Cheriton Hill G/SE-015 Demoted

Sorry to bring some sad news, but just found out that Cheriton Hill G/SE-015 is no longer a Marilyn. Although I will the doing the G Association update next month with the date of the update to be set as 2nd March 2021 to be in line with SOTA’s 19th Birthday, I will be setting the valid to date for Cheriton Hill G/SE-015 as 31st August 2021 to hopefully give chance for those who haven’t activated this summit yet to do so once we come out of Lockdown. Here is Alan Dawson’s announcement in regards to Cheriton Hill G/SE-015. You will noticed in this announcement that Cliffe Hill G/SE-014 was also re-surveyed and this is confirmed to remain as a Marilyn with a drop of 150m exactly. Other changes relate to none SOTA summit.

"Some Marilyn and Submarilyn updates as a result of more Lidar data
becoming available.

- Cheriton Hill in Kent has finally been demoted from the Marilyn list.
In 2016 I surveyed it and found it had 149.7m drop, but owing to the
margin for error at both summit and col I left it as 150m until further
evidence was available. Lidar has now provided that evidence. It
confirms 149.7m drop but with a col in a different location and the
summit location still somewhat uncertain. With two independent
measurements showing that the drop falls short of 150m, it has to be
reclassified. Cliffe Hill is also borderline but it remains with 150m
drop owing to the potential margin for error at the col. The summit is

  • Hen Comb in Cumbria has made way for Cheriton Hill. OS maps show it to
    be 509m but Lidar shows it to be only 506.2m, with 138.8m drop not 140m.

  • The Verne in Dorset, next to the prison, has a drop of 143.6m
    according to Lidar data, which is enough to convince me that
    fluctuations in the col on Chesil Beach are unlikely to raise it by over
    3.6m. At a mere 152.3m high, it replaces Glastonbury Tor as the lowest
    Submarilyn on the mainland. Portland just about counts as mainland. Bit
    of a nuisance really as I have not climbed it and have no reason to go
    all the way down there again.

  • Mochrum Hill near Maybole, south of Ayr, becomes another new
    Submarilyn, with height 270.7m, drop 140.4m. Turns out that we had
    recorded the wrong col location, which is easily done in flat areas.

These changes have just been applied to the Hill Bagging site, along
with various other updates, so if you notice that your total of Marilyns
has gone down by one, you know why.

Thanks to the DOBIH team for their detailed analysis of Lidar data to
prompt these updates.


Jimmy M0HGY
G - Association Manager


Just when you thought 2021 couldn’t possibly be worse than 2020…

If and when tour work resume for me as a musician, this comes as bad news. There’s a big venue in Folkestone that usually hosts one of the shows. Probably result in extra zero point activations of Detling Hill G/SE-013 from now on.

Saying that, Cheriton was always a rubbish hill. Good riddance.

I think I’m right in saying that it wasn’t in the original Marilyns list and got added later when it got “promoted”. Maybe it will become the “West Bromwich Albion” (yo-yo club) of SOTA!

Wasn’t the original Birk’s Fell or Horse Head Moor?

Good. There’s no rush to upload anything then.

You’ll probably find that Jimmy has updated new official heights, grid references of summits etc for the annual 2nd March G anniversary update Andy. I don’t know for certain (as he’s still in bed) but every year there’s usually a number of these updates, and he puts them in for the uploads you do at the end of February.

Please check your emails Andy, I will be doing the update next month and the publish date will be 2nd March 2021 so this will be on the uploads for the 28th February. I will have the valid to date as 31st August 2021 on the SL ready for this upload then. No extra work will be needed in August for this.

Jimmy M0HGY
G - Association Manager

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So, for the sake of 30cm, can someone carry enough rocks and soil up to the summit and thereby “walk up a hill and come down a SOTA mountain”?


There is already a man-made reservoir on this summit Andy and if that was counted the hill would be a Marilyn. However such man-made features do not count unless loads of vegetation grows through it meaning that it would not be considered man-made anymore which was the case for Hensbarrow Downs G/DC-008. As the reservoir on Cheriton Hill is classed as man-made, it doesn’t count and it you put rocks on soil on top, this wouldn’t count either as that would be classed as man-made. Cheriton Hill G/SE-015 is officially no longer a Marilyn so I will be removing it from the SOTA programme, but allowing activations to continue to happen on this summit until 31st August 2021.

Jimmy M0HGY
G - Association Manager

Quite a few of the previous changes would have resulted in no change to the observable data. If the only observable change is the end date of a summit which is 7 months in the future then there’s no need to bust a gut implementing it.

I wish I could find a way of preventing unwanted vegetation (weeds) from growing through my garden!

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Surprised that it’s been demoted based on 30cm ‘measurement’ when the margin of error on the LIDAR is +/- 15cm.

Unless it was a different survey?

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That’s about typical for LIDAR. We have removed summits for less!

Assuming the error isn’t lopsided anything less than 150m probably isn’t a Marilyn. In this case it’s backing up his earlier measurement, so the chances of it being “good” despite all this will be slimmer than the 1-in-20 one might (crudely) infer. I haven’t looked up how far away is the col, but one might also suppose that the relative error will be not so bad (correlated at least to some degree, so better than a quadrature addition).

The Relatives Hills of Great Britain have been the UK’s (and Ireland) leader surveyors for a numbers of years and the Ordnance Survey have always updated their maps in line with the latest Relatives Hills of Great Britain results. SOTA has always used the Relative Hills of Great Britain survey results ever since SOTA started nearly 19 years ago and will be continue to use Relative Hills of Great Britain’s data in SOTA.

I will be updating the SL’s soon, but I am happy for you hold of uploading these to the SOTA Database until nearer the deletion date for G/SE-015. However every year we have done the G - Association updates at the beginning of March each year so this is in line for SOTA’s Anniversaries. If you are holding of uploading the SL’s, I will update the SL’s in February and again when the next Hills-Database update is done which could be any-tine between now and August.

Jimmy M0HGY
G - Association Manager

I just thought I’d remind anyone that wishes to activate Cheriton Hill G/SE-015 for SOTA that they only have until 31st August to activate this summit. After 31st August Cheriton Hill G/SE-015 will no longer be a SOTA summit due to no longer being a Marilyn.

Jimmy M0HGY
G - Association Manager



Re altitude measurements.

I understand that you are beholden to the official Marilyn determiners but there is perhaps too great a belief that the numbers are sufficiently reliable to add or ditch a peak based on a 15 cm difference. There is an important difference between precision and uncertainty which likely makes such a decision wrong.

The LIDAR RMSE is claimed to be 15 cm so the uncertainty, the 95% confidence interval is +/- 30 cm and the 99% confidence interval is +/- 39 cm. If the height of the saddle is determined with the same uncertainty as the peak then the prominence has an uncertainty of 42 cm. If could be worse if the influence factors are correlated.

Given there is no mention of systematic errors or influence quantities, making decisions on prominence with differences of less than 0.3 m is wishful thinking.

I spent many years explaining to surveyors that precision was not the same as accuracy/ uncertainty. If your tape measure has unknowingly stretched making a hundred measurements and getting a fine average does not fix the tape error.

A GPS based survey instrument of suitable quality is the only way to measure these small height differences with any degree of reality.

LIDAR might be sexy but it is second best in terms of uncertainty. It is of course useful if a boots on the ground survey has not been done.

In VK we have some impressive claims for LIDAR based on average errors. This does not tell the whole story. There are individual errors between surveyed points and the LIDAR of over 40 m for orbiting mappers but the averaging of the errors makes this so diluted that it is not noticed. That’s no help if it’s your peak that has the bad result.

Aircraft based LIDARS typically have measured uncertainties of 0.5 m. Vegetation can contribute to greater uncertainties and height offsets. There are corrections applied but a residual always remains.

Drone based LIDAR mapping for local areas is a growing activity. Watch this space for more wonderful claims.

The life of a peak mapper is filled with uncertainty and doubt.


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Good, because survey grade GPS is what “The Tamperers” use when re-surveying. Glad you support their choice of instrument.

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I wasn’t being critical of their use of GPS.

I do think the LIDAR that was referenced is over rated for the reasons I have given. The surveyors did not act on their initial survey in at least one case but waited to see LIDAR results. That says they weren’t confident in their own GPS work. And for such a small shortfall, justly so.

My comment on “suitable” was an allusion to the fact that not all GPS surveying instruments are the same. Survey instruments use carrier phase rather than basic time stamps but the addition of differential mode GPS is necessary for measuring to better than a metre in a reasonable time. It’s that damn ionosphere. Post measurement data adjustment can also help but I’m unclear as to what was available to these guys.

I repeat I understand SOTA can’t go out on a limb and ignore the Marilyn surveyors.

There was some discussion on whether 0.3 m was a reasonable amount to condemn a peak. I was adding to this. If this offends some then as always TT.


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The surveyors that review these things know what they’re doing. They use the best available equipment and processes, and ensure that decisions are only made on the basis of strong mathematical confidence.

The shrinking number of these changes every year is a convincing indication that their methods are as good as they can be.

Our government national mapping agency, Ordnance Survey, accepts the data they produce - it’s that highly regarded.

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I am well versed in surveying mathematics and measurement uncertainty modelling and don’t doubt these surveyors are well trained and good at what they do. But best available does not mean error free or even adequate for what is being attempted. The apparent lack of inclusion of error sources other than random variations gives me little comfort in regard to the 15 cm accuracy claim for the LIDAR.

The fact that there are changes from year to year is an indication they are still trying to get the best numbers and when they demote or promote a summit they are admitting to errors.

If you are interested in measurement errors you should read about why the metre is the wrong length. It’s out by about 0.2 mm due to errors by the best available survey people with the best instruments.


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It’s an indication that someone is re-surveying the old measurements and confirming them or producing more accurate versions.


I won’t step into discussions about accuracy v. precision, but an earlier point intrigues me:

We have cases of both ancient and more recent man-made mounds getting classed as summits because they have become vegetated. In some cases they have become naturally vegetated, in other cases they have been artificially vegetated to stabilise them. In the case of Cheriton Hill an artificial mound no bigger than a garden rockery would restore it to Marilyn status as long as it was vegetated. It would take no more than a dumper-truck load of ballast and several square metres of turf and perhaps a sapling or two and some wild flower seed to produce a Marilyn as authentic as Hensbarrow Downs!