# Why Geoids Matter for Planning

The UK has a new Geoid OSTN15

What is a GEoid and why does it matter?

To obtain accuracy in any plan it needs to be referenced.  There are to, a projection and a datum/geographical coordinate system.  The former describes how the flat plan wraps around a sphere, the latter the mathematical difference between that sphere and the warped and bumpy shape of the earth.  Its derived from a gravity model so the more hilly or mountainous the terrain the bigger the problem.  You can use a global model – such as WGS84 – used in Google and by GPS, but that will be way too inaccurate at local level for infrastructure purposes. So ideally you develop a local geoid which is only really detailed in your local area.  Ideally each significant patch of the earth should have its own geoid.

I found the importance of this in three recent cases.  Firstly been doing a lot of work recently in Afghanistan – recently.  GPS signals were showing a road 25m in elevation off true, why, because the survey was based on US maps based on WGS84 not the local HArat North and the area was surrounded by 3,000m + high mountains.

Secondly I was managing a new masterplan for a city in Saudi near Mecca where the cut and fill calcs made the site uneconomic. The reason it was drawn on a google maps base and transposed to the local system.  Leading to a 25m shift the mountains were in the wrong place.  The matter was coumpounded by the military moving the whole coordinate system by several meters – believed to protect the Kabaa from Drone attack – a not uncommon tactic internationally I understand.

Finally I had an infrastructure plan for a new city – it was shifted as a whole by the corect amount.  The problem is the differences are different for every point in space.  It should have used a mathematical MLS model to calculate the correct shift for every point in space.  We had already done that for the Riyadh Metro.

So if you are shifting to OSTM15 from old survey data – be careful – you likely need specialist GIS advice to transform all yur survey and engineering data accurately.

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