Determining acreage

[Kentish Man]

I've been doing my own pacing around in a woodland that's for sale.

Each of the 4 sides is of different length and I just can't seem to get the same acreage with my calculations as the official acreage. I wouldn't actually care if I was out by a little bit, but I seem to be out by about 5 acres!



I'm sure I must be doing something wrong and have been sitting here for a while now puzzling over it, so its time to ask!



I've read the practical guide to calculating acreage here: http://www.woodlands.co.uk/blog/practical-guides/how-big-is-an-acre-measuring-your-woodland/ and with that in mind, I noted down my measurements.



Now, I make the sides of the woodland, 280 strides x 116 strides x 255 strides x 240 strides. I've calculated that a stride is about 3 foot, so that gives very roughly 840 ft x 348ft x 720ft x 765 feet.



I've then calculated two areas and added them together, so:



720ft x 348ft = 250,560 square feet

and

120ft x 417ft = 50,040 square feet




= 300,600 square feet.



The woodland for sale is 522,720 square feet, hence, I'm over 222,000 square feet - or over 5 acres out!



I know my paces might be a bit out and the whole exercise was just a quick one to see roughly how big the area was, but now its driving me a bit mad!



Can anyone spot what I am doing wrong?! Answers on a postcard please!

[MartinD]

The Forestry Commission have an interactive feature which allows you to measure the acreage of land - it's called LIS, and is available at http://fortingall.forestry.gov.uk/glade/ccheck/ConstraintsChecker.htm?map=england&l=e&c=e&controls=labels%3atrue (If this doesn't work, go to the FC Grants page, and follow links for LIS Advanced). Go to your woods using postcode or grid ref, zoom in to the area, and then use the icon to the left of the words 'measure draw' on the menu bar. Click the icon to activate, click as many points as you need to follow the boundaries, and then click the icon again to 'close' the area, and the measurement is displayed at the bottom of the page.

It's not unusual for sales measurements to be out - the woodland behind us was for sale with a claimed area of 42 acres. Using this tool I determined it to be 34 acres. I told the vendor, but they continued to claim 42 acres - buyer beware!

The FC tool does not work with the most recent version of Internet Explorer (version 8?)www.magic.gov.uk also have an acreage calculator, but I've not used it.

[jillybean]

I cant see anything wrong with your maths, but than im no mathematician. I go to google earth, draw a path round the land, and take the measurements off that, or get on the land registry website and download the map for £4.50 or £8.00 if its agricultural land its a big difference 5 acres as you say....

[Kentish Man]

Thanks for the help swoggers!



I went to the FC link and drew the area as it is being sold on the map and it does come out roughly to the correct acreage. This still leaves me wondering though - if I've measured it wrongly on foot; that the stakes are set out wrongly on the ground or that my maths is still wrong! Something's wrong somewhere!



For an apparent rectangular shape, on the ground, one of the sides really does seem to cut in at a less than 90 degree angle to the other edges - maybe this is where my "error" has crept in? Its so difficult to tell.



Are there any specialist surveyors who can come and take proper measurements, in the same way house surveyors come around?

[MartinD]

you can also measure the length of each side of the woodland using the icon/tool to the left af the area icon - this will tell you how far out your distances are - perhaps your strides are only 2ft!

[greyman]

I am by no means a mathematician (I had to look it up to spell it) but I wonder if the fundamental problem is that you need to treat the area as two triangles due to the differing lengths of the sides. When you do that I think you also have to use an angle in the proceedings.Normally when you do the area of a square the equation is X x Y = area. If you are carrying out two different equations, due to all sides being of differing lengths, I'm don't see how you can amalgamate them . I'm not sure I understand how your figures work either so I wonder if you have made adjustments in your calculation already? My limited understanding is that you have to divide up any unequaly shaped trapesoid ('cos there's no such thing as an unequal rectangle)in to measurable and calculable sections, triangles, squares etc. The triangles you have to use calculus on. Thats my two p'oth anyway! Sounds like the online calculating doo-dah thingamy wotsit is yer best bet.

Greyman

[DuncanB]

Rather than doing difficult sums, it's quite easy to measure the size of a property on Live search maps (now called Bing for some unknown reason!). If you got to:

http://www.bing.com/maps/

and play (dragging the map / changing the scale etc) until you can see your property (use the aerial button to look at a satellite pic)

then click on "collections", and on "open your collections"

you'll then get a box, click on the 4th icon at the bottom of this box - "Mark an area on the map"

Then click on all of the boundary points on your property, double clicking when you've finished. This will then colour in your land and give its area (in square feet). Sounds difficult, but is actually quite easy. As an aside try the the Birds eye view if you can - can give some interesting angles (albeit with limited coverage)

[Stephen1]

One way is to get your hands on a good gps. Until recently these didn't work well under tree cover, but some of the garmin ones do. I've got a 60 CSx which works pretty well under a moderate tree canopy, and very well in winter when the leaves are off!

https://buy.garmin.com/shop/shop.do?cID=145&pID=309

One of the functions allows you to walk around any area of land (whatever the shape of its boundary)and it then calculates the area in the units of your choice (including acres).

You can get these, garmin 60Cx pretty cheaply on ebay now that the latest gps systems have ordnance survey maps on them - these have no external antennae though, and do less well under trees than the 60Cx.

[Kentish Man]

Thanks for all the help everyone!



The official sales map does indeed stack up with the correct area using both the FC and Bing links, so officially, everything is in order (as I assumed!).




However, on the ground, I can't escape the feeling that things are different.



Unfortunately there's no Bird's Eye view of the wood, although the detailing is quite good at max zoom, but the picture was taken in full summer so the tree canopy is quite full, so I am not able to determine where the bordering posts have been placed. The woodland is on an incline with a dip in the middle too, just to make it a bit harder to work it out!



I wouldn't even know where to begin, Greyman in finding out the angle of the posts, save duffing up some school kid for his protractor! I did try in my calcs to build in the areas, but mine were rectangles rather than triangles and I can't find an easy to use online tool anywhere yet for my manual measurements.

[DaveTaz]

One of the problems you may be facing is accuracy at ground level. When taking measurements on this scale and then multiplying large numbers, any inaccuracies are multiplied as well with remarkable effect

ie 240 strides x 116 strides @ 3' per stride = 720' x 348' = 250,560 sq ft

if you measure the stride to be say 3.3 ft the answer is hugely different

(240 x 3.3 = 792) x (116 x 3.3 = 382.8) = 303,177.6

If you still want to convince your self by your own measurements get hold of large tape measures or possibly hire one that uses a wheel that you push along. Take measurements in metres and be as accurate as possible

Doing an approximate conversion from strides to feet won't work, hope this helps