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posted: 25 Oct 2007 05:41 from: Martin Wynne
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Dear all, Bob Reid asked on RMweb for some help in identifying the size of this turnout. I thought it would make an interesting exercise in using perspective correction in the GIMP image editor: 272_gf_1.jpg Before attempting to gimp it some general clues can be seen. This is a loose-heel straight switch rather than an REA flexible (A, B, C, etc). The heel location is very clear in the pic. This is a passenger running line (hence the facing-points lock). The loop on the right is a goods-only line (hence the catch points). So it's unlikely to be a long fast turnout, i.e. no longer than say 1:12. On the other hand short 9ft and 10ft loose-heel switches are very unusual in running lines. Ditto V-crossings less than about 1:7. So probably somewhere between 12ft and 18ft for the switch with a V-crossing angle somewhere between 1:7 and 1:12. I think I can count 33 timbers between the switch toe and the V-crossing, so this will be a useful guide to the turnout length. gimp3.png This is a crop of the turnout area from the original. I have added some guide lines in green. In addition to the rail edges, the marks on the left from top to bottom are the heel (pivot) location, the end of the blade planing, and the blade tip (point). On the right the mark is the end of the blade planing (where the rails diverge). All these are as best I can judge on a rather indistinct photo -- in particular the end-of-planing on both sides is a best guess. This is unfortunate because it is the only location showing on both sides, and at least one such location is needed to align the correction grid. gimp4.png This is a zoomed-in screenshot from Gimp, showing the perspective correction grid. I have dragged the grid corners to align the vertical grid lines down the rail edges, and also one horizontal grid line is aligned across between the end-of-planing green marks where they intersect the gauge lines. Strictly speaking we need another location to align the horizontal grid lines square across the rails and complete a full rectangle. The best that can be managed here is to align a grid line across the far ends of the check and wing rails -- or in this case the smudges representing them. On most turnouts the end of the check rails align with the end of the wing rails, if the timbering is square-on to the rails. Note the "most" and "if" in that statement. For perspective correction to work properly, it is important that everything to which the grid is aligned is in the same plane -- in this case the rail top. This means that we can't use the timbers for alignment, because they are in a different vertical plane. Sometimes you can get away with using the timbers in a higher shot (if they are square-on), but not in a low-angle shot such as this. gimp5.png This is the result in Gimp after performing a perspective transform (in Corrective mode). The aspect ratio of length to width is still completely wrong, of course. gimp6.png This is the same thing rotated into the more usual direction for Templot. gimp7.png And here it is loaded into a picture shape in Templot. I have shrunk it vertically and stretched it horizontally to fit the proper aspect ratio using the shift shape corner 2 mouse action. It is easy to get the height correct -- just match the rails to the track gauge. Getting the length right is much more tricky. We need to match a switch to the green guide marks (bearing in mind that they were very approximate originally, and are now even more so). And also find a turnout length which matches the timber count (bearing in mind that we don't know the prototype or exact timber spacings, so again it is very approximate). gimp8.png Working in 4mm scale, after much trial and error I have matched the image to a turnout having a 12ft switch and 1:9 crossing angle. The blade tips and heel position (purple radial end mark) match well. The end-of-planing less so, confirming that it was very much a guess on the original. gimp9.png With the timbering added. For this size there are 32 timbers between the blade tips and the crossing vee. Which matches well my best attempt to count them in the original (I make it 33 timbers -- anyone?). gimp10.png gimp11.png A couple of close-up views. Either this turnout is wide to gauge through the turnout road, or I didn't get the Gimp correction grid quite right. You can spend days doing trial and error gimping if you are not careful. gimp12.png The more usual Templot view of this turnout. For a 12ft switch you can use B-size blades, which have the same 1:32 planing angle. gimp13.png This is an alternative solution matching a 15ft switch and 1:11 crossing angle. Here the timber count is 38 -- rather more than I can see in the original. On the other hand this is modern timber spacing, and pre-grouping designs tended to use fewer timbers on wider spacings. To build a 15ft switch you can use C-size blades, which have the same 1:40 planing angle. So either of these can be regarded as good matches, but on balance I think the 12ft-9 is the more likely for a goods loop exit. 1:11 is an unusual size and unlikely to be used without good reason to fit a specific location, which doesn't seem to apply here. It would be possible to make a 12ft-8 fit well too, and it's a common size. But there does seem to be too many timbers in the original view for it to be a 1:8 turnout. There is more about gimping photos at gimping track and gimping for modelling. Anyone else having a go at gimping this turnout for Bob? regards, Martin. |
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