Templot Club Archive 2007-2020
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posted: 28 Aug 2020 18:12 from: DerekStuart
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Good afternoon all. The discrepancies between google aerial views and OS maps has been discussed before, but I can't see at any time anyone has reached a conclusion. I have engineering drawings of some buildings- copied from original records, which I'm inclined to accept as accurate, but these do not tally up with the aerial view from either Google or Bing. The OS map again shows something different. It's enough of a difference that I would be reticent to scale the whole plan based on this discrepancy. Does anyone have any ideas, please? Or can point me in the direction of a previous discussion that reached a conclusion? (I did try searching but found nothing conclusive). Thanks Derek |
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posted: 28 Aug 2020 18:30 from: DerekStuart
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Thanks to a post elsewhere from Ickdab on another matter, I will go along with his suggestion that the OS maps are more likely to be accurate than the aerial photos. Derek |
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posted: 28 Aug 2020 19:36 from: Martin Wynne
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DerekStuart wrote: I have engineering drawings of some buildings- copied from original records, which I'm inclined to accept as accurate, but these do not tally up with the aerial view from either Google or Bing. The OS map again shows something different.Hi Derek, If you are using the historic maps from the NLS, bear in mind that they have different map projections available. The original "County" series OS maps pre-1936 used a separate projection datum for each county, with the result that sheets from neighbouring counties do not line up at the sheet edges. The scans have been resampled and geo-referenced by the NLS so that they do align seamlessly (mostly) to create a single "slippy" map and can be cross-faded to modern maps and aerial images. The original unmodified scanned sheets are available in the Find by place function on the NLS web site, and are more likely to match your original drawings than the Georeferenced slippy map version. If you look at the geo-referenced maps for a long way west or north of Greenwich, you will notice that the OS grid lines have become slightly curved, and/or run at an angle across the page. Some railway tracks and long platforms which are dead straight on the original scans are shown slightly curved on the geo-referenced maps. Templot templates make no allowance for the curvature of the Earth, so this could be a problem for those wanting an accurate replica of a specific location.  cheers, Martin. |
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posted: 1 Sep 2020 09:50 from: ikcdab
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Hi Martin, I now cannot remember the datum details for county series maps. I last used these back in the late 1970s when I worked as a cartographer. On a technical point, the projection is the system used to project the curved lines of lat and long on a flat sheet of paper. Mercator, transverse Mercator, Cassini etc. I think the county series might have used Cassini. The "datum" is the reference position for those lines. OS used their 1936 datum for many years, obviously this wasn't around when the Victorians surveyed the county series. Now the modern datum is WGS84 and its various refinements. My original comments on the other thread are still relevant. The UK was precisely measured by the early 20th century and trig pillars precisely positioned. So although the maps may be drawn on different cup thh datums, they will all be internally accurate and a simple xy shift will bring them into agreement. It's when the scans get rubber-sheeted to fit universal Google projections that distortions begin to occur. If only the earth was flat then these issues wouldn't occur! |
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posted: 4 Sep 2020 18:39 from: DerekStuart
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Thanks Martin. Although I have seen the distortion you describe, I haven't found any in this case. I imported directly into Templot. I have put the templates over the map as usual and after a little 'fettling' what has pleasantly surprised me is how various key trackwork matches up *exactly* between Templot and the reference photos. For example, 7 years after I started this I have now found the Whitby station tandem is a B6.75/C9R- The partial templates together even provides the 'extra' timber that I can see in the photos. Considering the map I am using is no newer than 1914 and has been copied then digitized then imported into Templot, it's quite incredible really. Again, thanks Derek PS just to reiterate the accuracy of the mapping system: There is a straight piece of track containing a 'derail to centre' catch/trap. I put in a straight piece of plan track just as a placeholder- and when I've just come to replace it with a catch/trap... it was LESS than 40thou out on the distance. That's pretty good and demonstrates a great skill from cartographers in 1914 and the ability of Templot. Well done. |
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| Last edited on 4 Sep 2020 18:57 by DerekStuart |
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posted: 5 Sep 2020 18:39 from: roythebus
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ikcdab wrote: Hi Martin, I now cannot remember the datum details for county series maps. I last used these back in the late 1970s when I worked as a cartographer. On a technical point, the projection is the system used to project the curved lines of lat and long on a flat sheet of paper. Mercator, transverse Mercator, Cassini etc. I think the county series might have used Cassini. The "datum" is the reference position for those lines. OS used their 1936 datum for many years, obviously this wasn't around when the Victorians surveyed the county series. Now the modern datum is WGS84 and its various refinements.The Flat Earth society has members all round the globe. |
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