great blog, the content of which will add to interesting conversation about this very topic.


    1. Tasmanian traveller Post author

      I find it so interesting that in this day and age where ‘facts’ are deemed to be so important that the ‘fact’ of the length of the Derwent River hasn’t been given much focus. Of course I realise it depends where the start and end of a river is measured from, but I cannot find any other information source that explains how they came to their figures. As the result, and because I have measured the River (albeit in only a close to accurate way), I think other numbers are figments of imagination.



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