The MT Blog

Finding Summits - Part 1

19 June 2017
Simon, G4TJC
SOTA, in common with other amateur radio awards programmes, consists of collecting QSOs satisfying a particular criterion. For others this may mean islands, map squares or DXCC entities, for instance. For SOTA it means valid summits. One of the strengths of the SOTA programme is that we use a strong, objectively-defined test for validity. We do not pick and choose summits on a whim; they must have sufficient topographic prominence. Usually the minimum is 150m, hence P150.

What is Prominence?

This can cause confusion, but actually it's fairly simple, especially if you're only concerned about passing the minimal test. Technically the prominence is the difference between the elevation (altitude) of the summit and of its key col. Ok, but what is a key col, or any kind of col for that matter?

Cols and Key Cols

This is a French word that we use in English. If you prefer, think saddle point. I like to think bwlch because most of the mountains I climb are Welsh! Let's start with a mathematical definition just to bamboozle you. A col is a stationary point in the topography. This means in one direction its a minimum and in another it's a maximum. So, if you're passing along the ridge line joining two summits (could be a knife-edge ridge - an arête - or just a smooth grassy slope; it's all the same) the col is where you dip down to the lowest point before you start climbing back up. If you're coming up from the valley below, trying to pass between summits with the least effort, the col is the highest you have to climb before dropping back down.

Another way to explain this is with sheep - yes, honestly...

Imagine a flock of sheep strung out in a line part way up a hill, each one slightly further up the slope than the one below it. They munch away at the grass, gradually moving forwards, staying at the same elevation. Actually they are slowly proceeding along the contours. Sheep above the col will loop around the hill and finally find their way back to the start. Sheep below the col will cross onto the adjacent hill, munch their way all around that, and finally arrive back at the start point somewhat later than their brethren. The division between the sheep that go just around the start hill and around the other hill is the height of the col.

But what about sheep lower still? On their way around the second hill they might come to another col, with the lower sheep trudging around a third hill, and so on. Now the factor to bring in is how high are these other hills? In many cases they will be lower. Think about a typical ridge line with all manner of lumps and bumps. Every point where your ridge line heads up a bit on your (mostly) descent from the summit counts as a subsidiary summit with its own col. We're not bothered about those. What we're looking for is the first summit that is actually higher than our starting summit. This has its own col too - the point where the higher sheep loop around the first hill and the lower sheep go all the way around this higher hill. It's this that we call the key col, and the difference in elevation between this point and our starting summit (not the higher summit) is its prominence.

Comedy sheep above, at and below the col

Post-Apocalyptic Islands

In case the sheep have confused you I have an even better way to visualize this.

Imagine that there has been a catastrophic rise in sea level and your summit has now become an island, with just 150m down from the summit to the new shore. This is an easy test for P150; if your summit is the highest point on the island it passes. If you can now drop sea level until dry land just joins your island onto an island with a higher summit that is your key col, and the height of your summit above this new sea level is the prominence.

What's especially good about this way of looking at it is that you can easily try this out for yourself, and there's no need to wait for global warming!

You will need Google Earth and a special kml (or kmz) file from here. Start by locating your summit in Google Earth and then load the flood file. It's set to flood to 1000m, so if your summit is in the UK you'll likely be seeing a screen full of blue by now. Find the flood item in the list and right click to change its properties. You'll find that there is an altitude tab. Here you can type in the height of the flood or simply drag around the slider.

Actually it's good to start high, or perhaps at exactly your summit elevation. This way you can see where are the adjacent higher summits. You may need to zoom out your view, in which case the flood object will reset itself to the starting altitude, necessitating another visit to its altitude property tab. Next drop sea level to 150m below your summit altitude. Could you cross from your summit to any of those higher summits without getting your feet wet?

Examples

Let's try this out with an example of a hill in Dorset, southern England. Pilsdon Pen is topped by an iron-age hill fort and is noted as the second-highest hill in Dorset. So it should be one of the Dorset SOTAs, right?

Well, not so far away is Lewesdon Hill. This is the highest point in Dorset at 279m and is a SOTA (G/SC-009). Pilsdon Pen is just 2m lower, at 277m. So, let's find the col using the Flood object in Google Earth. In the image below I have dropped the flood level to the point where the dry areas of Pilsdon Pen and Lewesdon Hill just touch. This is at 180m. But what we need for P150 is 277m - 150m = 127m. So this is one nice hill fort that does not qualify for SOTA. There are lower SOTAs in Dorset despite this. Why? The surrounding terrain drops down more to a lower col, resulting in a prominence of 150m or more.

Google Earth flooded to locate the col between Pilsdon Pen (left) and Lewesdon Hill (right).

Let's look at a hill fort that does  pass -  Foel Fenlli in North Wales. The summit is at 511m, so we need to check with a flood to 361m. It turns out it's quite close to the pass/fail point. The flood is quite blocky when zoomed out, so I zoom in on the col towards the higher summit of Moel Famau, at the Bwlch Penbarra (remember bwlch is Welsh for col) car park.

Oh no, the car park is under water! 3m over the col.

With the flood set to 361m there is clear separation between Foel Fenlli and Moel Famau. Google Earth is great for guiding us to the col but for an accurate check, especially in a case like this, we must follow the contours or check spot heights on the best available mapping. For Wales we have Ordnance Survey mapping at 1:25,000 scale, which confirms 511m for the summit and conveniently has a col spot height. This is at 358m, so we have P = 153m - a good SOTA but not by much.

Found an Unlisted Summit?

So, if you have found a summit that is not in the appropriate SOTA summit list you can check it using this method. Be sure to check carefully as your local organizers (AM and RM) and the SOTA summits team will have already looked. If the flood test passes go on to check against contours and spot heights on a good, current, topographical map. If it still looks good the procedure is to contact your local region manger. There is an update cycle for all the associations. Don't expect the team to drop everything to get your summit in right away.

Other Methods

The flooding approach is handy for a quick test of an individual summit and is good for guiding the eye around a topo map (particularly if you can load the map into Google Earth as an overlay). But it's not so practical for checking large areas with many summits. Techniques for handling this will be outlined in another posting.