What is a datum

What’s in a datum?

Michael Krome Location Intelligence

And why asset managers should care.

Location plays a big part in managing assets. When you need to maintain kilometres of underground pipeline, or service submarine communication cables, being able to pinpoint the location of access points with a high degree of accuracy can have a huge impact on the effectiveness of your entire operation.

As enterprise asset management specialists with spatial and geomatics expertise, the crew here at Clarita are keen on helping our clients do exactly that. Which is why we’ve been having lots of conversations lately about the modernisation of Australia’s geocentric datum. From January this year, the process of replacing GDA94 with GDA2020 officially began with the aim of bringing Australia’s national latitude and longitude coordinates into line with global satellite positioning systems.

To understand exactly what is happening and why, it helps to have a handle on some of the key concepts involved. The Intergovernmental Committee on Surveying and Mapping (ICSM – the body responsible for overseeing the datum modernisation) has published an excellent set of Frequently Asked Questions that will help you get to the bottom of all things GDA2020.

The beginning is always a pretty good place to start, so in this post, we’ve distilled the highlights to explain what a geocentric datum is and its relevance to asset management systems and processes.

Geodetic Datums

There are many different datums worldwide, used to translate positions indicated on maps to their real position on Earth. In Australia, we have historically had two types of datums – Geodetic Datums and Geocentric Datums.

Geodetic Datums consist of a reference surface (or Ellipsoid), a coordinate system and a set of defined reference points to define a “best fit” to the earth’s surface over that part of the Earth.  Historical datums such as the Australian Geodetic Datum (AGD66, AGD84) were examples of a geodetic datum, as they best defined the shape and location of Australia in isolation from the rest of the world.

Geocentric Datums consist of a reference surface (or Ellipsoid) which uses the centre of the Earth’s mass as the base-point reference.  The Geocentric Datum of Australia (GDA94 and GDA2020) are examples of this.

Static vs dynamic datum

All current and historic datums in Australia have been defined as being “fixed” or static to the Australian tectonic plates.  The reference points move along with the tectonic plate and the coordinates appear to be unchanging, or static, over time. GDA94 and GDA2020 are both static datums.

In 2020, Australia is going to adopt a dynamic national reference framework.  The Australian Terrestrial Reference Framework (ATRF) will align with the International Terrestrial Reference Framework (ITRF). The ATRF will be defined as being “fixed” to the Earth as a whole (not just fixed to Australia), and aligned to the centre of the Earth’s mass.  The reference framework will move with the Earth, so the coordinates of reference points on a tectonic plate appear to be dynamic, or changing over time.  The ATRF will be adopted to deliver the highest accuracy possible between location references and navigation technologies such as GPS / GNSS.

Why you need to know your datum

Depending on how important location is to your asset management operations, you will likely have recorded location data in the form of geographic coordinates. It’s important to know which datum was used to generate those coordinates, as a specific location can have substantially different coordinates depending on the datum used to make the measurement.

For example, there is a 200 metre difference between coordinates captured using the GDA94 datum compared with the historical Australian Geodetic Datum (AGD84). This margin of error may be acceptable for broad scale applications, but is not suitable for managing asset infrastructure.

Storing asset location information with metadata identifying the underlying datum of spatial reference is widely considered best practice.  As we move toward the introduction of a dynamic reference framework (ATRF), the requirement to record the date of measurements and an estimate of its reliability and quality or accuracy will become a necessity to maintain the integrity of your organisation’s data.

Map Projections

Typically all asset managers operate their assets based upon a map projection using metric units of measure (Eastings, Northings, Elevation), as they are easier to understand than latitude / longitudes related to the datum.

Map Projections in Australia are derived by applying a Universal Transverse Mercator (UTM) projection over the relevant datum.  The corresponding map projection for GDA94 is Map Grid of Australia 94 (MGA94).  When GDA2020 is implemented, it too will have a corresponding UTM map projection MGA2020 which will be approximately 1.8 metres different to the MGA94 coordinates.  This reflects the projected differences between the GDA94 and GDA2020 latitudes and longitudes.

 

How does this affect you?

Understanding the terminology and reasons behind the modernisation of Australia’s datum is one step. The next is understanding how this change is going to impact your asset management operations.

There is a significant risk that if asset managers don’t manage their data in relation to the correct datum and projection, they could find their assets up to 1.8m away from where they thought they were. Clarita Solutions’ GDA2020 Readiness Assessment is a fixed price engagement, intended to help identify the scope of change required and plan an implementation approach. Click below or contact us for further details.

Click for details on Clarita's GDA2020 Readiness Assessment

 

Feature Image by NASA Goddard Space Flight Centre from Greenbelt, MD, USA (Melbourne, Australia) [CC BY 2.0 (http://creativecommons.org/licenses/by/2.0)], via Wikimedia Commons