RPA Calculations

[Tom D]

Just doing a Wee planning query, objecting to an application, one of the trees in question is multi-stemmed, calculating the RPA using the Helliwell method gives an RPA of 5.3 m using the other method of 10 x diameter above the root flare gives 12.5m. This difference could have a significant impact on the application.. 5837 gives a measurement guide showing the individual measurement of stems but does not supply a formula. Anyone know the 'approved' method?

[Gray git]

Sorry can't answer your question as years since I've had to use this sort of calculation but out of interest how come your getting involved with objecting to a application?

I'm not saying your wrong and mind your own business like we often hear on here.

Also if I started quoting helliwell to the local council they wouldn't have a clue what I was going on about.

[Tom D]

I'm acting for a family member, tbh I don't mind arguing the toss with another consultant. It's just business at the end of the day, I wouldn't take offence if someone took issue with a survey that I had done.

Plus I think I have a strong case...

[arnoldbuchsassinger]

BS5837 also says 12 times stem diameter, should also be measuring at 1.5m above the ground

 

page 10 ish of bs5387, rpa calcs, trees two to five stems

stem 1 squared + stem 2 squared + stem 3 squared + stem 4 squared + stem 5 squared. Then square root the final answer, i cant find the square root sign etc on my keyboard.

[Alan Webster]

Just doing a Wee planning query, objecting to an application, one of the trees in question is multi-stemmed, calculating the RPA using the Helliwell method gives an RPA of 5.3 m using the other method of 10 x diameter above the root flare gives 12.5m. This difference could have a significant impact on the application.. 5837 gives a measurement guide showing the individual measurement of stems but does not supply a formula. Anyone know the 'approved' method?

 

The 10 x diameter is from the 2005 standard and is no longer used; this and the Helliwell method is not to the current BS5837 British Standard. Arnold describes the correct calculation method in the above post.

It is the average dtameter for more than 5 stems.

Edited May 22, 2016 by Alan Webster
Added a bit

[daltontrees]

It is the average dtameter for more than 5 stems.

 

Slight clarifiction, over 5 stems you use the average diameter, square it, multiply this by teh number of stems and then take the square root of that. The method in the current BS relies on finding the total cross sectional area of all the stems added together, then finding the equivalent single stem diameter for that cross sectional area. The 12.5x 'rule' is then applied to that hypothetical diameter to find the RPA radius.

[Tom D]

Thanks. I had missed that section, I've been using the same excel base one for years.

[Chris at eden]

Sorry can't answer your question as years since I've had to use this sort of calculation but out of interest how come your getting involved with objecting to a application?

I'm not saying your wrong and mind your own business like we often hear on here.

Also if I started quoting helliwell to the local council they wouldn't have a clue what I was going on about.

 

You'll go a long way to find a TO which doesn't know who Heliwell is!!!

[Chris at eden]

Almost there. Essentially there is no 12 x diameter = root protection area, I know it says there is but... Once you have the diameter of the single stemmed tree, or its equivalent for multi stemmed trees, measured in accordance with Annex C of BS 5837, and calculated as per the formulae in para 4.6.1 of BS 5837, you use table D1 in Annex D of BS 5837 to give you your root protection area. And in most cases that isn't exactly 12 x the diameter of the stem.

 

Ed

 

Tell you what I find odd. The fact that you measure stem diameter to the nearest 10mm and then calculate in accordance with annex D which is rounded to 25mm. Why not just measure to the nearest 25mm and then multiply by 12 for the radius.

 

Also, I may be wrong but it doesn't say whether to calculate to the nearest 25mm or round up to nearest 25mm. Or at least I haven't been able to find it. What you think Ed?

[daltontrees]

Almost there. Essentially there is no 12 x diameter = root protection area, I know it says there is but... Once you have the diameter of the single stemmed tree, or its equivalent for multi stemmed trees, measured in accordance with Annex C of BS 5837, and calculated as per the formulae in para 4.6.1 of BS 5837, you use table D1 in Annex D of BS 5837 to give you your root protection area. And in most cases that isn't exactly 12 x the diameter of the stem.

 

Ed

 

Nope, I was there. The original post was about arriving at a RPA radius. The area is a pretty useless quantity to know unless you are planning to deviate from the default circular RPA. So the key thing in this posting was the RPAr. Annex D calls this the 'Radius of nominal circle" ('RoNC'). In all cases in Annex D the single stem diameter is exactly 1/12th of the RoNC.

 

But as subsequently mentioned, the RPA is rounded to the nearest m2. This is necessary because the area is always pi x (RPAr)2, which can never be a whole number.

 

I have never used Annex D, nor do I intend to because I don't need to. RPAr = 12 x DBH. And RPA = pi x (RPAr)2. Then I do the rounding at the end, manually or by Excel.

 

The discrepancy caused by rounding relative to calculation is greatest for small values of DBH and RPAr. The greatest is for RPAr 0.90, where the error is 17%. This error disappears to a trivial 0.04% by RPAr 15.00.

 

I don't see that 5837 directs the user to round the DBH up or down to the nearest 25mm. As has been said, this is inconsistent anyway with Clause 4.4.2.6 which says to record the DBH to nearest 10mm.

 

For completeness, the greatest error that could be caused by rounding the DBH to nearest 25mm would be rounding 85mm actual DBH up to recorded 90mm, then rounding this up to 100mm to use Annex D. The true RPA would be 3.26m2, but Annex D gives 5m2. A whopping 53% error (hmm)!.

 

Clearly Annex D will be useful if you don't have a calculator but just as clearly (to my pedantic mind) interpolation is expected or at least possible. And in hindsight it would be better if Annex D used intervals of DBH 10mm. I have put one together quickly if anyone wants it (attached).

alt annex D.xls

Edited May 23, 2016 by daltontrees