The Derwent River is 215 km long – the authoritative answer!

I am so excited.

Blog followers know I have been frustrated in my attempt to discover the official length of the Derwent River. Now I have found the answer in Australia’s most reliable source of information.  Today I found that the Australian Bureau of Statistics gives the length as 215 km.  I feel vindicated.  Previously, using an Opisometer on 1:25,000 Tasmanian maps, I had calculated the length of the River at 214 km – all the time knowing that some inaccuracy was possible with this device. You can read my earlier post here.  Now all I have to do is to persuade Google to change their information!  What chance do I have?

6 thoughts on “The Derwent River is 215 km long – the authoritative answer!

    1. Tasmanian traveller Post author

      You may wonder why I didn’t find it earlier. In the past I have searched on the ABS site but obviously not with the right words so I didn’t get what I wanted – well that’s my take on it. Somehow today I googled in the right way and up it came. Voila! And thank goodness. Finally. But I think getting Google to change will be much more challenging.


  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 17 sheets, there were at least 16 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 214 kms 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.



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