I have just added a line of sight calculator to my website.
I wanted something that could accept a summit reference for either or both points.
You can also use an os grid reference, lat long or a uk postcode.
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That’s brilliant. Thanks Paul.
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There are a number of on-line line-of-sight calculators. Many of them do not take account of the Earth’s curvature. They are accurate only for short distances between summits. Many will be surprised to discover, no matter how high your summits are, like an Everest to another ‘Everest’ how the Earth’s curvature makes LoS impossible beyond a certain distance. And that’s assuming the Earth is a perfect sphere (with no ‘bumps’) between summits.
Even fewer models take account of of the intervening terrain (like a higher mountain) that would obstruct a LoS path. Usually that needs a high-resolution Digital Elevation Model (DEM).
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I am only using a medium resolution EU DEM as that is available from open topo for free 
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…I found this only works in the uk. Not VK.
Geoff vk3sq
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Just tried it from VK3/VN-016 Mt. Alexander to VK3/VU-009 Mt. Ida. Known to be line of sight (have been up them both 100’s of times, sometimes even for SOTA). Shows as “not LOS”. Looks like the maths has problems with VK. Hope it gets fixed ‘cos it looks like it could be a good resource. Cheers, Phil
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It appears to work fine in DL - perhaps this is a northern vs southern hemisphere issue?
73 Ed.
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It should be fixed now.
The problem was to be honest, that I hadn’t considered outside of Europe. I was only using an EU elevation model. The tool now selects the correct elevation model, depending on what part of the world the summit is in.
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And it’s rather good Paul 
I spent the last 20 years modelling. (pre-silicon semiconductor simulations as opposed to glamour or clothes 
) and what you learn backs up the old joke “the difference between theory and practice in practice is greater than the difference between theory and practice in theory”. I have run your calculator on some known true LOS paths and some which were scratchy on 13cm and it’s giving results that are very close to what I observed for flat band conditions. I have only run it using the 23cm option and on 13cm the Fresnel zone is narrower, results maybe better. But as it stands, it is agreeing closely with observations. By the time you get to such scratchy/marginal contacts on 13cm, there is a good chance you and your QSO partner may well be able to use a common reflective area for a better signal.
This is therefore a rather excellent tool for those of us who play on the higher bands never mind its usefulness for 2/79.
Thanks
I have on many occasions throughout my career had to explain that to people.
See my post below for another fringe example
Everyday is a school day
For some years, I thought could just squeeze my 2m wave along the side of a lump of rock that stands between my house and GW/NW-043. I have struggled with many a QSO on that summit, but always assumed it was all down to the transmitter station.
Well it turns out that I seem to bounce over the shoulder of that lump of rock. This probably accounts for some of the variability I get with QSO’s on Cairn y Brain. But this also shows that in marginal cases other factors come into play beyond the theory.
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