It seems nothing is fixed if you look closely enough, a few months ago we posted The Planning Paradox How much detail is too much? Which looked at the ‘coastline paradox’, which in summary states, the smaller your measurement unit, the longer a coastline becomes. This post is a corollary – the fact a reference point moves does not invalidate the reference.
Every height in the in the Great Britain is stated as a height above (or below) the Ordnance Datum Newlyn (ODN) – Ireland uses a different datum. The ODN is defined as the mean sea level as recorded by the tidal gauge at Newlyn in Cornwall between 1915 and 1921, and is marked by a brass bolt fixed to the harbour wall in Newlyn.
(a) The brass bolt benchmark (OS BM 4676 2855) which is located in the Tidal Observatory and from which the ODN national datum is defined as being 4.751 m below the mark, and
(b) the cover of the historic mark.
While ODN was a measurement of mean sea level in 1915-1921, it is important to recognize that mean sea level has risen since then, so it is best to think of the ODN as an arbitrary height reference point that has been used for the past 100 years rather than a reflection of the actual current mean sea level.
Given the Cornish peninsula is made of solid granite, as is the harbour wall, Britain has a fixed reference point for all levels, so everything should be good….. or is it?
The problems start when you measure heights from space using accurate GPS, these measurements show that the whole Cornish Peninsular rises and falls by several centimeters twice every day in response to the tidal loading caused by the very high tidal range in the region moving gigatons of water in and out of the English and Bristol Channels. While moving up and down by as much as 13 cm sounds dramatic, and is measurable, the deflection is very small when compared to the scale of the earth. In contrast, an error of 13 cm in an engineering survey would be very significant.
The effect of tidal loading is not restricted to Cornwall, an academic paper by P. J. Clarke and N. T. Penna has determined ocean tide loading (OTL) affects all parts of the British Isles to varying degrees, causing peak-to-peak vertical displacements of up to 13 cm in South-West England (Cornwall), reducing to a few millimeters as you move further inland. OTL also causes lateral displacements as the earth’s crust flexes, these are typically around one-third of the magnitude of vertical displacements.
All of this flexing means there will be measurable differences between surveys done using the Global Navigation Satellite System (GNSS) and those based on the Ordnance Survey datums across the UK, And the difference will change continually throughout the day. The differences are calculable but which reference system is correct? Most modern survey equipment uses GNSS, but the location of everything shown on maps and plans is based on measurements derived from the Ordnance Survey datums.
Combine this with the ‘continental drift’ discussed in Knowing (exactly) where you are is not that simple! and the challenge of creating an accurate survey becomes apparent.
The first question is which reference point really matters for what you are doing. Most terrestrial surveys are positioning things on land, property boundaries, foundation levels, etc. Given all of the ‘land’ in a location will be moving more-or-less as a single unit, measurements from the datums fixed to the land are usually going to be the most useful. It is only if you are operating at a global level, the GNSS data becomes more useful. Surveyors the world over use equipment based on GNSS (it makes their job much easier), but typically reference their equipment’s datum back to a local survey mark – they calculate the relative differences based on this fixed reference point – the fact everything is moving becomes irrelevant.
So, what has all this got to do with project controls? I would suggest two things:
- First excessive detail is the enemy of useful information. When you are using a map on a hike, you don’t care if the reference points a moving a few centimeters, you just want to know how many miles to the next pub!
- Second, you need a valid reference point and metric. If you are looking a measuring ‘velocity’ in a project using Scrum, or productive effort in an engineering facility, using hours of effort are likely to be more meaningful than the overall cost which can be distorted by the variable pay rates earned by different people. But, if you are measuring the overall viability or profitability of a project, then the overall costs do matter.
For more on project controls see: https://mosaicprojects.com.au/PMKI-SCH.php
This post is part of a series looking at The Origins of Numbers, Calendars and Calculations used in project management: https://mosaicprojects.com.au/PMKI-ZSY-010.php#Overview
Pat Weaver
Lynda Bourne
Aavssitedev's Blog
A very interesting article. We tend to think the earth is fixed… the idea that the earth’s crust flexes with tide load introduces other considerations… For example what happens when you remove millions of tons of ore from one area or build mega cities in places that was originally open fields… If the ground level sinks in one area what happens elsewhere?