What is the difference between a local and global coordinate?

This comes up a few times so I thought I woukd capture it. When you create a point you can create a Grid or Grid Only point and it can have a Local or Global Coordinate. What does this all mean?

When you create a point you can define a point as Global and that is its latitude/ longitude and height defined in the WGS 84 coordinate system using the standard reference ellipsioid of the earth.

If you create a point in some coordinate systems eg US State Plane on NAD83 the global and local coordinates are usually the same. This is because the WGS84 and US State Plane Coordinate Systems utilize the same ellipsoid. If however you work in Sydney Australia the New South Wales coordinate system uses a different ellipsoid for the coordinate system so points there will have a different local oordinate in Lat Long Ht than the Global Coordinate Lat Ling Height. So the difference between Global and Local is that the two coordinate systems do / do not use the same ellipsoid definition.

So Grid and Grid Only points - what are the differences here. When you create points by importing point data the import script allows you to create Grid or Grid Only points. A number of other TBC commands but not all, that create points allow you to do the same thing. So what is the difference. A Grid Only point is like a CAD point, it has a position that is unaffected by a change in coordinate system. A Grid point is defined and its position is directly connected to the coordinate system - if the coordinate system is changed then the points coordinates will be recompiled in the new coordinate system ie they will move.

For construction, we recommend the use of Grid Only points for most things primarily because the coord system for a project should be the first thing you lock down and it should never change once you start work. You for sure do not want your measured or stakednor design locations moving once created.

Surveyors who want to progressively update the coord system while developing control andbtopo survey on a project however likely want to use Grid points because as the control and site calibration evolves or is repeatedly computed, you want your measured points to also update as those changes happen.

Hope this helps


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Very clear and lucid as always. Thanks.