This problem has occurred again. I was going to start looking for local iron workings to each Wealden furnace site, but the first one I chose - Burningfold Furnace in the far west of Surrey - seems to have a whole load of West Sussex minepits sites within 10 km. I don't think so, matey! They are some 40km further east.


http://www.aditnow.co.uk/mines/Burningfold-Furnace/ 

If I remember correctly didn't Simon say it was problem with certain OS sheets and the actual GR? I have a vague memory it had something to do with starting with a zero. Mind, knowing the problem and getting it sorted is in two different ball parks.

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I think I have spotted the cause of the problem. The NGR of the sites in West Sussex that I have looked at recently have an NGR that starts with a zero, such as 041nnn. The sites that are identified as being nearest have an NGR such as 41xnnn. In other words the algorith that is working out the closest locations is stripping off the leading zero at some point and therefore getting a locational error of some 40 km.

When do I receive my prize? 🙂

When do I receive my prize? 🙂

Some time after you fix it. 😉

I can't fix it, but here is my guess of how it goes wrong.

The algorithm is based on a simple bit of trig. But it has to be fed the correct values to work properly.

I think the NGR is read from the database as six figures. Unfortunately, it is saved in a simple numeric field, so if it has any leading zeros these are ignored. In order to obtain the Eastings, it then reads the first three digits which no longer includes the leading zero, and therefore picks up one of more of the Northings digits. To obtain the Northings it extracts the last three digits, which will always work unless the coordinate starts with four or more zeros.

The correction will therefore require the algorithm to preserve all six digits by treating them as characters and not as numeric. Either that or accommodate the missing zeros by checking the size of the numeric being read. If it is less than 100000, remember to put the appropriate number of zeros back into the string before extracting the Easting.

Now do I get my prize? 🙂

Now do I get my prize? 🙂

Cannot fault the logic but the prizes are handed out by a higher authority than me. 🙂

Makes sense, some Swaledale and Cleveland locations are linked despite there being a fair distance between them, for example Fremington Edge (NZ055002) and Snotterdale (NZ518014).